FOUS30 KWBC 052017
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
316 PM EST Fri Mar 05 2021

Day 1
Valid 16Z Fri Mar 05 2021 - 12Z Sat Mar 06 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Otto

Day 2
Valid 12Z Sat Mar 06 2021 - 12Z Sun Mar 07 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann


Day 3
Valid 12Z Sun Mar 07 2021 - 12Z Mon Mar 08 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann


$$
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FOUS30 KWBC 062013
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
312 PM EST Sat Mar 06 2021

Day 1
Valid 16Z Sat Mar 06 2021 - 12Z Sun Mar 07 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

...Eastern Florida Peninsula...
There is a non-zero (but sub-Marginal Risk) threat for flash
flooding along central portions of the eastern Florida Peninsula
today and early tonight, in advance of a low-mid-level trough
approaching from the eastern Gulf Coast. Total precipitable water
values are near average for this time of year (1.0 to 1.3 inches)
but kinematic profiles support slow mean storm motions and weak
low level flow but 50 to 60 kt near 500 mb from the west. Vertical
motions may be augmented by the right entrance region of a
forecast 130 kt upper level jet streak just east of the Florida
coast. Stronger cells elevated above cold pools will have the
greatest threat to produce localized totals in excess of 3 inches,
but only weak instability is expected throughout the day/night.
Given drier than average antecedent conditions over the past 2
weeks, the small spatial threat of heavy rainfall and limited
coverage, the threat for flash flooding is less than 5 percent.

Otto


Day 2
Valid 12Z Sun Mar 07 2021 - 12Z Mon Mar 08 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann


Day 3
Valid 12Z Mon Mar 08 2021 - 12Z Tue Mar 09 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann

$$
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FOUS30 KWBC 072013
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
312 PM EST Sun Mar 07 2021

Day 1
Valid 16Z Sun Mar 07 2021 - 12Z Mon Mar 08 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Otto


Day 2
Valid 12Z Mon Mar 08 2021 - 12Z Tue Mar 09 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann



Day 3
Valid 12Z Tue Mar 09 2021 - 12Z Wed Mar 10 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann

$$
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FOUS30 KWBC 082040
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
339 PM EST Mon Mar 08 2021

Day 1
Valid 16Z Mon Mar 08 2021 - 12Z Tue Mar 09 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Chenard


Day 2
Valid 12Z Tue Mar 09 2021 - 12Z Wed Mar 10 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Otto


Day 3
Valid 12Z Wed Mar 10 2021 - 12Z Thu Mar 11 2021

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE CENTRAL PLAINS AND MIDWEST...

...Eastern Kansas into western Illinois...
Thunderstorms are expected to form along a cold front moving
southeastward through the Central Plains and Middle Mississippi
Valley Wednesday evening into Thursday morning. A resurgence of
850 mb flow from the southwest, from 40-60 kt) is expected from
the Southern Plains into the Middle Mississippi Valley between
00Z/11 and 06Z/11 as subtle low-mid level height falls push out
into the Plains after 00Z/11. Thunderstorms are forecast to
develop with the increase in 850 mb flow, within a corridor of
CAPE expected to range between 1000 and 2000 J/kg ahead of the
cold front from central Kansas into central Oklahoma. Precipitable
water values are forecast by the 12Z model consensus to climb to
near 1.25 inches which should support rainfall rates of at least 1
in/hr, locally higher. The concern for training arises given mean
steering flow is parallel to 850 mb flow and of similar magnitude.
This would support an axis of heavy rain oriented from southwest
to northeast, with the greatest potential for higher amounts to
the south and west where frontal movement will be slower, and
instability will be higher.

Current flash flood guidance values are as low as 2 inches in 3
hours for a large portion of Kansas into Missouri and Illinois.
Factors against flash flooding include the dry antecedent
conditions and somewhat modest moisture values, despite the
moisture being rather anomalous for this time of year (+2 to +3
standardize anomalies of PW). The Marginal Risk area was
coordinated with area WFOs and encompasses some spatial spread as
seen in the latest deterministic/ensemble guidance, extending from
the eastern Kansas/Missouri border into central/northern Missouri,
western Illinois and far southeastern Iowa.

Otto

$$
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FOUS30 KWBC 092048
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
347 PM EST Tue Mar 09 2021

Day 1
Valid 16Z Tue Mar 09 2021 - 12Z Wed Mar 10 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Pagano


Day 2
Valid 12Z Wed Mar 10 2021 - 12Z Thu Mar 11 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN WISCONSIN...

East of a surface low pressure system tracking northeastward
across the Central Plains into the Upper Midwest, a front is
forecast to meander over northern Wisconsin during the day on
Wednesday, with moisture pooling along it. Precipitable water
values should be over 1", over the 95th percentile for this time
of year. Some elevated CAPE of 100-250 J/kg could enter the region
in the evening, leading to rain rates exceeding 0.5". These
marginally heavy rain rates and 1-2" of rain total could become a
problem particularly because the rain is falling on snowpack
containing over 2" of liquid equivalent. The rain could cause
rapid melting and thus runoff concerns, allowing for especially an
areal flooding threat/potential for riverine flooding, but flash
flooding cannot be ruled out with these conditions.

A Marginal Risk was removed this cycle from in and around
Missouri, as it appears the cold front should progress a little
faster through the period than previously forecast. This will lead
to a shorter time frame for any localized heavy rain to fall in
any particular area, and thus less potential for training through
12Z Thu.

Tate


Day 3
Valid 12Z Thu Mar 11 2021 - 12Z Fri Mar 12 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MIDDLE MISSISSIPPI AND LOWER OHIO VALLEYS...

The main change to the previous cycle's Marginal Risk area was to
extend it farther west into Missouri due to the frontal system
slowing down after 12Z Thursday. Instability is forecast to make
its way into the area; the ECMWF for example has pockets of over
1000 J/kg of MUCAPE in southern Missouri, which could increase
rain rates, and the slow-moving front could lead to training
issues.

See the previous discussion below for more details.

Tate

...Previous Discussion...

A front slows down as it moves into the OH Valley as the shortwave
urging it along progresses across the Great Lakes and an upper low
digs across the Southwest. This becomes a problem late in the
period as heavy rain shifts into a region that has received above
average rainfall where streamflow remains near to above average
per National Water Model output and the flow becomes increasingly unidirectional out of the west, with low-level inflow remaining
substantial (40-50 kts). Precipitable water values rise to
1.25-1.5" and a pool of instability lies upstream across the
Southern Plains and Mid-Mississippi Valley. While there is model
dispersion in this region, the threat of organized and potentially
training convection late in the period (00-12z on the 12th)
remains sufficient to raise a Marginal Risk area for the region as
a precaution. Guidance appears to be trending south, which if it
continued would lead to an uptick in the QPF in the region on
later WPC cycles and potentially raise the flash flood threat.

Roth

$$
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FOUS30 KWBC 102024
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
323 PM EST Wed Mar 10 2021

Day 1
Valid 16Z Wed Mar 10 2021 - 12Z Thu Mar 11 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN WISCONSIN AND THE UPPER PENINSULA OF MICHIGAN...

...16Z Update...
Moisture continues to stream into the Upper Midwest from the
Central Plains this morning ahead of a deepening surface low that
is progged to track along a stationary boundary through the day.
There is some concern that as the surface low approaches, the once
stationary front will advance a little farther north than
previously forecast (as a warm front). This will likely result in
the axis of heavier precipitation shifting north as well; mainly
far northern WI. Therefore, expanded the Marginal Risk slightly to
account for this. Based on the current radar, estimated rain
rates are around or below 0.50 inches per hour, which is below
FFG. Anticipate this trend to continue with a dry slot possible by
mid afternoon. Regardless, with multiple rounds of precipitation
expected through the next several hours falling atop cold surface
temperatures and snow pack, there could be run-off and thus
localized flooding concerns. Apart from the aforementioned minor
adjustment to the Marginal Risk area, the forecast is on track.

Pagano

...Previous Discussion...
There will be a threat of some excess runoff and flooding today
and tonight in the warm sector of a deepening low, due to a
combination of rain and snowmelt. Warm conditions over the past
couple days have already lead to some snowmelt, but there is still
a significant amount of liquid equivalent (over 2 inches in some
places) in the snow pack over northern Wisconsin and into the
Upper Peninsula of Michigan. Those are the areas with a chance of
some flooding, as rain rates are not likely to be sufficiently
high (or sustained long enough) to lead to flash flooding on their
own. Although models are projecting around 200-500 j/kg of MUCAPE
for convective rain bands or thunderstorms just east of the
surface low, hi-res models are showing hourly rain rates mostly
remaining below 1"/hr. In fact, 40km neighborhood probabilities of
just 0.5"/hr rain rates peak below 40 percent in northwest
Wisconsin.

Therefore, the Marginal Risk was concentrated in areas where the
highest probabilities of 1+ inch of rainfall in 6-12 hours overlap
with existing snow pack and the prospect of additional runoff from
snowmelt. It's worth noting there is still a substantial amount of
spread over the risk area related to rainfall totals and the
character of the warm sector air mass. Some models more
aggressively push higher dewpoints into the area, while others
more stubbornly retain colder air. At KPBH in north-central
Wisconsin by 16Z, the NBM has dewpoints +/- 1 standard deviation
from the mean at 50F and 40F respectively. Higher dewpoints and a
warmer air mass in general would promote more efficient snowmelt
if they materialize. And with respect to rainfall, some of the
newer hi-res model runs (particularly the HRRR) are pushing rain
through quickly and then enveloping most of the Marginal Risk area
in a dry slot after about 18-21Z. The global models have a
stronger signal for heavy rain. This will need to be monitored
closely, but the probabilities of 1+ inch of rain in 12 hours seem
to be high enough even on the HREF (around 60 percent) to warrant
the Marginal Risk.

Lamers


Day 2
Valid 12Z Thu Mar 11 2021 - 12Z Fri Mar 12 2021

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF
THE MIDDLE MISSISSIPPI AND LOWER OHIO VALLEYS...

A slow moving front that eventually stalls and washes out over
portions of mid-MS and lower-OH river valleys will become the
focus for heavy rainfall during the day 2 time frame. Along and
ahead of the front, a surge of higher than normal precipitable
water (1.25-1.5") and sufficient instability (upwards of 1000 J/kg
MUCAPE) will accompany the warm sector. Precipitation should break
out early in the period across portions of IL/IN then the focus
sags south/southwest as the primary low/shortwave quickly lifts
through the Great Lakes and the trailing front stalls over MO.
Here, the mean flow will become increasingly more parallel (westerly/southwesterly) to the expected storm motions and this
could favor some localized training convection. The heaviest QPF
axis is expected to fall along/north of the Ohio River and
westward into southeastern MO, generally north of the heaviest
rainfall footprint from a couple weeks ago in KY. Other areas in
MO and southern IL have also seen relatively drier than normal
soil conditions. Despite this, the environmental conditions and
potential training does suggest a localized flash flood threat and
given these reasons, the Marginal Risk was maintained for the
update.

Taylor


$$
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FOUS30 KWBC 112012
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
310 PM EST Thu Mar 11 2021

Day 1
Valid 16Z Thu Mar 11 2021 - 12Z Fri Mar 12 2021

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF
THE MIDDLE MISSISSIPPI AND LOWER OHIO VALLEYS...

...16Z Update...
A line of showers and thunderstorms will continue to cross the OH
Valley (mainly IN into western OH) through the next several hours.
This activity should wane by early/mid afternoon as it gets
farther removed from the best frontogenesis and upper level
forcing. As warm air advection, moisture flux and instability
increases coupled with the approach of mid-level impulses, expect
thunderstorm activity along the front (especially across mid-MS
Valley region) to blossom through the afternoon. This
precipitation will advance east into the OH Valley along the cold
front resulting in training of moderate to heavy rain. Based on
the latest 12Z high resolution model guidance, the previous
forecast is on track with only minor changes made to the risk
area.

Pagano

...Previous Discussion...
A slow moving front that eventually stalls and washes out over
portions of mid-MS and lower-OH river valleys will become the
focus for heavy rainfall during the day 1 time frame. Along and
ahead of the front, a surge of higher than normal precipitable
water (1.25-1.5" or around +2 standard deviations above normal)
and sufficient instability (500-1000 J/kg MUCAPE) will accompany
the warm sector. Low-level frontogenesis will be increasing from southwest-northeast across the outlook area, especially overnight
as the upper level jet streak (150+ kts at 250 mb) slowly migrates
across the Upper Midwest and the Lower Great Lakes. Low-level
moisture transport into the frontal zone is at best modest;
however, as the surface frontal progression begins to slow down
(becoming more w-e oriented), the potential for cell training will
be on the increase. This would especially apply later Thursday
afternoon and overnight, coinciding with the aforementioned
increase in low-level frontogenesis and as the 30-40+ kt 925-850
mb southwesterly flow begins to align more closely to the mean
850-300 mb wind. Deep-layer instability, mainly elevated, will be
somewhat limited as MUCAPEs generally top out between 500-1000
j/kg. This would limit the rainfall rates somewhat, however given
the enhanced signal for training convection, probabilities of
1.5-2 inches within a 3 hour period have come up per the most
recent HREF 40km neighborhood probabilities. Despite drying out
somewhat over the past week, the 14-day precipitation remains
between 150-300% of normal over much of the outlook area. Given
the aforementioned thermodynamic parameters, minor/isolated runoff
issues will be possible, and as a result, the Marginal Risk from
yesterday's Day 2 ERO remains in place, though with a slight
southward adjustment based on the latest high-res CAM trends.

Hurley/Taylor


Day 2
Valid 12Z Fri Mar 12 2021 - 12Z Sat Mar 13 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MID SOUTH TO THE CENTRAL PLAINS...

20Z Update: The majority of the threat areas for Day 2 remain
similar for this forecast cycle. However, the axis of the Slight
Risk area was adjusted slightly southward by about 25 miles to
account for better 12Z CAM guidance clustering from north-central
Oklahoma to far western Kentucky. Both versions of the ARW are
indicating maxima on the order of 3 inches in the general vicinity
of extreme southeast Kansas, and into southwestern Missouri and
far northern Arkansas. The HRRR is likely the most pronounced
with the coverage of 2-3 inch rainfall totals. The previous
discussion is appended below for reference. /Hamrick

An an upper low moves through the Southwest and an upper trough
moves through the Northeast, low-level inflow backs to become more
south to southwest across the Southern Plains. Temperatures at
700 hPa slowly rise in the warm advection pattern, with the 6C
isotherm barely shifting into southern OK. This should act as the
southern fringe for active convection this period. The guidance
seems to be having its usual warm season issue with breaking out
convective precipitation in the High Plains (TX Panhandle and
vicinity) so manually increased amounts there from the model blend
utilized (which has been coming up recently as well). MU CAPE of
up to 2000 J/kg along with enough low-level inflow converging
into/over the front in the region and precipitable water values of
1.5" should allow for hourly rain totals to 2" at the northeast
side of the instability pool where non-traditional cell training
-- movement at a decent angle to the low-level inflow but along
the lines of the mid to upper level/mean wind flow -- occurs. The
rain in MO has begun and should continue for the next few days,
beginning to add up by Saturday. Despite initial dry conditions
over the past couple of weeks, soils should have saturated
sufficiently to lead to a greater flash flood/excessive rainfall
risk across southern MO, the epicenter of the Slight Risk area.
The wettest pieces of guidance continue to indicate local 4-5"
amounts across portions of OK and MO. The Slight Risk area is
similar to continuity, with few adjustments made this cycle.


Roth/Hamrick

Day 3
Valid 12Z Sat Mar 13 2021 - 12Z Sun Mar 14 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL PLAINS...

20Z Update: There has been a slight northwestward adjustment in
the Slight Risk area based on trends in the 12Z model guidance
suite. The deterministic guidance generally agrees that central
Kansas to south-central Nebraska would have the greatest rainfall
totals. The GFS stands out as a particularly heavy solution with
some 3-4 inch maxima within that general area, whereas other
guidance is more on the order of 1.5 to 3 inches. In addition,
the western edge of the existing Marginal Risk was trimmed back
slightly to account for more winter weather across eastern
Colorado and west-central Nebraska. The previous discussion is
appended below for reference. /Hamrick

Sufficient inflow within a cyclone's warm conveyor belt
circulation brings 500-750 kg/m/s of IVT through TX and OK towards
KS. This leads to precipitable water values to 1.25-1.5"
generally east of the 100th meridian, with lower amounts into the
High Plains within the low's developing comma head. Guidance
indicates that MU CAPE of 500+ J/kg is possible within the
system's warm sector, which could be enough, combined with
rainfall the previous day, to lead to short duration heavy rain
issues (hourly totals up to 1-1.5") across portions of central and
eastern KS. Have raised a Slight Risk in this area to accommodate
this possibility and account for rainfall from the previous day.
A broad Marginal Risk covers areas that rained heavily on previous
days and should be more sensitive by Saturday and Saturday night.
Overall amounts in the 2-3" range, with locally higher totals, are
expected in and near the Slight Risk area.

Roth/Hamrick

$$
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FOUS30 KWBC 130828
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
327 AM EST Sat Mar 13 2021

Day 1
Valid 12Z Sat Mar 13 2021 - 12Z Sun Mar 14 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL PLAINS...

Based on the latest trends in the guidance, there were a few minor
adjustments to the Slight and Marginal Risk areas from yesterday's
Day 2 ERO across portions of the Central Plains. An expansive deep
and dynamic cyclone will migrate slowly east across the Southwest
and Four Corners, then into the southern Rockies by early Sunday.
During this time the warm conveyor belt will continue to
mature/amplify, leading to a deepening TROWAL extending
north-northwest across much of KS/NE Saturday night. A robust LLJ,
especially Saturday night as S-SE 850 mb flow increases to 50-60
kts, will result in equally vigorous moisture transport into the
outlook areas. 850-700 mb moisture flux standardized anomalies are
around +5 across the Plains per the SREF and GEFS. Models still
depict IVT values in the range of 500-750+ kg/m/s from portions of
Texas and Oklahoma into Kansas, with the nose of the IVT making
its way north-northwestward with time. Overall, the models still
point to PW values peaking around 1.25-1.35 inches being in place
ahead of a cold front associated with the synoptic scale cyclone,
with warm-sector instability on the order of 1000-1500 j/kg of
MUCAPE per the latest CAMs lifting across western-central OK and
KS. Deep-layer instability is expected to taper to around 500 j/kg
east of the Slight Risk area, i.e. within the Marginal Risk area
across eastern KS and southern MO. Strong dynamical forcing along
with the more impressive thermodynamical profiles will support 24
hour areal-average rainfall totals of 1.5-2.5 inches across much
of the Slight Risk area, with localized totals of 3-5 inches
expected per the latest CAMs. Short-term rainfall rates will be
somewhat limited (capped) by the relative lack of instability and
PWs under 1.5 inches. However, the slow-moving WCB and eventual
pivoting TROWAL will maintain the strong deep-layer moisture flux
over the Slight Risk area for several hours, with training of
elevated convective segments likely to enhance the flash flood
risk over time (especially overnight Saturday).

Hurley


Day 2
Valid 12Z Sun Mar 14 2021 - 12Z Mon Mar 15 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE LOWER MISSOURI VALLEY...

The potential remains for some heavy to potentially excessive
rainfall over portions of the Upper Midwest into the Lower
Missouri Valley on Sunday as a deep synoptic-scale cyclone begins
to make its way out onto the Plains. There continues to be
general agreement with respect to strong south to southeasterly
flow in the low levels drawing an airmass with PW values
approaching an inch northward ahead of the system. This results
in decent agreement among the models for an axis of heavy precip
in the 1-2"+ range in the axis of much above average pw values
along and ahead of the above mentioned front. Precipitation
should be more progressive to the northeast during day 3 compared
to day 2, which is resulting in lesser areal average amounts
compared to day 2. However, with ffg values lower across this
region than areas upstream, there will continue to be at least a
marginal risk of runoff issues. This may be especially so across
portions of southern Missouri where a narrow axis of heavy rains
occurred over the past 24 hours.

Bann

Day 3
Valid 12Z Mon Mar 15 2021 - 12Z Tue Mar 16 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann

$$
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FOUS30 KWBC 131934
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
233 PM EST Sat Mar 13 2021

Day 1
Valid 16Z Sat Mar 13 2021 - 12Z Sun Mar 14 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL PLAINS...

1530Z update: Minor adjustments made to the Slight Risk area for
this update with some southward expansion into portions of the
Texas Panhandle and western Oklahoma, where previous 24 hour
rainfall totals of 1-2" (locally 3") were observed. Latest
guidance remains locked on the idea of blossoming deep convection
later in the period as low pressure organizes and deepens over the
southern Plains. Anomalously high moisture transport and low-level
jet approaching 60 kts this evening/tonight with sufficient
instability in place should focus the strongest convection over
the TX Panhandle into western OK where the latest HREF
probabilities hint at 2"+ hour rates. This could result in a
localized flash flood threat where training develop given some
parallel orientation to the deep layer mean flow. Further north,
the WCB and TROWAL setup will pivot an area of widespread moderate
to embedded heavier elevated convection over portions of KS/NE.
While rates will be lower (generally less than 1"/hr), the longer
duration poses a flood threat too.

Taylor

previous discussion:
Based on the latest trends in the guidance, there were a few minor
adjustments to the Slight and Marginal Risk areas from yesterday's
Day 2 ERO across portions of the Central Plains. An expansive deep
and dynamic cyclone will migrate slowly east across the Southwest
and Four Corners, then into the southern Rockies by early Sunday.
During this time the warm conveyor belt will continue to
mature/amplify, leading to a deepening TROWAL extending
north-northwest across much of KS/NE Saturday night. A robust LLJ,
especially Saturday night as S-SE 850 mb flow increases to 50-60
kts, will result in equally vigorous moisture transport into the
outlook areas. 850-700 mb moisture flux standardized anomalies are
around +5 across the Plains per the SREF and GEFS. Models still
depict IVT values in the range of 500-750+ kg/m/s from portions of
Texas and Oklahoma into Kansas, with the nose of the IVT making
its way north-northwestward with time. Overall, the models still
point to PW values peaking around 1.25-1.35 inches being in place
ahead of a cold front associated with the synoptic scale cyclone,
with warm-sector instability on the order of 1000-1500 j/kg of
MUCAPE per the latest CAMs lifting across western-central OK and
KS. Deep-layer instability is expected to taper to around 500 j/kg
east of the Slight Risk area, i.e. within the Marginal Risk area
across eastern KS and southern MO. Strong dynamical forcing along
with the more impressive thermodynamical profiles will support 24
hour areal-average rainfall totals of 1.5-2.5 inches across much
of the Slight Risk area, with localized totals of 3-5 inches
expected per the latest CAMs. Short-term rainfall rates will be
somewhat limited (capped) by the relative lack of instability and
PWs under 1.5 inches. However, the slow-moving WCB and eventual
pivoting TROWAL will maintain the strong deep-layer moisture flux
over the Slight Risk area for several hours, with training of
elevated convective segments likely to enhance the flash flood
risk over time (especially overnight Saturday).

Hurley


Day 2
Valid 12Z Sun Mar 14 2021 - 12Z Mon Mar 15 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM FAR NORTHEAST
OKLAHOMA INTO SOUTHERN MISSOURI...


2100 UTC update

A slight risk area was added to the updated day 2 Excessive
Rainfall Outlook from far northeast Oklahoma into southern
Missouri. The marginal risk was also extended westward by 75-175
nm across southeast Kansas into northeast Oklahoma. These changes
were to cover the axis of where heavy rains have fallen over the
past 24 to 48 hours (2-5"), resulting in high stream flow as per
the National Water Model and lowered ffg values. Well defined
frontal convergence in an axis of above average pw values, 2 to 3
standard deviations above the mean, along and ahead of the cold
front forecast to push eastward Sunday from the eastern portions
of the Southern Plains into the lower Mississippi will support an
organized area of moderate to heavy rainfall pushing across these
areas. While the precip will be progressive overall, the lower
ffg values and high stream flows will increase the risk of runoff
issues from additional precip totals of 1"+.

Oravec



Day 3
Valid 12Z Mon Mar 15 2021 - 12Z Tue Mar 16 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Oravec

$$
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FOUS30 KWBC 140852
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
451 AM EDT Sun Mar 14 2021

Day 1
Valid 12Z Sun Mar 14 2021 - 12Z Mon Mar 15 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM FAR NORTHEAST
OKLAHOMA AND SOUTHEAST KANSAS INTO SOUTHERN MISSOURI AND NORTHERN
ARKANSAS...

The Slight Risk area that was added to yesterday's Day 2 ERO was
expanded to include more of the Ozarks in southern MO and northern
AR, while extending farther east toward the Bootheel of MO. This
was based largely on the heavy rainfall footprint this region has
observed since Thursday (3 to 7+ inches). The recent heavy
rainfall has resulted in high stream flow per the National Water
Model, along with lowered FFG values. Additional areal-average
rainfall of 1-2 inches, with localized amounts of 3+ inches per
the CAMs, are expected within the Slight Risk area. Rainfall rates
will be capped somewhat given the limited elevated instability
(MUCAPEs ~500 j/kg) along with PWs ~1.25" ahead of the surface
cold/occluded fronts. However, per the latest CAMs, spotty 1-1.25
inch/hr and 1.5 inch/3hr rates are likely, and would pose a more
enhanced flash flood threat within the Slight Risk area given the
recent heavy rains and resultant lowering FFGs (down to 1"/hr and
1.5"/3hr over most locations).

Elsewhere, the marginal risk was extended westward across eastern
KS and NE, in light of the widespread moderate-heavy rain during
the predawn hours along with an additional 1-2+ inches expected
after 12Z Sunday. Deep-layer elevated instability within the
robust Warm Conveyor Belt (WCB) and eventually pivoting TROWAL is
meager at best north of the Slight Risk area (MUCAPEs generally
below 500 j/kg); however, the persistent, widespread rainfall with
bursts of heavier rain rates (0.50-1.00"/hr) could certainly lead
to isolated runoff issues by Sunday afternoon and overnight.

Hurley






Day 2
Valid 12Z Mon Mar 15 2021 - 12Z Tue Mar 16 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann


Day 3
Valid 12Z Tue Mar 16 2021 - 12Z Wed Mar 17 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
PLAINS TUESDAY AND TUESDAY NIGHT OVER PARTS OF THE PLAINS...

...Kansas...
Another area of low pressure will be ejecting from the Rockies
during the day on Tuesday. Low level winds back and strengthen
ahead of the low pressure system, drawing moisture back into
Southern and Central Plains by Tuesday evening which sets the
stage for another round of moderate to heavy rainfall. The
mid-level closed low will be deepening as it moves east of the
Rockies, which results in 850 mb flow backing and accelerating
into the 30 to 45 kt range over Oklahoma late Tuesday night and
early Wednesday. It is expected that precipitation will become
more widespread over southern Kansas during the night in response
to the increasing flow of moisture interacting with a
quasi-stationary or slowly moving warm front at 850 mb. In
addition, the GFS showed a well-define divergence maximum in the
400-200 mb layer moving out of New Mexico and Colorado which
becomes even better defined over southwest and south-central
Kansas by 17/09Z. The model QPF signal is mixed in terms of
placement and amounts were generally under 1.5 inches. That may be
an artifact of the PW values not being highly anomalous or in the
way the any particular model handles the placement of the surface,
low level synoptic-scale boundaries or the magnitude of the upper
level divergence. But a Slight Risk area appears to be warranted
given the agreement of the GEFS ensemble fields fitting the mass
fields/QPF of the ECMWF and UKMET runs combined with the fact that
some of these areas in Kansas have received 1 to 2+ inches of
precipitation since late Friday.

Bann

$$
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FOUS30 KWBC 141950
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
350 PM EDT Sun Mar 14 2021

Day 1
Valid 16Z Sun Mar 14 2021 - 12Z Mon Mar 15 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM FAR NORTHEAST
OKLAHOMA AND SOUTHEAST KANSAS INTO SOUTHERN MISSOURI AND NORTHERN
ARKANSAS...

1600 UTC discussion

No changes made to the previous Excessive Rainfall Outlook. The
1200 UTC hi res guidance continue to emphasize an organized area
of heavy rains pushing eastward this period from the eastern
Portions of the Southern to Central Plains toward the Lower
Mississippi Valley with overall good continuity from the prior
forecast cycle. The forecast rationale listed below has not
changed.

Oravec

0900 UTC discussion

The Slight Risk area that was added to yesterday's Day 2 ERO was
expanded to include more of the Ozarks in southern MO and northern
AR, while extending farther east toward the Bootheel of MO. This
was based largely on the heavy rainfall footprint this region has
observed since Thursday (3 to 7+ inches). The recent heavy
rainfall has resulted in high stream flow per the National Water
Model, along with lowered FFG values. Additional areal-average
rainfall of 1-2 inches, with localized amounts of 3+ inches per
the CAMs, are expected within the Slight Risk area. Rainfall rates
will be capped somewhat given the limited elevated instability
(MUCAPEs ~500 j/kg) along with PWs ~1.25" ahead of the surface
cold/occluded fronts. However, per the latest CAMs, spotty 1-1.25
inch/hr and 1.5 inch/3hr rates are likely, and would pose a more
enhanced flash flood threat within the Slight Risk area given the
recent heavy rains and resultant lowering FFGs (down to 1"/hr and
1.5"/3hr over most locations).

Elsewhere, the marginal risk was extended westward across eastern
KS and NE, in light of the widespread moderate-heavy rain during
the predawn hours along with an additional 1-2+ inches expected
after 12Z Sunday. Deep-layer elevated instability within the
robust Warm Conveyor Belt (WCB) and eventually pivoting TROWAL is
meager at best north of the Slight Risk area (MUCAPEs generally
below 500 j/kg); however, the persistent, widespread rainfall with
bursts of heavier rain rates (0.50-1.00"/hr) could certainly lead
to isolated runoff issues by Sunday afternoon and overnight.

Hurley






Day 2
Valid 12Z Mon Mar 15 2021 - 12Z Tue Mar 16 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Taylor


Day 3
Valid 12Z Tue Mar 16 2021 - 12Z Wed Mar 17 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
PLAINS TUESDAY AND TUESDAY NIGHT OVER PARTS OF THE PLAINS...

In response to a closed mid/upper level low tracking from the Four
Corners region to the southern Plains and its associated upper
level divergence, an area of low pressure is forecast to develop
and deepen over portions of TX/OK Tuesday. Low level flow will
increase Tuesday night and bring with it anomalously higher
precipitable water values (1-2 standard deviations above normal).
This higher moisture transport nosing impinging on an east/west
boundary draped in the area will allow for convection to fire up
across southern Kansas/northern Oklahoma. The latest model
guidance suggests upwards of 1000-1500 J/kg of MUCAPE will be
available. Overall model guidance has not changed too much from
the previous cycle where 1-2 inches (locally 3") will be possible.
This activity falling on areas that are running slightly above
normal for precipitation the last 14 days (thanks to the recent
rainfall from March 13), could result in some instances of flash
flooding. The Slight Risk was maintained and adjusted minimally to
account for the latest trends.

Taylor

$$
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FOUS30 KWBC 151949
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
349 PM EDT Mon Mar 15 2021

Day 1
Valid 16Z Mon Mar 15 2021 - 12Z Tue Mar 16 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHERN ALABAMA INTO NORTHWEST GEORGIA...

16Z update:
No changes to the ERO based on the latest guidance. A nose of
higher instability and moisture is expected to stall/slow over
Alabama later today, resulting in a couple rounds of convection
that could produce hourly totals in the 1-2" range. The 12Z hi-res
models support the idea of localized 3" totals. Just some minor
changes made to the Marginal Risk area.

---previous discussion---
The deep, occluded cyclone across the central CONUS early Monday
will begin to open at mid and upper levels later this afternoon
and tonight while lifting north and encountering the deep ridge
over the Upper MS Valley. Meanwhile, the Warm Conveyor Belt (WCB)
via the southern/subtropical branch will become increasingly
displaced from the cyclone -- eventually becoming more w-e
oriented across the Southeast by Monday night. At the surface, the
approaching cold front over the Lower MS Valley will begin to
stall/weaken with time, while the weak boundary south and
southwest of the Mid Atlantic CAD wedge remains quasi-stationary
over or near the outlook area. Despite the lackluster
frontogenesis, moisture transport across the outlook area will be
rather vigorous during the period, as 850 mb moisture flux
standardized anomalies range between +2 and +2.5 per the 00Z SREF
and GEFS. The guidance, especially the CAMs, continue to depict
multiple rounds of convection this afternoon and again tonight,
when in both instances a thermodynamic profile characterized by
MUCAPEs of 1000-2000 j/kg and PWs of 1.25-1.50" would support
pockets of 1-2"/hr rainfall rates.

The dry period over the last 7-10+ days has resulted in below
normal streamflow anomalies and a gradual reduction in soil
saturation per the latest National Water Model (NWM) output. The
expectation is that the multiple rounds of convection will likely
offset the antecedent soil conditions, especially overnight as the
increasing southwest low-level inflow (30-40 kts at 850 mb)
becomes nearly parallel to the mean 850-300 mb wind, thereby
enhancing the potential for training of convective segments.
Additional activity with peak rates between 1-2"/hr between 06-12Z
Tuesday might be sufficient to cause isolated/localized flash
flooding in areas that would encounter rapid runoff.

Hurley/Taylor



Day 2
Valid 12Z Tue Mar 16 2021 - 12Z Wed Mar 17 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
PLAINS TUESDAY AND TUESDAY NIGHT OVER PARTS OF THE PLAINS...

21Z update... The latest guidance persists in targeting
central/southern Kansas and the Oklahoma panhandle with the
highest rainfall; with a majority of the global guidance with
areal averages of 1 to 2 inches with isolated maximums near 3
inches. There was an increasing trend across southwest Kansas and
the Oklahoma panhandle with this update. The WPC QPF and
adjustments to the day 2 ERO reflected this trend.

Campbell

Previous discussion... In response to a closed mid/upper level low
tracking from the Four Corners region to the southern Plains and
its associated upper level divergence, an area of low pressure is
forecast to organize and deepen over portions of Texas and
Oklahoma on Tuesday. In response to the deepening area of low
pressure, low level flow will back and strengthen Tuesday night
and bring with it anomalously higher precipitable water values
(1-2 standard deviations above normal). This higher moisture
transport nosing impinging on an east/west boundary draped in the
area will allow for convection to fire up across southern
Kansas/northern Oklahoma. Overall model guidance has not changed
too much from the previous cycle where 1-2 inches (locally 3")
will be possible. This activity falling on areas that are running
slightly above normal for precipitation the last 14 days
(especially in response to the rainfall on March 13), could result
in some instances of flash flooding. The Slight Risk was
maintained and adjusted minimally to account for the latest
trends.

Bann

$$
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FOUS30 KWBC 160846
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
446 AM EDT Tue Mar 16 2021

Day 1
Valid 12Z Tue Mar 16 2021 - 12Z Wed Mar 17 2021

...THERE IS A MARGINAL RISK AREA FOR EXCESSIVE RAINFALL OVER PARTS
OF THE PLAINS AND DEEP SOUTH...

...Much of Kansas into northern Oklahoma and eastern portions of
the TX/OK Panhandles...
In response to a closed mid/upper level low tracking from the
Southwest and Four Corners region into the southern High Plains, a
compact area of robust upper level difluence and divergence will
allow a lee-side surface low to strengthen as it crosses into the
Red River Valley by early Wednesday morning. In response to the
deepening area of low pressure, low level flow will back and
strengthen Tuesday night and bring with it modestly anomalous
precipitable water values (1-1.5 standard deviations above
normal). This higher moisture transport nosing impinging on an
east/west boundary draped in the area will allow for convection to
fire up across southern Kansas/northern Oklahoma after 00Z Wed.
The guidance overall has not changed too much from the previous
cycle, where 1-2+ inches was noted. This activity falling on areas
that are running slightly above normal for precipitation the last
14 days (especially in response to the rainfall on March 13),
could result in some instances of flash flooding. However, per the
latest HREF exceedance probabilities, any instances of rapid
runoff potentially leading to flash flooding are expected to be
isolated or localized at most, and as a result, a Marginal Risk
was noted in the Day 1 ERO.


...Portions of Mississippi into Alabama and Georgia...
Within an active subtropical jet, PWs of 1.5-1.75" will average
around 2 standard deviations above normal. The degree of low-level
inflow and thus moisture transport/flux are not overly anomalous;
however, an uptick in deep-layer forcing will take place across
the Deep South within the right-entrance region of the upper jet
streak (110-130 kts at 250 mb) across the mid Atlantic. As this
occurs, the initially weak surface frontal segments over/near the
Deep South will eventually give way to a quasi-stationary,
less-diffuse front draped WNW-ESE across the Deep South by the end
of the period. MUCAPEs of 1000-2000 j/kg along with the available
moisture will support hourly rainfall rates of 1-2+ inches, as the
latest high-res guidance continues to indicate a heavy rainfall
footprint with isolated-scattered pockets of 3-5+ inch totals
during the 24 hour period. By the same token, the models,
especially the CAMs, continue to exhibit quite a bit of areal
spread with the axes of heavier QPF, even as the event is now
within the day 1 window. As a result, the Day 1 ERO casts a fairly
wide net, at least with the Marginal Risk area, with the Slight
Risk encompassing the higher ensemble mean QPF along with more
prolonged higher HREF probabilities of >1"/hr rainfall rates.

Hurley





Day 2
Valid 12Z Wed Mar 17 2021 - 12Z Thu Mar 18 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID MISSISSIPPI VALLEY INTO THE TENNESSEE VALLEY...

...Portions of Iowa/Nebraska/Kansas and Northern Missouri...
Moderate to heavy rainfall is possible over parts of
Kansas/southeast Nebraska into adjacent portions of southern Iowa
and northern Missouri as system lifts northeast from the
Central/Southern Plains on Wednesday night/early Thursday morning.
The models are doinig be better job tonight in depicting a
deformation zone forming over Kansas into far southeast Nebraska
which helps enhance rainfall rates along and northwest of the main
low pressure system. Some of the guidance QPF fields suggests
that some of the models were suffering from convective
feedback...so their exact deterministric precipitation amounts
seems suspect. Even so, the arrival of an airmass with PW values
close to an inch and enhanced mesoscale forcing in and near the
deformation zone supports the idea of 1 to 2 inches with isolated
amounts exceeding 3 inches during the period...with highest
amounts from far northwest KS or northeastern Missouri
northeastward.

...Mid-Mississippi Valley into the Tennessee Valley...
A well-defined front associated with the same area of low pressure
moving northeast across the Plains will help focus convection in
areas of eastern Missouri into the Tennessee Valley during the
latter part of the day on Wednesday and persisting into Wednesday
night/early Thursday. Deeper moisture, as shown by precipitable
water values increasing to nearly 1.5 inches, and 850 mb flow of
40 to 50 kts, and a low level boundary to work with suggests some
locally intense rainfall rates are possible. Opposing that for
flash flooding will be the dry conditions across a large part of
the area recently and enough steering flow to keep cells
propagating. Suspect that there will be some potential for
short-term rainfall rates to challenge the relatively high 3-hour
flash flood guidance.

Bann


Day 3
Valid 12Z Thu Mar 18 2021 - 12Z Fri Mar 19 2021

...THERE IS MARGINAL RISK OF EXCESSIVE RAINFALL FROM PARTS OF THE
OH VALLEY EASTWARD TO PENNSYLVANIA AND PARTS OF THE MID ATLANTIC
REGION...

Rain will spread eastward across portions of the southern Great
Lakes and Midwest region into parts of West Virginia and
Pennsylvania as well as parts of the northern Mid-Atlantic during
the period. Rainfall amounts and rainfall rates should be
diminishing as the system reaching this part of the country during
the period, in part because the system will be in the
filling/weakening stage and because the better instability, deeper
moisture and best upper divergence/core of the upper jet get
shunted away from the region. The models did show some half inch
per 6 hour amounts (implying some locally heavier rates in the short-term)...but these values were within the 3-hourly flash
flood guidance even in regions where topography could be a factor
in focusing the flow. Some 1 to 2 inch amounts are still
possible...and that could result in some isolated problems but the
recent dry conditions should help mitigate any broad-scale
concerns,

Bann

$$
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FOUS30 KWBC 172029
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 PM EDT Wed Mar 17 2021

Day 1
Valid 16Z Wed Mar 17 2021 - 12Z Thu Mar 18 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER A LARGE AREA
FROM THE LOWER MISSOURI VALLEY AND MID MISSISSIPPI VALLEY INTO THE
TENNESSEE VALLEY AND PARTS OF THE DEEP SOUTH...

16z Update: Only minor tweaks made to the ERO with this
update...as the broad Slight risk looks in good shape with
isolated to scattered flash flooding possible over a large spatial
extent today into tonight.

The most immediate risk is from northeast AR into western/central
TN and far northern MS/AL. There is a pretty good setup for some
west to east training of convection across this corridor over the
next several hours. An additional 2-4" of rain here should led to
isolated to scattered flash flooding this afternoon.

The second area of focus is southern/central MO into southern IL.
Rainfall rates should never get as intense here given less
instability, and the environmental setup is not as conducive for
training cells. However this area will see two rounds of locally
heavy rainfall...one which is ongoing now and another this
afternoon/evening ahead of the low pressure. Soil saturation is
already well above normal over this corridor...and the two rounds
of heavy rainfall should be enough to bring an increasing flood
risk into tonight.

The third area of focus is within the warm sector over MS/AL. The
primary risk here is sever weather, as depicted by the Moderate
and High risk areas in the SPC outlook. However, isolated to
scattered flash flooding will also be a possibility...as the
supercells will be capable of dropping intense rainfall rates and
there may be some repeating of cells. Also multiple rounds are
expected here (and into northern GA), with the discrete cells this afternoon...and a more linear mode tonight. With soil conditions
saturating here as well, these multiple rounds will likely lead to
isolated to scattered instances of flash flooding.

So all in all the broad Slight risk seems like the correct
messaging at the moment, with isolated to scattered flash flooding
possible. While some localized areas of more focused flash
flooding are possible...not seeing anything at the moment that
warrants any Moderate risk upgrades. But will continue to monitor
trends through the day.

Chenard

...Previous Discussion...

...Lower Missouri Valley and Mid Mississippi Valley...
A broad area of moderate to embedded heavier rain is expected
across portions of Kansas, Missouri, and southern to central
Illinois as the system lifts northeast out of the central/southern
Plains. A deformation zone of enhanced rainfall rates is expected
with the highest amounts forecast around the Kansas City area
through Missouri into west-central Illinois. Widespread amounts of
1-2" are expected, with localized totals of 3-5" per the high-res
CAMs. This is where some wetter than normal soils exist and
streamflows are higher than normal, so additional heavy rainfall,
while lacking the intensity in the short-term as rates are limited
by the relative lack of instability, could lead to mainly
localized flash flooding.


...Lower Mississippi Valley...Tennessee Valley...and parts of the
Deep South...
Deepening cyclone tracking across the southern Plains and
lower-mid MS Valley during the period will lead to robust
deep-layer forcing and rapidly improving thermodynamic profiles in
generating numerous areas of heavy rainfall from the Lower MS
Valley into the Tennessee Valley and Deep South. The hi-res
guidance continues to indicate the potential for at least a couple
of rounds of convection, associated first along and north of the warm/stationary fronts and again ahead of the strong height falls
and approaching cold front. Strong deep-layer ascent owing to the
increasingly favorable difluent flow aloft will get a boost in the
left exit region of a 90-100 mb 250mb jet streak rounding the base
of the trough. At the same time, the thermodynamic environment
will quickly become conducive for heavy rainfall ahead of the
approaching cold front. PWs will quickly spike to 1.5-1.75+ inches
as the low-level southerly inflow increases (50-55kts at 850 mb).
The accompanying robust moisture transport (850 mb moisture flux
standardized anomalies increasing to +4 to +5) will coincide with
ample deep-layer instability (MUCAPEs 1000-2000+ j/kg) in
supporting hourly rainfall rates of at least 1-2", with certainly
some locally higher rates within well organized clusters
(especially supercells).

Fast-moving storm motions and drier-than-normal soils may preclude
a greater flash flood threat, though attention will need to be
given to areas that received quite a bit of localized heavy rain
yesterday (portions of MS into AL). Flash flood guidance in those
areas is reduced quite a bit, and later shifts will monitor for
the potential upgrade to a Moderate Risk for the Day 1 ERO. At
least for the time being, given the considerable spread among the
guidance (especially the CAMs) with respect to the axes of
heaviest rainfall, the Slight Risk was maintained.

Hurley


Day 2
Valid 12Z Thu Mar 18 2021 - 12Z Fri Mar 19 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PARTS OF
THE SOUTHERN/CENTRAL APPALACHIANS NORTHEASTWARD INTO THE NORTHERN
MID-ATLANTIC REGION AND SOUTHERN NEW YORK/CONNECTICUT...

...2030Z Update...

The main change to the overnight Excessive Rainfall Outlook was to
expand the Marginal Risk farther northeast into southern portions
of New York and Connecticut. Model guidance is agreeable with
higher rain amounts over this area given precipitable water values
3+ standard deviations above normal per the ECMWF. Limiting
factors will be low instability as well as dry antecedent
conditions. But, the urban corridor should make for less receptive
ground conditions and enhance runoff.

No significant edits were needed to the southern part of the
Marginal Risk through the Appalachians. It appears some localized
heavy rain may linger after 12Z Thursday in the Southern
Appalachians, while another round of rain on the back end of the
system could cause some modest to locally heavy rain rates in the
Central Appalachians later Thursday with higher instability. See
the previous discussion for additional details.

Tate

...Previous Discussion...

...Upslope Region of the Central Appalachians to the
Mid-Atlantic...
Maintained a Marginal Risk area for excessive rainfall from the
upslope regions of the Central Appalachians northeastward into
southeastern Pennsylvania on Thursday as low pressure approaches
the region from the west. The 17/00Z runs of the ECMWF and UKMET
depict an area of QPF with a characteristic bulls eye max for
convective grid-scale feedback in the upslope region of the
Appalachians. While the exact QPF from those models may be
suspect, the idea of some localized convective processes can not
entirely be ruled out with correspondingly higher rainfall
rates/amounts. That, combined with the terrain, suggests there is
at least some potential for problems with runoff or ponding in
poor drainage areas. The Marginal Risk area was extended to the
northeast where the model has the best QPF signal as a result of
Precipitable Water values approaching an inch and a quasi-coupled
upper jet especially seen on the 290K/295K surfaces. Thinking is
that the risk of excessive rainfall should be confined to the
first 12-18 hours of the forecast period before the system
translates far enough away and high pressure starts to funnel
drier air into the region.

Opted to remove the Marginal Risk area from places farther to the
west in the deformation zone. While there may be some short-term
enhancements to the rainfall rates as a result of the fgen
mesoscale forcing, modest instability and Precipitable Water
values at or below 0.75 inches should hold the rainfall
rates/amounts in check. In addition, the area has not had
significant rainfall in a while and it is presumed that this will
also help to mitigate excessive rainfall concerns.

Bann

$$
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FOUS30 KWBC 181540
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1139 AM EDT Thu Mar 18 2021

Day 1
Valid 16Z Thu Mar 18 2021 - 12Z Fri Mar 19 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE
SOUTHERN TO CENTRAL APPALACHIANS INTO THE MID-ATLANTIC...

The strong closed low moving into the Lower Mississippi Valley
this morning will continue to push eastward this period through
the Tennessee Valley and into the Southern Appalachians. A large,
well defined precipitation area is expected to the north and
northeast of this system from the combination of a well defined
comma head/deformation precip area to the north of this closed
low, north of the Ohio River, and in a region of strong isentropic
lift in the moist south southwesterly flow from the Appalachians
into the Northeast. Precipitation across much of these regions
has been below average over the past few weeks, resulting in
stream flows beginning the event below to much below average.
Some of the northern portion of the Marginal Risk across northern
PA was removed as the 12z HREF showed very little probability of
0.25" an hour totals over the area of low flash flood guidance as
precipitable water values remain below an inch and MU CAPE is
expected to remain negligible north of the Mason-Dixon line. A
marginal risk area continues from the Southern to Central
Appalachians into the Mid-Atlantic where the probabilities of
0.5"+ in an hour are highest. Adjusted the southern portions
somewhat southeast as the recent guidance shows heavy rain along a
warm front/leading edge of a cold air dam attempting to advance
northward through the coastal plain this afternoon with a notable
impact seen within the National Water Model output in that area of
Tidewater Virginia.

Roth/Oravec



Day 2
Valid 12Z Fri Mar 19 2021 - 12Z Sat Mar 20 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann


Day 3
Valid 12Z Sat Mar 20 2021 - 12Z Sun Mar 21 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann

$$
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FOUS30 KWBC 181928
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
327 PM EDT Thu Mar 18 2021

Day 1
Valid 16Z Thu Mar 18 2021 - 12Z Fri Mar 19 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE
SOUTHERN TO CENTRAL APPALACHIANS INTO THE MID-ATLANTIC...

The strong closed low moving into the Lower Mississippi Valley
this morning will continue to push eastward this period through
the Tennessee Valley and into the Southern Appalachians. A large,
well defined precipitation area is expected to the north and
northeast of this system from the combination of a well defined
comma head/deformation precip area to the north of this closed
low, north of the Ohio River, and in a region of strong isentropic
lift in the moist south southwesterly flow from the Appalachians
into the Northeast. Precipitation across much of these regions
has been below average over the past few weeks, resulting in
stream flows beginning the event below to much below average.
Some of the northern portion of the Marginal Risk across northern
PA was removed as the 12z HREF showed very little probability of
0.25" an hour totals over the area of low flash flood guidance as
precipitable water values remain below an inch and MU CAPE is
expected to remain negligible north of the Mason-Dixon line. A
marginal risk area continues from the Southern to Central
Appalachians into the Mid-Atlantic where the probabilities of
0.5"+ in an hour are highest. Adjusted the southern portions
somewhat southeast as the recent guidance shows heavy rain along a
warm front/leading edge of a cold air dam attempting to advance
northward through the coastal plain this afternoon with a notable
impact seen within the National Water Model output in that area of
Tidewater Virginia.

Roth/Oravec



Day 2
Valid 12Z Fri Mar 19 2021 - 12Z Sat Mar 20 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Tate

Day 3
Valid 12Z Sat Mar 20 2021 - 12Z Sun Mar 21 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Tate


$$
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FOUS30 KWBC 191942
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
341 PM EDT Fri Mar 19 2021

Day 1
Valid 16Z Fri Mar 19 2021 - 12Z Sat Mar 20 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Oravec


Day 2
Valid 12Z Sat Mar 20 2021 - 12Z Sun Mar 21 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Taylor


$$
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FOUS30 KWBC 200801
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
401 AM EDT Sat Mar 20 2021

Day 1
Valid 12Z Sat Mar 20 2021 - 12Z Sun Mar 21 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Oravec


Day 2
Valid 12Z Sun Mar 21 2021 - 12Z Mon Mar 22 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Campbell

Day 3
Valid 12Z Mon Mar 22 2021 - 12Z Tue Mar 23 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Campbell

Scattered to widespread rainfall is expected across the Central
Plains and portions of the Mississippi Valley. While rain rates
will likely be less than 0.50 inch/hour, periods of heavy rainfall
over central Kansas, eastern Nebraska, western Iowa and northwest
Missouri will result in the highest QPF occurring over this
region. The latest guidance continues to show southerly low-level
flow advecting PW values near 1 inch northward along a frontal
boundary over the central U.S. Instability over the central U.S.
will be enough to produce some areas of enhanced precipitation
efficiency. A majority of the guidance did show a western shift
with the maximum QPF, averaging 1 to 2 inches with isolated higher
values. Overall, the threat for excessive rainfall and flash flood
conditions is relatively low for this period, but is a non zero.
The forecast QPF more than likely will not reach or exceed local
FFG during this period. Therefore, a Marginal Risk was not
introduced.


$$
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FOUS30 KWBC 201931
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
331 PM EDT Sat Mar 20 2021

Day 1
Valid 16Z Sat Mar 20 2021 - 12Z Sun Mar 21 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Chenard


Day 2
Valid 12Z Sun Mar 21 2021 - 12Z Mon Mar 22 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Pagano

Day 3
Valid 12Z Mon Mar 22 2021 - 12Z Tue Mar 23 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

...Central Plains...
A closed mid-level low will emerge out of the Central/Southern
Rockies early Monday and pivot through the Plains approaching the
Mid-MS Valley by Tuesday morning. Shortwave energy will round the
trough axis allowing it to become negatively tilted which will
increase synoptic scale lift and surge moisture and instability
north into the Southern/Central Plains and the Lower MS Valley.
As a result, showers and thunderstorms will become widespread
across the midsection of the country as the surface low develops
and deepens through the period.

Precipitable water values will surge to around 1.25 inches in the
Southern Plains and around 1 inch in the Central Plains. This is
1.5-2 standard deviations above the mean aided by 40+ knot low
level southern flow. While instability will be more robust ahead
of the trailing cold front in the Southern Plains and Lower MS
Valley, the large scale lift is quite impressive just to the
north. The coupled upper level jet will promote strong ascent
enhancing the moisture transport from the Gulf of Mexico. In
response, hourly rain rates could surge to over 0.50 inches and
with training/multiple rounds of precipitation activity, this
could result in some flooding concerns.

Based on the precipitation analysis over the past week, the
Central Plains have received an abundance of rain. However, with
a few drier days of late and little in the way of flooding during
the last heavy rain event, the soils will likely be able to
withstand the 1-3+ inches of rain expected. If anything, the mode
of flooding will be areal in nature with very localized flash
flooding issues. Therefore, while a Marginal Risk was considered
among the offices, it was decided to hold off for now. If rain
rates increase to around 1 inch/hour and/or a more prolonged rain
event is forecast, a Marginal Risk may be introduced at subsequent
updates.

As an aside, another factor considered was the potential for
winter precipitation. While some models are suggesting that there
may be enough dynamic cooling within the deformation axis of the
developing low, it appears eastern KS/NE will be all liquid.

...Southern Plains/Lower MS Valley...
The trailing cold front associated with the aforementioned storm
system will usher in better moisture content and instability
across this region. Therefore, anticipate more showers and
thunderstorms to blossom ahead of the front across eastern TX/OK
Monday afternoon and into AR/LA through the overnight. This
activity will be progressive and occur over higher FFG. However,
locally enhanced upslope across the Boston and Ouachita Mountains
could result in higher hourly totals. The rates do not appear to
meet the threshold for even a Marginal Risk at this time, but we
will continue to monitor this area and the latest model trends.

Pagano

$$
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FOUS30 KWBC 210830
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
429 AM EDT Sun Mar 21 2021

Day 1
Valid 12Z Sun Mar 21 2021 - 12Z Mon Mar 22 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Oravec


Day 2
Valid 12Z Mon Mar 22 2021 - 12Z Tue Mar 23 2021

...A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF THE
CENTRAL AND SOUTHERN PLAINS...

...Central Plains...

During this period a closed, mid-level low will pivot across the Southern/Central Plains into the Mid-Mississippi Valley by early
Tuesday morning. Shortwave energy traveling through the trough
will cause it to become negatively tilted- this will lead to
increasing low-level flow/moisture advection along with
broad-scale lift and instability northward over the Plains. As the
surface low develops and deepens, the areal coverage of the
showers and thunderstorms will become more widespread.

Precipitable water values will surge to around 1.25 inches in the
Southern Plains and around 1 inch in the Central Plains. This is
1.5-2 standard deviations above the mean aided by 40+ knot low
level southern flow. While instability will be more robust ahead
of the trailing cold front in the Southern Plains and Lower
Mississippi Valley, the large scale lift is quite impressive just
to the north. The coupled upper level jet will promote strong
ascent enhancing the moisture transport from the Gulf of Mexico.
In response, hourly rain rates could surge to over 0.50 inches and
with training/multiple rounds of precipitation activity, this
could result in some flooding concerns.
Most model guidance has an southwest-northeast oriented axis of
higher QPF over central Kansas to eastern Nebraska/western Iowa
with forecast amounts of 1 to 2.5 inches.

...Southern Plains/Lower Mississippi Valley...

Organized convection is expected to fire along the trailing cold
front, where there is better moisture content and instability
across this region. Hi-res guidance are showing hourly rain rates
upwards of 1.5 inches per hour for several hours as the storms
track across Texas, Oklahoma, Louisiana and into Arkansas.
However, locally enhanced upslope across the Boston and Ouachita
Mountains could result in higher hourly totals. While the nature
of these storms will be more progressive and over locations with
higher FFG, localized ponding/runoff may become problematic.

Campbell/Pagano

Day 3
Valid 12Z Tue Mar 23 2021 - 12Z Wed Mar 24 2021

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR THE GULF
COAST...

...CENTRAL GULF COAST...

The trailing cold front mentioned in for the Day 2 period will
slow its eastward progression and stall across parts of the Lower
Mississippi Valley and Central Gulf Coast. Meanwhile, deep Gulf
moisture will lift over the frontal boundary and continue to feed
thunderstorm development during this period. Storms will likely
move slower and potentially over previous storm tracks during this
period. While the latest model guidance keeps the highest QPF just
offshore, areal averages of 2 to 4 inches may be possible across
parts of the Mississippi Delta and surrounding areas of
Mississippi. A Marginal Risk area was introduced for parts of the
Gulf Coast as the local FFG may be reached or exceeded during this
period.

Campbell

$$
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FOUS30 KWBC 211944
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
343 PM EDT Sun Mar 21 2021

Day 1
Valid 16Z Sun Mar 21 2021 - 12Z Mon Mar 22 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Some rather impressive, very localized, heavy rainfall this
morning over portions of eastern GA/SC. Despite little to no
instability in model forecasts, we have seen some localized
rainfall over 2" in an hour, with a 5.75" total east of Savannah,
GA. This appears to be tied to a narrow/tight axis of strong
925-850mb moisture convergence. This is resulting in strong lift
focused below the freezing level...which is enhancing warm rain
processes. Meanwhile the presence of the 500mb vort overhead is
likely leading to enough cooling aloft to support updraft growth
despite weak instability. These heavier cells are very localized,
and likely tied to the convergence axis...with rain elsewhere more
of a steady light to moderate rain.

Unclear how long these embedded heaver cells will last...with most
of the 12z CAMs suggesting a westward push of the convergence axis
and subsequent weakening. The positive feedback of the heavier
cells may keep the convergence axis better defined for longer than modeled...but nonetheless a general weakening trend is expected.
This fact, combined with the very localized nature of the heavier
rains, does not seem to support a Marginal risk at this
time...although very localized flash flooding can not be ruled out
into the afternoon hours over southeast GA.

Chenard


Day 2
Valid 12Z Mon Mar 22 2021 - 12Z Tue Mar 23 2021

...A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF THE
CENTRAL AND SOUTHERN PLAINS...

...20Z Update...
Some minor changes to the Marginal Risk area to account for the
latest 12Z model guidance and expected impacts. It will likely be
more of a soaking rain event across the Central Plains associated
with the developing/deepening surface low. However, with a
prolonged event and some moderate rain at times (hourly rain rates
not expected to exceed 0.50 inches), localized flash flooding may
occur due to lowering FFG values. Across the Southern Plains and
Lower MS Valley, it appears the trailing cold front has slowed a
bit within the models resulting in most of the thunderstorms
activity occurring across eastern OK and northeast TX as well.
This line of storms will advance quickly into the Lower MS Valley
Monday night into early Tuesday. There are higher FFG values
across this region, but training could easily overcome these drier
basins with rain rates in excess of 1 inch/hour. So, localized
flash flooding is also possible.

Pagano

...Central Plains...

During this period a closed, mid-level low will pivot across the Southern/Central Plains into the Mid-Mississippi Valley by early
Tuesday morning. Shortwave energy traveling through the trough
will cause it to become negatively tilted- this will lead to
increasing low-level flow/moisture advection along with
broad-scale lift and instability northward over the Plains. As the
surface low develops and deepens, the areal coverage of the
showers and thunderstorms will become more widespread.

Precipitable water values will surge to around 1.25 inches in the
Southern Plains and around 1 inch in the Central Plains. This is
1.5-2 standard deviations above the mean aided by 40+ knot low
level southern flow. While instability will be more robust ahead
of the trailing cold front in the Southern Plains and Lower
Mississippi Valley, the large scale lift is quite impressive just
to the north. The coupled upper level jet will promote strong
ascent enhancing the moisture transport from the Gulf of Mexico.
In response, hourly rain rates could surge to over 0.50 inches and
with training/multiple rounds of precipitation activity, this
could result in some flooding concerns.
Most model guidance has an southwest-northeast oriented axis of
higher QPF over central Kansas to eastern Nebraska/western Iowa
with forecast amounts of 1 to 2.5 inches.

...Southern Plains/Lower Mississippi Valley...

Organized convection is expected to fire along the trailing cold
front, where there is better moisture content and instability
across this region. Hi-res guidance are showing hourly rain rates
upwards of 1.5 inches per hour for several hours as the storms
track across Texas, Oklahoma, Louisiana and into Arkansas.
However, locally enhanced upslope across the Boston and Ouachita
Mountains could result in higher hourly totals. While the nature
of these storms will be more progressive and over locations with
higher FFG, localized ponding/runoff may become problematic.

Campbell/Pagano

Day 3
Valid 12Z Tue Mar 23 2021 - 12Z Wed Mar 24 2021

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE NORTHERN GULF COAST...

...20Z Update...
Only minor adjustments made to the Marginal Risk area. While
models are in fairly good agreement, the exact amounts and
location of heavy rain focused along a residual surface boundary
will have fairly big implications on the potential for flash
flooding. And while there are model solutions that suggestion 6+
inches of storm total QPF, given the aforementioned mesoscale
uncertainties and higher FFG (despite the previous mid-week storm
system), decided to not introduce a Slight Risk at this update.
We need to see a bit more consensus among the guidance in terms of
a prolonged training event before this region would meet criteria
for more scattered to widespread flash flooding concerns.

Pagano

...CENTRAL GULF COAST...

The trailing cold front mentioned in for the Day 2 period will
slow its eastward progression and stall across parts of the Lower
Mississippi Valley and Central Gulf Coast. Meanwhile, deep Gulf
moisture will lift over the frontal boundary and continue to feed
thunderstorm development during this period. Storms will likely
move slower and potentially over previous storm tracks during this
period. While the latest model guidance keeps the highest QPF just
offshore, areal averages of 2 to 4 inches may be possible across
parts of the Mississippi Delta and surrounding areas of
Mississippi. A Marginal Risk area was introduced for parts of the
Gulf Coast as the local FFG may be reached or exceeded during this
period.

Campbell

$$
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FOUS30 KWBC 222349
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
748 PM EDT Mon Mar 22 2021

Day 1
Valid 01Z Tue Mar 23 2021 - 12Z Tue Mar 23 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHERN PLAINS...

The Marginal Risk area has been contracted this iteration based on
the 18z HREF probabilities of 50% chance of 0.25"+ and 0.5"+ an
area totals due to precipitation progression and declining
instability across the Central Plains. This removed northern KS
and southeast NE whose probabilities of 0.25"+ in an hour do not
exceed 30% due to extremely scant MUCAPE. The Southern Plains and
ArkLaTex lie ahead of the amplifying and increasingly negatively
tilted mid to upper level trough. Strengthening southerly low
level flow ahead of the system will increase precipitable water
values to 2 to 2.5+ standard deviations above the late March mean
in the region -- mainly 1-1.25" in the outlook area.

There are two regions where precipitation totals should maximize
this period: 1) a northern max forecast to be centered from
central Kansas northward into eastern Nebraska composed of longer
duration moderate rainfall in the vicinity of the forecast
position of a north-south oriented stationary front and 2) a
southern maximum along and ahead of a cold front across central to
eastern Oklahoma into northeast Texas/northwest Louisiana. The
best instability expected with the southern maximum will be where
a well-defined line of convection is pushing along and ahead of
the above mentioned cold front. Rain totals across much of
central to eastern Oklahoma into northeast Texas/northwest
Louisiana have been below average over the past two weeks (and
basically nothing from LBB-->OKC), resulting in stream flows
beginning this period below average as per the National Water
Model. There is a better chance of convection with the southern
max supporting hourly precip totals of 1"+ should fast-moving
convection train long enough or mesocyclones embed within the
convective line. The brisk thunderstorm movement and initially
dry conditions should keep the excessive rainfall threat marginal.
The best chances of runoff issues may be in the urbanized regions
from Dallas and Oklahoma City to Tulsa and Shreveport.

The northern precip max region over central Kansas into eastern
Nebraska has had above average precipitation over the past two
weeks with stream flow as per the National Water Model beginning
the period average to above average in spots. Precipitation rates
expected to be less than .20" due to virtually no MU CAPE present.
With the lower hourly rain rates, believe the excessive rainfall
threat is non-zero, but submarginal across the northern region
from here on out.

Roth/Fracasso/Oravec


Day 2
Valid 12Z Tue Mar 23 2021 - 12Z Wed Mar 24 2021

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL GULF COAST...

...20Z Update...
A deepening storm system across the Central U.S. will advance
thunderstorm activity through the Lower MS Valley region through
Tuesday afternoon. A residual surface boundary will become draped
across the region, especially southeastern LA into southern MS/AL.
The position of this front will be the focus for heavier
showers/thunderstorms through the overnight hours as mid-level
impulses, upper level divergence and instability increase and
align under strong moisture flux. Based on the latest 12Z model
guidance, only minor adjustments made to the Marginal and Slight
Risk areas.

Pagano

...Previous Discussion...

The latest model guidance has a better consensus on where the
heavier QPF will occur, but there is still some spread on where
the Mesoscale features will be. The exact amounts and location of
heavy rain focused along a residual surface boundary will have
fairly big implications on the potential for flash flooding. Most
solutions show 1 to 4+ inches across the Mississippi Delta and
surrounding areas, while a couple show localized maximums closing
in around 6 inches. Hi-res guidance maintains embedded cells with
rain rates around 1/1.25 inches through most of this period from
far eastern Texas, across Louisiana and into Mississippi. In
coordination with the local forecast offices a Slight Risk area
was raised for southeast Louisiana and southern Mississippi.
Training of storms may quickly reach or exceed local FFG.

Campbell


Day 3
Valid 12Z Wed Mar 24 2021 - 12Z Thu Mar 25 2021

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL GULF COAST...

...20Z Update...
The evolution of convection along the residual surface boundary on
Day 2 will influence the severity and spatial extent of flash
flooding within the region on Day 3 (Wednesday into early
Thursday). Based on the latest 12Z model guidance and where we
anticipate the heavier rain to occur on Day 2, some adjustments
were made to the Marginal and Slight Risk areas. While the amount
of QPF will likely be less within the forecast period, it could
occur over already saturated soils and thus lead to continued
flooding impacts. It should be noted that conditions will likely
improve late Wednesday into early Thursday as a shortwave
transitions northeast from the Lower MS Valley into the Deep
South, which will help to lift a bulk of the moisture north.

Pagano

...Previous Discussion...
Showers and thunderstorm will persist from day 2 through the day 3
period over the Mississippi Delta region. Widespread moderate to
heavy rain will have thoroughly soaked the soils across this
region and significantly lower FFG across southern Louisiana and
Mississippi. Local streams and main-stem rivers will likely have
already responded to prior runoff. The latest guidance is showing
another 1 to 3 inches of rainfall to occur over much of the same
area with a slight westward shift in where the local maximum is
expected. It will not take much to cause flash flooding concerns
across this area. A Marginal and Slight Risk was hoisted for this
period.

Campbell

$$
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FOUS30 KWBC 231559
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Tue Mar 23 2021

Day 1
Valid 16Z Tue Mar 23 2021 - 12Z Wed Mar 24 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG PORTIONS OF
THE CENTRAL GULF COAST...

Area of convection was pushing out of south central Louisiana this
morning ahead of a cold front back through eastern Texas and along
and just south of a warm front near the LA/MS border. Narrow axis
of heavier rain over Lake Charles this morning (nearly 5" rain in
5 hours) that was initially slow to move eastward was overtaken by
a trailing line of weakening convection and has moved east of the
LCH area. Upper jet over central LA will cease its eastward
progression this afternoon leaving areas to its south and east
underneath divergent flow with mid-level heights parallel to the
upper flow. With surface dew points in the low 60s expected to
rise further into the mid/upper 60s, coupled with precipitable
water values 1.50-1.75" forecast to rise to near 2" this evening
(about +2 standard deviations) from southeastern LA across
southern MS/AL and into the western FL panhandle, rain rates may
maximize 1-3"/hr in some locations with training possible off the
Gulf as any eastward movement likely pauses or at least slows. CAM
guidance continues to show max rainfall totals through 12Z Wed
anywhere from 3-10" which will cause flash flooding concerns
despite the overall dry antecedent conditions (7-day rainfall
along and north of I-10 generally an inch or less). Consensus (12Z
HREF probs) seems to stall the moisture axis into southeastern LA
(just southeast of New Orleans) where a Moderate Risk area could
be prudent though it would be over a small area. Will maintain the
Slight Risk as similar to the previous shift.


Fracasso



Day 2
Valid 12Z Wed Mar 24 2021 - 12Z Thu Mar 25 2021

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL GULF COAST...

The convection firing along the residual surface front during the
Day 1 period will influence the severity and spatial coverage of
flash flooding over the region on Day 2. Local streams and
main-stem rivers will likely have already responded to prior
runoff. Thunderstorm activity will persist along the Gulf Coast,
soaking the region with an additional 1 to 3 inches of rain. It
will not take much to cause flash flooding concerns across this
area. Marginal and Slight Risks were already in effect for parts
of the central Gulf Coast and only very minor adjustments were
needed based on the latest WPC QPF. It should be noted that
conditions will likely improve late Wednesday into early Thursday
as a shortwave transitions northeast from the Lower MS Valley into
the Deep South, which will help to lift a bulk of the moisture
north.

Campbell

Day 3
Valid 12Z Thu Mar 25 2021 - 12Z Fri Mar 26 2021

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FROM PORTIONS
OF THE CENTRAL GULF COAST TO THE TENNESSEE AND OHIO VALLEY WITH A
SLIGHT RISK AREA FROM NORTHERN ALABAMA/MISSISSIPPI TO SOUTHERN
KENTUCKY...

The surface low pressure will track north and east during this
period which will cause the stalled frontal boundary to begin
moving toward the eastern U.S. Deep Gulf moisture will continue to
advect northward ahead of and over the front, continuing to fuel
the moderate to heavy rainfall over parts of the Gulf states, as
well as, lift them into the Tennessee/Ohio Valleys and into the
Appalachian terrain. The additional rainfall over portions of the
Gulf states will aggravate areas with ongoing flooding. Meanwhile,
the local FFG has been lowered across parts of the Tennessee/Ohio
Valley region from recent rains. With areal average forecasts of 1
to 3.5 inches expected during the Day 3 period, local 1/3/6-hr FFG
(around 1.5 to 3 inches) may be reached or exceeded as the storms
lift northward. A broad Marginal Risk area was introduced from
southeast Louisiana, northwest to central Illinois and east toward
western Virginia. A Slight Risk area spans from northern
Mississippi/Alabama to southern Kentucky, where amounts may exceed
3 inches. Rapid runoff, urban ponding and localized flash flooding
will be possible across this area.


Campbell

$$
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FOUS30 KWBC 241550
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1149 AM EDT Wed Mar 24 2021

Day 1
Valid 16Z Wed Mar 24 2021 - 12Z Thu Mar 25 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG PORTIONS OF
THE CENTRAL GULF COAST...


...Central Gulf Coast...

Ongoing rainfall over Louisiana late this morning continues to
slowly lessen in coverage while lifting northward. Surface trough
to the south of the area will lift northward this afternoon in
tandem with an increase in precipitable water values (back over
1.50" per the RAP after decreasing through the early morning
hours) and instability, but upper dynamics will be less conducive
for organized rainfall/convection. However, with rainfall the past
24 hrs of 3-10" in southeastern Louisiana, any additional rainfall
can lead to runoff/flooding issues as FFG values are at the floor
(basically under an inch). 12Z guidance has trended toward less
rainfall coverage this afternoon over LA coincident with the
moisture return in favor of a bit of eastward extension toward
Mobile, AL where recent rainfall was less (though still 1.5-3")
and FFG values were higher. Over southern MS, maintained the
Slight Risk area as QPE > 3" along with lower FFG values
overlapped and where rainfall activity could expand this
afternoon/early evening. Any rainfall should move out of the area
after 06Z in response to the shifted focus to the northwest over
Arkansas.

...Northeast Texas, Arkansas, northern Mississippi, southeast
Missouri and western Tennessee...

A Marginal Risk area is noted from northeast Texas into Arkansas,
northern Mississippi, southeast Missouri and western Tennessee for
the potential of organized frontal/pre-frontal convection tonight
into early Thursday. The height falls pushing into the Southwest
this morning will be swinging eastward into the Southern Plains
this afternoon into early Thursday. The low level southerly flow
is expected to strengthen tonight into early Thursday along and
ahead of the strengthening frontal zone from eastern portions of
the Southern Plains into the lower to middle Mississippi Valley
region as PW values rise from under 0.5" to over 1.25". Convection
is likely to enhance around 00Z Thu over eastern Oklahoma into
northeast Texas, spreading fairly quickly northeastward across
Arkansas, southern Missouri, northern Mississippi and western
Tennessee. The expected quick northeastward movement of this
convection will likely be a detriment for widespread runoff
issues. However, with well-defined coupled jet dynamics and strong
convergence into the frontal zone, locally heavy rains of 1-2"/hr
(and total 1-3" 00Z-12Z) are possible resulting in isolated runoff
issues.

Fracasso/Oravec


Day 2
Valid 12Z Thu Mar 25 2021 - 12Z Fri Mar 26 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTH-CENTRAL
KENTUCKY TO NORTHERN ALABAMA/NORTHEASTERN MISSISSIPPI...

During this period the surface low will track from central
Arkansas to Michigan/Great Lakes region, lifting a warm front
through the Ohio Valley and a cold front through the Mississippi
Valley, Gulf states and the Tennessee/Ohio Valleys by Friday
morning. Meanwhile, a deep plume of Gulf moisture will continue to
advect northward over the front resulting in an axis of moderate
to heavy rainfall from portions of Mississippi/Alabama to
Indiana/Ohio. Recent rainfall has reduced the 3-hr flash flood
guidance across this region to 1.5 to 2.5 inches and the latest
WPC areal average QPF for this period is 1 to 3 inches with
locally higher amounts possible. Therefore, the flash flood
guidance may be reached or exceeded as the storms lift
northeastward in strong and deep southwesterly flow.

The Slight Risk area was extended further east into eastern
Tennessee and far northwest Georgia as some the newer guidance
suggests the footprint of the highest QPF has expanded into this
area, given the continued placement of the better instability.
Rapid runoff, urban ponding and localized flash flooding will be
possible across this area. The Marginal was adjusted further into
western North and South Carolina to encompass more of the terrain,
as the latest guidance show local enhancement and QPF nearing 2
inches.

Campbell

Day 3
Valid 12Z Fri Mar 26 2021 - 12Z Sat Mar 27 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Campbell

$$
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FOUS30 KWBC 250720
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
319 AM EDT Thu Mar 25 2021

Day 1
Valid 12Z Thu Mar 25 2021 - 12Z Fri Mar 26 2021

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR SOUTHERN
PORTIONS OF THE TENNESSEE VALLEY...

Southeast....
Per coordination with FFC/Peachtree City GA, HUN/Huntsville AL,
MRX/Morristown TN, and GSP/Greer SC forecast offices, a new
Moderate Risk has been added for portions of the southern TN river
valley, an area that has received 125-200%+ of its average
rainfall over the past two weeks. A deepening surface low will
track from central Arkansas to Michigan/Great Lakes region,
lifting a warm front through the Ohio Valley and a cold front
through the Mississippi Valley, Gulf states and the Tennessee/Ohio
Valleys by Friday morning. Influence from the subtropical and
southern stream jets along with significant IVT (1000+ kg/ms) into
the region is expected to lead to heavy rainfall north/on the cool
side of the 700 hPa 6C isotherm where it is less capped aloft
across portions of the Southeast, Deep South, and southernmost
Appalachians this period. Precipitable water values are expected
to rise to 1.5-1.75", if not higher. ML CAPE of 1000-2000 J/kg is
expected to develop during the heating of the day. An area of
broad, convergent flow at 850 hPa (up to 50-60 kts with effective
bulk shear to match), becomes more aligned with the mean 850-300
mb flow by this afternoon (18z onward), which will likely lead to
some training of convective segments prior to the passage of the
cold front. Areas in and near the southernmost Appalachians could
see three rounds of convection today, part of the reason for the
upgrade to Moderate Risk. Because of the degree of effective bulk
shear, mesocyclones are expected, which can also be heavy rain
producers, especially cells that form within the best/highest
instability across the southern fringe of the risk areas. While
hourly rain totals to 2" are generally expected, which in the
Moderate Risk area would approach the 3-hour flash flood guidance
in one hour, there is a non-zero chance for 3-4" an hour totals
within the area of strongest instability should a few mesocyclones
align/merge. While the uncertainty there (central MS and central
AL) in QPF is the greatest, the flash flood risk there can't be
ignored. Rapid runoff, urban ponding and localized flash flooding
will be possible across this area.


Mid-Mississippi Valley...
The guidance indicates that as the aforementioned surface low
spins up and tracks near the region, MU CAPE of 500-1000 J/kg gets
imported into eastern MO and IL. Precipitable water values rise
to 1-1.25", and the combination of the two could lead to 1.5" of
rain in an hour (approaching the 3-hourly flash flood guidance
values). While the system itself is progressive, there are
indications in the 00z HREF probabilities of 0.5"+ in an hour that
a TROWAL/comma head of the strengthening surface low could pivot
briefly/slow down the exit of the heavy rainfall. Since portions
of northeast MO and central IL have received 300%+ of their
average two week rainfall, coordination with LSX/St Louis MO and
the ILX/Central IL forecast offices led to a new Slight Risk of
excessive rainfall/flash flooding for this area. Local amounts in
the 2-3" are anticipated (HREF probabilities of 3" totals are near
50% in central IL during the 24 hour period).

Roth


Day 2
Valid 12Z Fri Mar 26 2021 - 12Z Sat Mar 27 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Campbell


Day 3
Valid 12Z Sat Mar 27 2021 - 12Z Sun Mar 28 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Another round of scattered to widespread thunderstorms are
expected to track through the Gulf states and into the
Tennessee/Ohio Valleys region, producing another 1 to 3 inches
over many of the locations forecast to receive heavy rains on Day
1. Day 2 is expected to be fairly quiet, allowing for some
recovery on local creeks and streams. However, a few ongoing
swollen streams (possibly flooded) will be rather sensitive to any
additional rainfall. The latest guidance has a decent signal for
moderate to heavy rainfall to set up in a W/SW to E/NE axis from
the Gulf states into the Tennessee Valley, but there is enough
spread in the amounts and location of the maximums to have a
reduced sense of confidence. The location of this maximum swath
will be dependent on the boundary placement and mesoscale
features. In coordination with the local forecast offices a
Marginal Risk area for excessive rainfall was not issued with this
package but is an area that will be closely monitored as the flood
risk is non-zero.

Campbell


$$
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FOUS30 KWBC 261548
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1147 AM EDT Fri Mar 26 2021

Day 1
Valid 16Z Fri Mar 26 2021 - 12Z Sat Mar 27 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

16z Update: No change in thinking over the Northeast for today.
Elsewhere, a narrow axis of periodic training cells over portions
of AL and GA will continue to produce localized rainfall over 2"
through the afternoon. However this is generally south and east of
areas with saturated soil conditions...thus the flash flood risk
is expected to remain below 5 percent (although localized ponding
of water/urban flooding is possible). Convection over the central
Plains and mid MS Valley may produce an axis of 1-2" of rain, but
quick cell motions and below normal soil saturation supports
little to no flash flood risk. Convection should develop late
tonight (after 06z) across portions of AR into northern MS/AL into
TN. Soil conditions are much more saturated over most of this
region. At the moment the activity looks too
disorganized/progressive to warrant any Marginal risk. But these
nocturnal convective events can sometimes get organized a bit
quicker than guidance indicates, which combined with the more
sensitive soil conditions, means this area will need to monitored
for a possible isolated flash flood risk towards the end of the
day 1 period. For now will maintain no risk areas.

Chenard

...Previous Discussion...

Northern New York/Northern New England...
Despite receiving less than 50% of normal precipitation over the
past couple of weeks, anticipated moderate-heavy rainfall
coinciding with more rapid snowmelt as surface dew point
temperatures approach or exceed 50F are expected to lead to a
longer duration flood concern, with a non-zero risk of flash
flooding. Highest areal-average QPF per the CAMs was 1-2" within
areas more likely to receive orographic enhancement. Modest
instability (mixed-layer CAPEs possibly as high as 500-1000 j/kg),
along with pooling of 1-1.25" PW values ahead of the cold front,
will allow for isolated hourly rainfall rates of ~1.0" (nearing 1
hour FFG values), especially through mid afternoon; fast storm
motions should prevent much more. The heavy rainfall will be
possible over a 6-12 hour time frame this morning and afternoon
and complicated by the possibility of additional frozen
precipitation in northern ME late in the period/tonight. The more
rapid snow melt is likely to offset the lack of precipitation as
of late, and combined with higher short-term rainfall rates with
any convective segments, isolated runoff issues cannot be ruled
out. Based on coordination with ALY/Albany NY, BTV/Burlington VT,
GYX/Gray, ME, and CAR/Caribou ME forecast offices, went without a
Marginal Risk for the day 1 ERO as concerns are expected to be
mostly hydrologic.

Hurley/Roth


Day 2
Valid 12Z Sat Mar 27 2021 - 12Z Sun Mar 28 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE ARKLATEX
REGION INTO PORTIONS OF THE LOWER MISSISSIPPI VALLEY, THE
TENNESSEE VALLEY, AND THE LOWER OHIO VALLEY...

Another round of widespread showers and thunderstorms are expected
Saturday into early Sunday across the outlook area ahead of the
next mid-upper shortwave trough and surface cold front. Models
continue to signal heavy rainfall across portions of the Lower MS
Valley into the TN/OH Valley with a decent QPF footprint of 2-4+
inches through the forecast period. As with the Thursday system,
thermodynamic parameters will be quite favorable for heavy rain,
though the 850-700 mb moisture transport/flux anomalies aren't
expected to be quite as high, 850 mb southwesterly flow increasing
to 45-55 kts, PWs of 1.5-1.75", and strong, deep layer instability
(mixed-layer capes peaking between 2000-3000+ j/kg) will support
hourly rainfall rates of 1.5-2.0+ inches within the Slight Risk
area.

The latitudinal spread among the model guidance continues.
Therefore, retained the broad Slight Risk area with a shift
northward based on the latest trends. It is the expectation that
subsequent EROs will likely be refined, while also addressing the
potential need for a Moderate Risk depending on the antecedent
soils along with added forecast confidence in terms of location of
the heaviest QPF. While early indications seemed to show that much
of the expected rain could fall atop the recent heavy rains
received on Thursday/early Friday across the Deep South/TN Valley,
the majority of the 00Z model guidance has shifted a bulk of the
precipitation activity north. This is a bit of good news, though
based on the latest soil moisture analysis (0-40cm) from NASA
SPoRT, much of the risk area is above the 98th percentile
(extremely saturated). So despite the axis of heavy rainfall being
slightly displaced from Thursdays storm system, there will likely
be localized to scattered flash flooding centered across the TN
Valley. This is also supported by the highest probabilities
(25-35%) of the 24hr rainfall exceeding the 2-year ARI.

Hurley/Pagano

Day 3
Valid 12Z Sun Mar 28 2021 - 12Z Mon Mar 29 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

...TN Valley/Southern Appalachians...

Showers and thunderstorms along or just ahead of a cold front will
continue to advance eastward Sunday morning through eastern
portions of the TN Valley and into the Southern Appalachians. Due
to the saturated soils in place, there is some concern for flash
flooding. However, the limiting factors are (1) the uncertainty
regarding the precipitation intensity and (2) progressive nature
of the cold front. Some guidance shows the activity diminishing
as the better dynamics exit. However, other pieces of guidance
suggest a more defined line of heavy rain traversing the region
through early afternoon. The latter situation would prove more
impactful to the region, though confidence on this occurring is
quite low at this update given the aforementioned
uncertainty/model spread. Regardless, most models show anomalous
precipitable water values with lingering instability. We will
continue to monitor the latest trends and introduce a risk area if
consensus shows a signal for heavy rainfall/flash flooding.

Pagano


$$
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FOUS30 KWBC 270823
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Sat Mar 27 2021

Day 1
Valid 12Z Sat Mar 27 2021 - 12Z Sun Mar 28 2021

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF
THE MID-SOUTH AND TENNESSEE VALLEY...

A complex synoptic pattern is expected to evolve across portions
of the Mississippi, Ohio, and Tennessee Valleys this period. A
warm front at the surface surges northward through the Mid-South
to the Ohio River with time as a surface low/cold front approach
from the west, urged along by a phasing trough across the Plains
which moves across the Midwest. The complicating factor here is
that even though the warm front itself appears to instigate the
convection early on, convergence at 850 hPa/near the leading edge
of the better 500 hPa height gradient ahead of the cold front and
south of the retreating warm front appears to tie it in place
across the Lower Mississippi and Tennessee Valleys south of the
Ohio River, likely leading to a forming/potentially stalled
outflow boundary in the warm sector with time. This is also near
the location of a possible atmospheric river with IVT values >1000
after 28/00z, which portends a broad, wet pattern for Saturday
into early Sunday. The atmosphere is uncapped north of LA and
central MS as 700 hPa temperatues are generally below 6C north of
those locations (which factored into being fairly wet on the day 1
QPF across the ArkLaTex and the northern half of MS, most in line
with the 26/12z ECMWF). The RAP guidance shows a broad pool of
1000-3000 ML/MU CAPE in this area while precipitable water values
surge above 1.5", potentially closing in on 2" across AR. With
the instability overspreading the active convection as the warm
front marches northward, a troubling situation sets up as any
outflow boundary which develops in the warm sector would become a
very effective boundary for heavy rainfall/cell training or
backbuilding. Low-level inflow rises to 50+ kt with effective
bulk shear to match, so convection should attempt organization.
Hourly rain totals to 2.5" are expected, though with so much
instability around, less organized convection and organized
mesocyclones could collide/merge, leading to a non-zero chance of
3-4" an hour totals. Some of the mesoscale guidance calls for
local amounts in the 3-7" range, which is quite reasonable. The
00z HREF probabilities of 5" in 24 hours rise into the 30-40%
range across spots of TN. The QPF was ramped up some in this
region when compared to continuity, with JKL/Jackson KY leading to
further increases along their section of the Ohio River. The
entire area has seen 150-400% of average precipitation over the
past couple of weeks, which has left soils sensitive. The
Moderate Risk in place from continuity is well supported, and few
changes were made to the Slight or Moderate Risk areas.

Meanwhile, up to the north closer to the warm front, there is
potential of heavy rainfall across portions of IL and IN, another
area with a significant positive rainfall anomaly over the past
couple of weeks and potentially saturated soils. RAP guidance
shows MU CAPE of 500-1000 J/kg sneaking around the southern
convection and potentially aiding heavy rainfall across that
portion of the Midwest. With some signal for 2" amounts in that
area from the 00z GFS/00z Canadian Regional/00z high-resolution
NAM/00z FV3CAM, which could fall quickly, a Maringal Risk was
added as a precaution.

Roth


Day 2
Valid 12Z Sun Mar 28 2021 - 12Z Mon Mar 29 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE
CUMBERLAND PLATEAU AND EASTERN PORTIONS OF THE TENNESSEE VALLEY
INTO THE CENTRAL APPALACHIANS...

Heavy bands of rain are expected along and ahead of a fairly
progressive cold front, with the forward speed increasing on
Sunday as the upper shortwave trough pivots into the Ohio Valley
and central Appalachians. Due to the saturated soils in place,
there is some concern for flash flooding. However, the limiting
factors are (1) the uncertainty regarding the precipitation
intensity and (2) progressive nature of the cold front. Recent
heavy rain has lowered local 3-hr FFG to 1/1.5 inches (1-hr FFG as
low as 0.75 inches). The pre-frontal convective line segments are
expected to move through the area during the morning hours
(especially 12-15Z). There continues to be some spread in the
guidance, where some suggest convection wanes by the afternoon
while others show a more defined line of heavy rain passing
through region during the early afternoon. Should the solutions
pan out with afternoon convection (low confidence), the risk for
flash flooding will remain elevated across much of the eastern
Tennessee Valley and central Appalachian region. A majority of the
guidance has anomalous precipitable water values with lingering
instability. The inherited Marginal Risk area on required minor
adjustments to reflect the latest WPC QPF, with the northern bound
extending further north into western Maryland and Pennsylvania.

Campbell


Day 3
Valid 12Z Mon Mar 29 2021 - 12Z Tue Mar 30 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Campbell


$$
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FOUS30 KWBC 272025
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
424 PM EDT Sat Mar 27 2021

Day 1
Valid 16Z Sat Mar 27 2021 - 12Z Sun Mar 28 2021

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF
THE MID-SOUTH AND TENNESSEE VALLEY...

16z Update: No major changes needed to the ERO at this time. 12z
HREF members and recent HRRR runs still support a Moderate risk
from eastern AR, into northern MS/AL and much of TN. Portions of
this area have already seen 1-3" of rain earlier today...and going
into the event soil saturation values were already well above
average. Thus antecedent conditions remain favorable for
additional flash flooding. Currently we are generally in between
forcing mechanisms, and a slight decrease in low level moisture
transport should result in some weakening/broadening of the low
level convergence axis. However, by mid to late afternoon we
should begin to see an uptick in mid/upper forcing from the west,
along with a renewed increase in low level moisture transport,
helping re-intensify the convergence axis. Thus the flash flood
risk will likely begin ramping up by mid/late afternoon into the
evening hours. This uptick by mid/late afternoon into the evening
is of high confidence. It is just late morning into early
afternoon where the risk may temporarily become more
isolated/scattered...but even during this period some flash flood
risk is still expected to persist.

The multiple convective rounds, combined with favorable antecedent
conditions, supports a scattered to widespread flash flood event
as we head through the evening hours. Embedded within this
Moderate risk there may very well be a narrower axis that receives
6-9" of rainfall. If this does occur then a focused corridor of
more significant and dangerous flash flooding is possible...with
the greatest chance of this from far northern MS/AL into
southwest/south central TN. Given amounts of this magnitude and
already saturated conditions, High risk level impacts are
certainly a possibility over a narrow axis. However, no upgrade
with this update, as the localized nature and uncertainty supports
maintaining a broader Moderate risk at this time.

Chenard

...Previous Discussion...

A complex synoptic pattern is expected to evolve across portions
of the Mississippi, Ohio, and Tennessee Valleys this period. A
warm front at the surface surges northward through the Mid-South
to the Ohio River with time as a surface low/cold front approach
from the west, urged along by a phasing trough across the Plains
which moves across the Midwest. The complicating factor here is
that even though the warm front itself appears to instigate the
convection early on, convergence at 850 hPa/near the leading edge
of the better 500 hPa height gradient ahead of the cold front and
south of the retreating warm front appears to tie it in place
across the Lower Mississippi and Tennessee Valleys south of the
Ohio River, likely leading to a forming/potentially stalled
outflow boundary in the warm sector with time. This is also near
the location of a possible atmospheric river with IVT values >1000
after 28/00z, which portends a broad, wet pattern for Saturday
into early Sunday. The atmosphere is uncapped north of LA and
central MS as 700 hPa temperatues are generally below 6C north of
those locations (which factored into being fairly wet on the day 1
QPF across the ArkLaTex and the northern half of MS, most in line
with the 26/12z ECMWF). The RAP guidance shows a broad pool of
1000-3000 ML/MU CAPE in this area while precipitable water values
surge above 1.5", potentially closing in on 2" across AR. With
the instability overspreading the active convection as the warm
front marches northward, a troubling situation sets up as any
outflow boundary which develops in the warm sector would become a
very effective boundary for heavy rainfall/cell training or
backbuilding. Low-level inflow rises to 50+ kt with effective
bulk shear to match, so convection should attempt organization.
Hourly rain totals to 2.5" are expected, though with so much
instability around, less organized convection and organized
mesocyclones could collide/merge, leading to a non-zero chance of
3-4" an hour totals. Some of the mesoscale guidance calls for
local amounts in the 3-7" range, which is quite reasonable. The
00z HREF probabilities of 5" in 24 hours rise into the 30-40%
range across spots of TN. The QPF was ramped up some in this
region when compared to continuity, with JKL/Jackson KY leading to
further increases along their section of the Ohio River. The
entire area has seen 150-400% of average precipitation over the
past couple of weeks, which has left soils sensitive. The
Moderate Risk in place from continuity is well supported, and few
changes were made to the Slight or Moderate Risk areas.

Meanwhile, up to the north closer to the warm front, there is
potential of heavy rainfall across portions of IL and IN, another
area with a significant positive rainfall anomaly over the past
couple of weeks and potentially saturated soils. RAP guidance
shows MU CAPE of 500-1000 J/kg sneaking around the southern
convection and potentially aiding heavy rainfall across that
portion of the Midwest. With some signal for 2" amounts in that
area from the 00z GFS/00z Canadian Regional/00z high-resolution
NAM/00z FV3CAM, which could fall quickly, a Maringal Risk was
added as a precaution.

Roth


Day 2
Valid 12Z Sun Mar 28 2021 - 12Z Mon Mar 29 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN APPALACHIANS NEAR THE TENNESSEE-NORTH CAROLINA
BORDER...

2030Z Update...

Guidance maintains the expected synoptic scenario with some
ongoing uncertainty over specifics of convection and/or offsetting considerations. Within the existing Marginal Risk area that was
maintained for this issuance (eastern Tennessee Valley into
central Appalachians), a composite of the 12Z GFS/NAM/NAM
Nest/ECMWF would suggest a relatively better signal for locally
heavy rainfall potential Sunday morning and perhaps into afternoon
from near the Kentucky/Virginia border southward. Overlaying this
region with locations receiving the greatest rainfall recently,
and the possibility for at least a brief period of training aided
by upslope flow even with the associated front being fairly
progressive, led to the addition of a Slight Risk area centered
near the Tennessee-North Carolina border. Forecast offices GSP
and MRX provided input/support for this upgrade.

Rausch


Previous discussion...

Heavy bands of rain are expected along and ahead of a fairly
progressive cold front, with the forward speed increasing on
Sunday as the upper shortwave trough pivots into the Ohio Valley
and central Appalachians. Due to the saturated soils in place,
there is some concern for flash flooding. However, the limiting
factors are (1) the uncertainty regarding the precipitation
intensity and (2) progressive nature of the cold front. Recent
heavy rain has lowered local 3-hr FFG to 1/1.5 inches (1-hr FFG as
low as 0.75 inches). The pre-frontal convective line segments are
expected to move through the area during the morning hours
(especially 12-15Z). There continues to be some spread in the
guidance, where some suggest convection wanes by the afternoon
while others show a more defined line of heavy rain passing
through region during the early afternoon. Should the solutions
pan out with afternoon convection (low confidence), the risk for
flash flooding will remain elevated across much of the eastern
Tennessee Valley and central Appalachian region. A majority of the
guidance has anomalous precipitable water values with lingering
instability. The inherited Marginal Risk area on required minor
adjustments to reflect the latest WPC QPF, with the northern bound
extending further north into western Maryland and Pennsylvania.

Campbell


Day 3
Valid 12Z Mon Mar 29 2021 - 12Z Tue Mar 30 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Rausch

$$
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FOUS30 KWBC 280747
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
345 AM EDT Sun Mar 28 2021

Day 1
Valid 12Z Sun Mar 28 2021 - 12Z Mon Mar 29 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR THE
SOUTHERN APPALACHIANS...

Bands of rain and thunderstorms are expected along and ahead of a
fairly progressive cold front mainly Sunday morning, with the
forward speed increasing on
Sunday as the upper shortwave trough pivots into the Ohio Valley
and central Appalachians. Due to the saturated soils in place,
some of which due to rain over the past 24 hours, there is some
concern for flash flooding. However, the limiting factor is the
progressive nature of the cold front. Recent heavy rain has
lowered local 3-hr FFG to 1-2". The pre-frontal convective line
segments are expected to move through the area during the morning
hours and the 00z HREF 24 hour probabilities of 2"+ highlight the
mountains of westernmost NC. There continues to be a mild
suggestion in the 00z HREF probabilities of 0.25"+ in an hour that
some convection could develop in and near the southern
Appalachians in the afternoon. Should that idea pan out (low
confidence), the risk for flash flooding will remain non-zero
across much of the eastern Tennessee Valley and central
Appalachian region past noon. A majority of the guidance has
anomalous precipitable water values into the early afternoon with
a touch of lingering instability. The inherited risk areas
required minor expansion to reflect the latest WPC QPF, recent
rainfall, and recent model guidance.

Roth/Campbell


Day 2
Valid 12Z Mon Mar 29 2021 - 12Z Tue Mar 30 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Campbell


Day 3
Valid 12Z Tue Mar 30 2021 - 12Z Wed Mar 31 2021

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE LOWER MISSISSIPPI AND TENNESSEE VALLEYS AND THE NORTHERN
PART OF THE GULF STATES...

Moisture from the Gulf of Mexico will be steadily increasing
northward through the Day 3 period. The southwesterly low level
winds of 25 to 40 kts will draw PW values around 1.25-1.5 (+1.5
standard deviations) over the area just prior to the approaching
cold front. Most of the guidance suggest convection firing up
around/after 18Z Tuesday near the Lower Mississippi Valley, but as
the front nears the South the coverage will likely become more
scattered to widespread. The best window for moderate to heavy
rainfall appears to be 06Z-12Z Wednesday, with the axis of maximum
amounts setting up from eastern Arkansas to eastern Tennessee.
Areal averages for this section is forecast to be 0.75 to 1.3
inches. Isolated higher amounts may be possible.

The recent rain over much of the Tennessee Valley and surrounding
areas have reduced local FFG, some areas as low as 0.25 but the
majority fall in the 1 to 2.5 inch range. This part of the country
will have roughly 24 to 36 hours of quiet weather where the soil
saturation can recover. However, due to the shear volume of water
over the region local streams and rivers will likely still be
swollen or have ongoing flooding by this period. Although this
scenario will more than likely be more of a long duration riverine
flooding event, the reduced FFG along with additional
precipitation expected may not take much for runoff or flash
flooding to develop. In coordination with the local forecast
offices a Marginal Risk area was introduced from Arkansas/northern
Louisiana to Middle Tennessee.

Campbell

$$
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FOUS30 KWBC 282028
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 PM EDT Sun Mar 28 2021

Day 1
Valid 16Z Sun Mar 28 2021 - 12Z Mon Mar 29 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Orrison


Day 2
Valid 12Z Mon Mar 29 2021 - 12Z Tue Mar 30 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Rausch


Day 3
Valid 12Z Tue Mar 30 2021 - 12Z Wed Mar 31 2021

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE LOWER MISSISSIPPI AND TENNESSEE VALLEYS AND THE NORTHERN
PART OF THE GULF STATES...

2030Z Update...

The afternoon update maintained continuity with the identical
Marginal Risk area introduced in the previous issuance, with
heaviest rainfall potential generally in the Tuesday night-early
Wednesday time frame. There are background uncertainties that
offer potential for future adjustments depending on guidance
continuity/trends. Some 12Z models offer potential for
significant rainfall to extend as far north as Kentucky. The
question here is whether precipitable water values will reach as
high as what the ECMWF, for example, suggests if convection exists
farther south toward lower stability. Meanwhile the current
Marginal Risk area still captures the area with best probability
of highest rainfall rates, but there is still enough spread for
where one or more axes of heaviest rain may intersect lowest FFG
values to temper confidence in placement of any embedded Slight
Risk area at this time. An improvement in guidance clustering in
the future would provide support for an eventual upgrade, with at
least some areas of runoff/flash flood issues possible given the
high soil moisture.

Rausch


Previous discussion...

Moisture from the Gulf of Mexico will be steadily increasing
northward through the Day 3 period. The southwesterly low level
winds of 25 to 40 kts will draw PW values around 1.25-1.5 (+1.5
standard deviations) over the area just prior to the approaching
cold front. Most of the guidance suggest convection firing up
around/after 18Z Tuesday near the Lower Mississippi Valley, but as
the front nears the South the coverage will likely become more
scattered to widespread. The best window for moderate to heavy
rainfall appears to be 06Z-12Z Wednesday, with the axis of maximum
amounts setting up from eastern Arkansas to eastern Tennessee.
Areal averages for this section is forecast to be 0.75 to 1.3
inches. Isolated higher amounts may be possible.

The recent rain over much of the Tennessee Valley and surrounding
areas have reduced local FFG, some areas as low as 0.25 but the
majority fall in the 1 to 2.5 inch range. This part of the country
will have roughly 24 to 36 hours of quiet weather where the soil
saturation can recover. However, due to the shear volume of water
over the region local streams and rivers will likely still be
swollen or have ongoing flooding by this period. Although this
scenario will more than likely be more of a long duration riverine
flooding event, the reduced FFG along with additional
precipitation expected may not take much for runoff or flash
flooding to develop. In coordination with the local forecast
offices a Marginal Risk area was introduced from Arkansas/northern
Louisiana to Middle Tennessee.

Campbell

$$
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FOUS30 KWBC 291536
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1135 AM EDT Mon Mar 29 2021

Day 1
Valid 16Z Mon Mar 29 2021 - 12Z Tue Mar 30 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Oravec


Day 2
Valid 12Z Tue Mar 30 2021 - 12Z Wed Mar 31 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE LOWER MISSISSIPPI AND TENNESSEE VALLEYS...

A digging longwave trough across the central U.S. will usher in
Gulf moisture and instability across portions of the Lower MS
Valley, TN Valley, and the Deep South. As a result, anticipate
multiple rounds of showers and thunderstorms across the region
late Tuesday afternoon into early Wednesday. This set up is
somewhat reminiscent of the previous storm system that brought
widespread heavy rain and flash flooding to the region. However,
there are some differences between these two systems and among the
model guidance itself resulting in slightly less confidence
regarding the spatial extent and severity of flash flooding across
the region. Regardless, there is enough signal to support the
potential for scattered flash flooding due to heavy rain occurring
over saturated soils.

As the aforementioned trough continues to dig south through the
Central Plains, better upper level divergence and mid-level
shortwaves will promote large scale forcing for ascent. Weak
surface low development along an associated cold front will also
promote lift along the boundaries. A warm front will lift north
through the Lower MS Valley/Deep South Tuesday afternoon bringing
showers and thunderstorms to the region. This front will
eventually lift through the TN Valley Tuesday evening/overnight.
It is ahead of the approaching cold front where the strongest
return flow will occur as precipitable water values surge above
1.5 inches aided by 40-50 kt southwesterly low level flow through
the evening hours. This is 2 standard deviations above the mean.
Instability will also be present, though it appears it may be
modest with cloud debris limiting diurnal heating and the cold
front passing overnight. It should be noted that the low level jet
will strengthen overnight to help sustain any thunderstorm
activity that does develop ahead of the front as it moves east.

Rain rates will be around 0.5-1+ inches/hour associated with the
precipitation ahead of the cold front. And given the already
saturated soils across much of the TN Valley, this region will be
susceptible to additional flash flooding/runoff. It is unlikely
that the wet antecedent conditions will improve much over the next
36-48 hours with the NASA SPoRT showing above 98th percentile of
saturation between 0-40 cm. Therefore, a Slight Risk was
introduced and takes into account not only the deterministic WPC
QPF, but also the degree of uncertainty through the probabilistic
guidance and ensembles. Depending on the expected rain intensity
and placement, a Moderate Risk area may be considered at future
updates.

Pagano

Day 3
Valid 12Z Wed Mar 31 2021 - 12Z Thu Apr 01 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHERN AND CENTRAL APPALACHIANS...

...Southern/Central Appalachians and Mid-Atlantic...
A digging longwave trough will continue to pivot east across the
MS Valley into the OH/TN Valleys through early Thursday.
Meanwhile, a strong cold front will be advancing east across the
OH/TN Valleys and through the Deep South/Lower MS Valley.
Shower/thunderstorm activity is expected to be ongoing at the
start of the forecast period (Wednesday morning), approaching the southern/central Appalachians and crossing the Deep South where
better large scale lift, moisture and instability align. Through
Wednesday afternoon, divergence aloft will increase thanks to a
strengthening upper level jet coincident with a strong mid-level
shortwave. As a result, a surface low will develop over the
Mid-Atlantic ushering in Gulf and Atlantic moisture and robust
instability northward. As a result, showers and thunderstorms
could continue beyond the mountains and into the Mid-Atlantic
allowing for multiple rounds of precipitation over fairly
saturated soils.

Precipitable water values will surge to over 1.25-1.5 inches ahead
of the aforementioned cold front aided by 30-40 kts southwesterly
low level flow. This is over 2 standard deviations above the
mean. With some lingering instability across the mountains
Wednesday morning expect heavier showers/thunderstorms to
continues to progress east, perhaps enhanced by the topography.
Rain rates could exceed 0.75 inches/hour. Given such wet soils
across the region (3 hourly FFG values below 1 inch and some
locations receiving over 300% of normal precipitation over the
past several days), it will not take much to cause flooding/runoff
concerns. Therefore, a Marginal Risk was introduced across
portions of the southern/central Appalachians.

By late Wednesday afternoon, instability will be climbing across
portions of the Southeast (MUCAPE > 1000 J/kg) ahead of the front;
though it is unclear how far north the higher values will reach.
This could limit the thunderstorm activity and also the rain rates
observed across the Mid-Atlantic. So while the region has observed
over 150% of normal precipitation over the past week with lower
FFG values, there is not enough evidence to support a Marginal
Risk area. We will continue to monitor the latest model trends.

Pagano


$$
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FOUS30 KWBC 302027
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
426 PM EDT Tue Mar 30 2021

Day 1
Valid 1717Z Tue Mar 30 2021 - 12Z Wed Mar 31 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE LOWER MISSISSIPPI AND TENNESSEE VALLEYS...

1800 UTC update

Some minor changes made to the marginal risk area across North
Florida. The southern edge was extended about 50 nm farther to the
south as per the latest model guidance suggesting additional
activity may fire farther south than the earlier activity.

Oravec

1600 UTC update

...North Florida...
A small marginal risk area was added over North Florida in the
vicinity of the warm front currently across this region. Slow
moving cells have been occurring along this boundary in a region
of upper difluence on the east and southeast side of the weak trof
moving off the Southeast coast. The latest hi res guidance from
1200 UTC shows potential for additional slow moving cells into
this afternoon, possibly slightly farther to the south where
instability is expected to increase. While FFG values are high,
the slow movement of the cells and potential for cells to move
across areas that received heavy rains early, warrants at least a
marginal risk area.

Over central Mississippi...a slight southward adjustment made to
the marginal and slight risk areas after collaboration with the
National Water Center. This slight southward adjustment was to
cover areas where the NWC rapid onset flooding product showed
potential later today. Otherwise, the new 1200 UTC hi res
guidance fit well in the previous risk areas.

Oravec


0900 UTC discussion

A digging longwave trough across the central U.S. will usher in
Gulf moisture and instability across portions of the Lower MS
Valley, TN Valley, and the Deep South. As a result, anticipate a
couple rounds of showers and thunderstorms across the region late
Tuesday afternoon into early Wednesday. This set up is somewhat
reminiscent of the previous storm system that brought widespread
heavy rain and flash flooding to the region. However, there are
some significant differences between these two systems but some
confidence that the severity of flash flooding across the region
would be less than the previous event as the heavier amounts are
expected to fall a little farther south. A warm front will lift
north through the Lower MS Valley/Deep South Tuesday afternoon
bringing showers and thunderstorms to the region. This front will
eventually lift through the TN Valley Tuesday evening/overnight.
It is ahead of the approaching cold front where the strongest
return flow will occur as precipitable water values surge above
1.5 inches aided by 40-50 kt southwesterly low level flow through
the evening hours. This is 2 standard deviations above the mean.
Instability will also be present, but much less than the previous
event, though it appears it may be modest with cloud debris
limiting diurnal heating and the cold front passing overnight. On
a broad scale, thickness diffluence is best from the ArkLaTex
eastward into northern/possibly central MS, which is where the
heavier rains/more organized convection should be favored. With
700 hPa temps fairly low north of central LA, the atmosphere is
more uncapped than the previous event which also argues for a more
southerly solution, perhaps south of what is currently indicated.

Rain totals could still peak at 2" inches/hour associated with the precipitation ahead of the cold front/along and near the warm
front within organized convection or within any training
convective segments. Although the max is indicated south of TN,
given the already saturated soils and very wet conditions over the
past couple of weeks across much of the Mid-South/TN Valley, this
region will be susceptible to additional flash flooding/runoff.
Therefore, a Slight Risk remains and takes into account not only
the deterministic WPC QPF, but also the degree of uncertainty
through the probabilistic guidance and ensembles.

Roth


Day 2
Valid 12Z Wed Mar 31 2021 - 12Z Thu Apr 01 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NORTHEASTERN ALABAMA, NORTHERN GEORGIA, FAR EASTERN TENNESSEE, AND
WESTERN NORTH CAROLINA...

2030 UTC Update...

Have bumped up a portion of the outlook area to a Slight Risk,
based on the slight uptick in deterministic/probabilistic QPF
amounts per the guidance (especially high-res CAMs), along with
the antecedent wet soils and above normal streamflows resulting
from recent heavy rainfall. The risk will be confined mainly
during the day Wednesday, prior to the cold frontal passage which
is expected to make more swift eastward progress by Wed evening as
the longwave trough approaches the TN Valley. Increasing
southwesterly low-level flow ahead of the front (30-40 kts at 850
mb) will become better aligned and of nearly the same magnitude of
the mean 850-300 mb wind, thereby enhancing the potential for
training convective segments prior to the frontal passage.
Certainly not the degree of cell training as was observed over the
TN Valley in recent days, however mixed-layer CAPEs of 1000-2000
j/kg along with PWs peaking between 1.5-1.75" will likely lead to
2-3" rainfall within a few hours per the high-res CAMs, which
again may pose more runoff issues within the Slight Risk area
given the recent heavy rains.

0830 UTC Discussion...

...Southern/Central Appalachians into the Mid-Atlantic...
A digging longwave trough will continue to pivot east across the
MS Valley into the OH/TN Valleys through early Thursday.
Meanwhile, a strong cold front will advance east across the OH/TN
Valleys and into the Southeast and Mid-Atlantic.
Shower/thunderstorm activity is expected to be ongoing early
Wednesday morning, approaching the Southern/Central Appalachians
and traversing the Deep South where better large scale lift,
moisture and instability align. Through Wednesday afternoon,
divergence aloft will increase thanks to a strengthening upper
level jet coincident with a strong mid-level shortwave approaching
from the south. As a result, a series of surface lows will develop
along the front helping to usher in Gulf and Atlantic moisture and
robust instability northward. Based on this assessment, expect
multiple rounds of showers and thunderstorms to impact the region.


Precipitable water values will surge to over 1.25-1.5 inches ahead
of the aforementioned cold front aided by 30-40 kts southwesterly
low level flow. This is over 2 standard deviations above the
mean. With some lingering instability across the mountains
Wednesday morning expect heavier showers/thunderstorms to
continues to progress east out ahead of the apparent cold front.
Another round of showers and thunderstorm is expected through the
afternoon as a potent mid-level shortwave moves atop the
approaching surface boundary. Rain rates could exceed 0.75
inches/hour. Given such wet soils across the region (3 hourly FFG
values below 1 inch and some locations receiving over 300% of
normal precipitation over the past several days) and multiple
rounds of heavy rain, it will not take much to cause
flooding/runoff concerns. Therefore, a Marginal Risk was
maintained across portions of the southern/central Appalachians. A
Slight Risk may need to be hoisted at a subsequent update, but the
model spread regarding the QPF footprint is too large to have
confidence regarding more scattered to widespread flash flooding
concerns at this time.

Farther north, by late Wednesday afternoon, instability will be
climbing across portions of the Southeast (MUCAPE > 1000 J/kg) and
even into parts of the Mid-Atlantic ahead of the front. Convective
activity advancing across the OH Valley early Wednesday morning
will push out ahead of the apparent cold front through much of the
region. The second wave of precipitation will arrive toward
evening and linger into the early morning hours associated with
the aforementioned mid-level shortwave. There is a bit of
uncertainty with respect to the intensity of both rounds of
precipitation. Regardless, there is enough signal to support
localized flash flooding concerns given the multiple rounds of
precipitation expected, wet antecedent conditions currently in
place and the urban corridor/runoff issues. Therefore, the
Marginal Risk was maintained/refined.

Hurley/Pagano


$$
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FOUS30 KWBC 311746
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
145 PM EDT Wed Mar 31 2021

Day 1
Valid 1742Z Wed Mar 31 2021 - 12Z Thu Apr 01 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHEAST...

1800 UTC update

Changes for the 1800 UTC update based on current radar trends.
The north side of the slight risk area was trimmed about 25 nm to
the south. The marginal and slight were also extended
southwestward into central to southern Louisiana. The marginal
risk was trimmed to the east across the central Appalachians.

Oravec

1600 UTC update

Changes to the previous Excessive Rainfall Outlook based mostly on
latest radar trends and new hi res output through this afternoon.
The northern portion of the slight risk was trimmed southward by
approximately 75-100 nm. Extended the slight risk area slightly
southward across southern Mississippi, southern Alabama into north
central Georgia for latest hi res max qpf axes. Please see WPC's
mesoscale precipitation discussion # 0085 that will be issued
shortly and valid until 2200 UTC for additional information on
flash flood potential in the slight risk area.

Oravec

0900 UTC discussion

...Southern/Central Appalachians into the Mid-Atlantic and
Southern New England...
A digging trough will cause a strong cold front to advance into
the Southeast and Mid-Atlantic states. Increasing southwesterly
low-level flow ahead of the front (30-40 kts at 850 mb) will
become better aligned and of nearly the same magnitude of the mean
850-300 mb wind, thereby enhancing the potential for training
convective segments prior to the frontal passage, particularly
across the Southeast where the best 1000-500 hPa thickness
diffluence is expected. Matching effective bulk shear should lead
to organized convection. PWs peak between 1.5-1.75". The biggest
concern is the progressive line moving into a region of ML CAPEs
of 1500-2500 J/kg this afternoon. This sequence of events is
likely lead to some broadening/reorganization of the convective
line into a band and enhance hourly and overall storm totals.
Hourly rain totals to 2" and local 3-4" amounts are expected
within a few hours, which would breach flash flood guidance and
overwhelm partially saturated to saturated soils where two week
precipitation has been 200%+ when compared to average. The Slight
Risk area was maintained and expanded southward per coordination
with JAN/Jackson MS and MOB/Mobile AL forecast offices.

Farther north, by this afternoon, instability will be climbing
across portions of the Southeast (MUCAPE > 1000 J/kg) and even
into the southernmost Mid-Atlantic ahead of the front. A touch of
instability even gets up into southeast NY. Convective activity
advancing across the OH Valley early Wednesday morning will push
out ahead of the cold front through much of the region. The
second wave of precipitation will arrive toward evening and linger
into the early morning hours associated with a mid-level
shortwave. Hourly rain totals to 2" are possible in Virginia
while more northern areas should see 0.5-1" an hour totals. There
is enough signal to support localized flash flooding concerns
given the multiple rounds of rain expected, wet antecedent
conditions currently in place and the urban corridor/runoff
issues. Therefore, the Marginal Risk up the East Coast was
maintained/refined based on the latest guidance.

Roth


Day 2
Valid 12Z Thu Apr 01 2021 - 12Z Fri Apr 02 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF EASTERN MAINE...

A cold front will approach the Northeast Wednesday with a surface
low developing and deepening off the coast through Friday morning.
A warm front will advance north ahead of the surface low Wednesday night/Thursday morning bringing showers and thunderstorms to the
region through late morning. Given fairly low FFG values and
multiple rounds of heavy rain expected during the first half of
the forecast period, localized flash flooding may occur.

Precipitable water values will surge to over 1.25 inches across
far eastern ME, aided by 50+ knot low level southerly flow. This
is over 3 standard deviations above the mean. With some elevated
instability present, rain rates may exceed 0.50 inches/hour. It
should be noted that orographic enhancement along the northeast
coast of ME is expected with low level winds becoming orthogonal
to the coastline. Areal average precipitation will be around 1-2+
inches with locally higher amounts possible. A bulk of this
activity will likely occur within only a few hours.

While the soil saturation is not above normal based on NASA SPoRT
analysis, current FFG values are still hovering around 1-2 inches
within 3 hours with green up not yet occurring. Therefore,
multiple rounds of heavy rain could lead to rapid runoff in some
locations and thus, the Marginal Risk was maintained.

Pagano

Day 3
Valid 12Z Fri Apr 02 2021 - 12Z Sat Apr 03 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Pagano


$$
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FOUS30 KWBC 011547
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1146 AM EDT Thu Apr 01 2021

Day 1
Valid 12Z Thu Apr 01 2021 - 12Z Fri Apr 02 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS COASTAL
SOUTHEAST FLORIDA...

A marginal risk area was added in the urbanized regions of
southeast Florida from Miami northward to West Palm Beach. The
amplified eastern U.S. upper trof will help push a strong cold
front south through South Florida this afternoon. The latest
simulated hi res radars suggest there could be two rounds of
convection affecting southeast Florida in the 1800 UTC Thu to 0000
UTC Fri time frame ahead of this front in an axis of 1.75"+ PWs
and 1000+ j/kg MUCAPE. An initial slower moving pre frontal sea
breeze convection, followed by a faster southeastward moving
frontal convection. The latest HREF neighborhood probabilities
are high for 2"+ amounts (60-90%+) from Miami northward to Palm
Beach, 40-60% for 3"+ amounts and 10-20% for 5"+ totals. Isolated
hourly precip totals of 2-3"+ may result in urbanized flash flood
issues across coastal southeast Florida.

Oravec


Day 2
Valid 12Z Fri Apr 02 2021 - 12Z Sat Apr 03 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Pagano

Day 3
Valid 12Z Sat Apr 03 2021 - 12Z Sun Apr 04 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Pagano

$$
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FOUS30 KWBC 031522
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1121 AM EDT Sat Apr 03 2021

Day 1
Valid 16Z Sat Apr 03 2021 - 12Z Sun Apr 04 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Petersen


Day 2
Valid 12Z Sun Apr 04 2021 - 12Z Mon Apr 05 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Lamers

Day 3
Valid 12Z Mon Apr 05 2021 - 12Z Tue Apr 06 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Lamers

$$
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FOUS30 KWBC 040724
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
323 AM EDT Sun Apr 04 2021

Day 1
Valid 01Z Sun Apr 04 2021 - 12Z Sun Apr 04 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann



Day 2
Valid 12Z Mon Apr 05 2021 - 12Z Tue Apr 06 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Lamers

Day 3
Valid 12Z Tue Apr 06 2021 - 12Z Wed Apr 07 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

A Marginal Risk area was considered for portions of the Central
Plains, particularly in and near eastern Nebraska. Most models do
indicate a corridor of heavy rainfall, with 24hr QPF in excess of
1 inch, somewhere in the region. However, there is considerable
variability on the structure of the low pressure system and
placement of the rainfall from model to model. Therefore,
confidence in placement of heavy rain is too low to support a
Marginal Risk at this point, particularly as antecedent ground
conditions might prove crucial in terms of actual impacts. One
area to monitor closely is eastern Nebraska, where 14-day rainfall
has been more than double average levels and the elevated soil
moisture would potentially support greater runoff. However,
several global models currently indicate a QPF minimum in that
area despite heavier QPF nearby.

Lamers

$$
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FOUS30 KWBC 051536
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1135 AM EDT Mon Apr 05 2021

Day 1
Valid 16Z Mon Apr 05 2021 - 12Z Tue Apr 06 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Roth/Chenard


Day 2
Valid 12Z Tue Apr 06 2021 - 12Z Wed Apr 07 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

A Marginal Risk was considered for portions of SD into
south-central MN. With respect to the deepening Plains low, the
area of concern is the pivoting low-mid level deformation band to
the north of the low. It is the presence of this persistent
forcing mechanism coincident with marginal instability (MUCAPE
around 300-600 j/kg; tall and narrow CAPE profiles) and
precipitable water values above the 95th percentile for early
April that suggests the potential for slow-moving and somewhat
efficient convective rain bands. Models also generally show an
inverted trough extending from the surface low northward into E
SD, and it's not uncommon for nearly stationary heavy rain bands
to evolve over a period of a few hours in those areas (given
sufficient moisture and instability).

Nevertheless, given ongoing drought conditions and lingering
uncertainty on placement of heavy rain bands, collaboration with
WFOs in the region yielded lower than 5 percent probabilities for
this outlook. It's important to note that the probability of
excessive rainfall is not zero in this case, and the area will
continue to be monitored for a risk area if confidence in
placement of heavy rainfall increases sufficiently.

Lamers


Day 3
Valid 12Z Wed Apr 07 2021 - 12Z Thu Apr 08 2021

...MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE MID SOUTH
REGION...

An area of more concentrated convective activity is expected over
the Mid South from Wednesday afternoon into early Thursday morning
along an advancing cold front, and this should affect some areas
that have received significant rainfall over the past several
weeks. In fact, NASA SPoRT LIS 0-100cm soil moisture percentiles
over much of the region are between the 70th and 90th percentile.
The progressive nature of the cold front should limit residence
time of any convective clusters or lines in any one location.
Nevertheless, given the antecedent soil moisture anomalies, there
should be at least some risk of flash flooding. Precipitable water
values should be above the 90th percentile for early April, with
narrow CAPE profiles (MUCAPE generally around 500 j/kg), and this
will support heavy rainfall with any organized convection.

Lamers

$$
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FOUS30 KWBC 061953
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
352 PM EDT Tue Apr 06 2021

Day 1
Valid 16Z Tue Apr 06 2021 - 12Z Wed Apr 07 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL PLAINS...

1600 UTC discussion

The only changes made to the previous day 1 Excessive Rainfall
Outlook were to expand the marginal risk southeastward into
southeast Kansas. This was to cover the spread in the 1200 UTC hi
res guidance..

Oravec

0900 UTC discussion


We decided to carry a Marginal risk of excessive rainfall for
later today into tonight across portions of central KS into far
southeast NE. It is here where we see the greatest overlap in
instability and low level moisture transport, supportive of
intense rainfall rates. Severe weather is probably the bigger
risk, as highlighted by SPCs Slight risk...but localized flash
flooding can also not be ruled out. The orientation of moisture
transport in model fields this evening indicates some
training/backbuilding potential as a west/east convergence axis
develops in response to the approaching closed mid level low and
subsequent surface cyclogenesis. With plenty of upstream
instability available, and persistent moisture transport into the
region...some backbuilding potential seems plausible. Current
model QPFs are not really indicating much a flash flood
risk...however have seen models under do the degree of convective
initiation in setups like this. If convection is more widespread
than indicated...then a better chance we are able to organize a
small scale backbuilding convective cluster this evening. So while
not a high end risk...the potential for greater convective
coverage than currently modeled, combined with favorable
thermodynamic and moisture parameters...suggests a localized flash
flood or two can not be ruled out in spite of the relatively low
areal averaged QPF in the latest WPC forecast.

Higher WPC QPF is further north from SD into MN and northern WI.
Contemplated a Marginal risk here as well...but opted to hold off
for now. Locally heavy convection has moved across these areas
last evening...but the region as a whole has below normal soil
moisture. Unclear how much this past 12 hours of rain has put a
dent in that...but overall would still expect this additional rain today/tonight to be beneficial in nature given the dry antecedent
conditions and normal to below normal streamflows. Weaker
instability will keep rainfall rates lower here as well...although
pockets over 1" in an hour still appear probable. Some uncertainty
on the coverage and latitude of convective cells over MN/WI also
was a factor playing against a Marginal at this time. A decent
amount of spread is seen in the guidance...with the EC furthest
south and HRRR once of the furthest north solutions. More
confidence over southeast SD...where the 00z-06z time frame
tonight should see an increase in convection. Lower instability
and moisture transport compared to further south should cap the
upper bound on rainfall rates...but still could see localized
amounts approaching 1.5" in an hour. With that said, even the
wetter CAMs generally cap magnitudes below the 1 and 3hr FFG
values. Thus, while some localized urban and low lying ponding of
water seems probable from SD into MN and WI...the risk of true
flash flooding still seems to be below 5 percent (although non
zero). Will continue to monitor through the day and possible a
Marginal risk will eventually be needed for a portion of this
corridor.

Chenard


Day 2
Valid 12Z Wed Apr 07 2021 - 12Z Thu Apr 08 2021

...MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE MID SOUTH
REGION...

The approach of a cold front and a marginally unstable warm sector
ahead of it should support the initiation and progression of
convective lines through the evening and overnight hours. Despite
the progressive nature of the expected lines, the duration of
heavy rain is expected to be maximized over northern Mississippi
and adjacent portions eastern Arkansas and northeast Louisiana;
that is the region in which deep layer mean flow should be closest
to parallel with the initiating cold front (generally SW to NE).
Indeed, that is where forecast QPF is highest. The potential for
quasi-training convective lines and clusters in the presence of
precipitable water values around 1.5 inches (around the 95th
percentile for early April) should support rainfall potentially
reaching 2-3 inches in 3 hours (localized 4" totals possible) in
the strongest convection. This would generally meet or exceed
flash flood guidance across the region, where the top layers of
the soil are still wetter than average due to heavy rain in March.
Therefore, the Marginal Risk has been maintained with only small
adjustments to the risk contour.

Taylor

Day 3
Valid 12Z Thu Apr 08 2021 - 12Z Fri Apr 09 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Taylor


$$
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FOUS30 KWBC 081954
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
352 PM EDT Thu Apr 08 2021

Day 1
Valid 16Z Thu Apr 08 2021 - 12Z Fri Apr 09 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Oravec


Day 2
Valid 12Z Fri Apr 09 2021 - 12Z Sat Apr 10 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE LOWER MISSISSIPPI VALLEY...

12Z update...

Timing and placement of mesoscale features will likely determine
where locally heavy rain will occur. While this has been a
persistent challenge with the last few runs of the model guidance,
there does seem to be growing consensus for an area centering over
along the Mississippi River from Louisiana northward to southeast
Missouri and another area along the Gulf Coast to southern Alabama
and Georgia with areal averages of 1 to 3 inches. A few models
suggest isolated amounts surpassing 4 inches. With the areal
coverage having increased from the last forecast package, the
Marginal and Slight Risk areas were also expanded (particularly
over parts of Tennessee) to reflect this trend.

Campbell

Previous discussion...

Cyclogenesis over north Texas early Friday will set the stage for
a round of thunderstorms downstream into the Lower Mississippi
Valley. The warm front extending to the east (and associated nose
of the low-level jet) should be the focus for most of the
convective activity in the afternoon and evening, with organized
lines and clusters pushing east overnight. There is still some
uncertainty on placement of the heaviest rainfall, which will
likely depend on mesoscale details, so a broad Slight Risk was
maintained (and even somewhat expanded) on this outlook.

Excessive rainfall and flash flooding appears most likely in the
evening or early overnight hours along the instability gradient
associated with the warm front, as low level inflow begins to
increase and veer to a more SW or WSW direction. In combination
with strong instability (CAPE over 2000 j/kg) in the inflow
region, that would be a more favorable configuration for
backbuilding convection with time. And when precipitable water
values around 1.5 inches (approx. 90th percentile for early April)
are considered, rain rates in the 1-2 in/hr range would be
expected in the strongest convective clusters. And that could lead
to flash flooding, particularly if the heavy rain is sustained for
a couple hours or more in a given location.

For now, model consensus favors the warm front and instability
gradient setting up from near the AR-LA border east-southeast into
C MS. However, it would not take much for the warm front to lift
slightly further north, or for a stronger cold pool to eventually
push the most active convection further south than models are
indicating.

Lamers

Day 3
Valid 12Z Sat Apr 10 2021 - 12Z Sun Apr 11 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PRIMARILY
PORTIONS OF SOUTH ALABAMA AND THE FLORIDA PANHANDLE...

21Z update...

Antecedent moisture from day 2 will have likely reduced local FFG
across part of the Gulf states region. Organized convection is
expected to carry on through the day 3 period, over portions of
the same area affected on day 2 and over the eastern Gulf
Coast/Southeast. The scenario still favored with the 12Z/18Z
guidance has a broad area over moderate to heavy rain along the
Florida panhandle, southern Alabama and Georgia. Most of the
solutions are showing 2 to 4 inches over this area with a couple
isolated 4.5 to 6 inches. Rainfall of this magnitude, especially
in a multi-day event, can quickly lead to flooding and urban
ponding/runoff. The Slight risk area was expanded further east
over the Florida panhandle, as well as, the western edge moving
toward Louisiana. As such, the Marginal Risk area was also
adjusted further into north-central Florida.

Campbell

Previous discussion...

Organized convection should be ongoing at the beginning of the
period in the vicinity of AL and MS, and the excessive rainfall
and flash flooding potential should be a continuation from the
prior day. Model signals for the placement of heavy rain are
relatively dispersed, with some showing rainfall maxima out over
the Gulf of Mexico, and others much further inland. For now, an
intermediate scenario was favored similar to the 12Z ECMWF and 00Z
GFS, with the heaviest rainfall over S AL and the FL Panhandle.
Conceptually, this makes sense, with convection more or less
following the instability gradient to the east, and this gradient
is naturally favored to be situated close to the coast.

Provided the dominant convective clusters do not slip south over
the Gulf, there should be sufficient instability (CAPE over 1000
j/kg) and deep moisture (PWATs 1.6 to 1.7 inches) to yield heavy
rain rates of 1-2 in/hr and thus potential for flash flooding.
Ultimately, the structure of any convective clusters will play a
big role in the nature of the heavy rain threat. A more
forward-propagating line of storms (which is possible) would
reduce the duration of heavy rain at any one location and may lead
to a broader footprint of outflow, pushing the most intense
convection increasingly far to the south and east. Backbuilding
storms near a coastal front could lead to a swath of very heavy
rainfall, and backbuilding is also possible given the large
westerly component to the low-level inflow.

Given these mesoscale uncertainties, a relatively broad Slight
Risk was depicted over the Gulf Coast region at this time.

Lamers


$$
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FOUS30 KWBC 100818
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
417 AM EDT Sat Apr 10 2021

Day 1
Valid 12Z Sat Apr 10 2021 - 12Z Sun Apr 11 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE DEEP SOUTH AND NORTHERN GULF COAST...

A closed low will become stacked with the surface low deepening
across the mid-MS Valley, slowly lifting northeast into the
Midwest through the forecast period. A trailing cold front will
advance organized thunderstorms across portions of the Deep
South/northern Gulf Coast region through morning, with evening
convection into the southern/central Appalachians and moderate
rain within a deformation axis across portions of MO/IA/IL.

...Deep South/Northern Gulf Coast Region...
An organized line of convection ahead of a cold front will
continue to propagate southeast across portions of the Deep South
and along the northern Gulf Coast region this morning. This
activity will be moving rather quickly with most of the
precipitation offshore by early afternoon. With the potential for
very heavy rain associated with this line of thunderstorms, brief
training and pockets of wet antecedent conditions, flash flooding
is possible closer to the coast. This main line will advance
quickly east across portions of the Southeast through the
afternoon with very little concern for flash flooding beyond
southern GA.

Precipitable water values will surge to around 1.5 to 1.8 inches
this morning (which is supported by the latest TPW) across the
Deep South/Gulf Coast with ample instability over 1000 J/kg of
MUCAPE. In addition, divergence aloft will provide sufficient
large scale lift to keep this convection ongoing. While the
convection will be progressive, rainfall rates of 1-2 in/hr are
expected along with brief training of heavier embedded cells. This
is also supported by the latest HREF neighborhood probabilities.
In addition, some overlap of these higher rain rates may occur
atop Friday's rainfall, increasing the flash flood threat locally
over portions of the Lower Mississippi Valley into southern
Alabama and the Florida Panhandle. Therefore, the Slight Risk area
was retained and modified based on the latest observations and
model trends.

...Southern/Central Appalachians...
The warm front associated with the deepening surface low in the
central U.S. will advance northward through into the Mid-Atlantic
bringing much of the Southeast within the warm sector. Ahead of
the approaching cold front, moisture will surge with instability
also climbing. The mid-level impulse advancing the convection
across the Deep South/Gulf Coast this morning will lift toward the
Appalachians within the southwest flow aloft helping to sharpenen
the trough with it becoming negatively tilted. Therefore,
anticipate thunderstorms to develop across the western Carolinas
moving north and east ahead of the approaching cold front this
evening.

Precipitable water values will increase to around 1.25-1.5 inches
(which is 2+ standard deviation above the mean) aided by the
nocturnal low level jet. While instability will not be robust,
MUCAPE values will climb close to 1000 J/kg. Given these factors
and the aforementioned strong forcing for ascent, rain rates may
approach 1 inch per hour. While the line of thunderstorms should
advance north and east fairly quickly, there will be brief
training and also orographic enhancement along the southern facing
slopes of the terrain. Areal average precipitation across the
higher terrain will range between 1-2+ inches.

Given Fridays convection (wet antecedent conditions) and lower
flash flood guidance values, there is enough potential for heavy
rain to lead to localized runoff concerns/flash flooding.
Therefore, a Marginal Risk was introduced at this update.

...Midwest...
As the aforementioned surface low deepens through the morning, the
deformation on the northwest flank will provide continual moderate
rain across northeast MO, southeast IA into northwestern IL. As
the upper level closed low and associated trough becomes
negatively tilted, Gulf moisture will surge north wrapping around
the eventual stacked low. The surface low will become wound up,
slow to progress northeast through the forecast period. This will
result in an axis of higher QPF under the deformation zone/pivot
point. So, while the areal average precipitation over the next 24
hours looks quite robust (around 2-4 inches), this will occur
throughout the day/overnight. Rain rates will likely be at the
highest this morning (0.50-0.75 inches/hour) as elevated
instability briefly increases. Otherwise, anticipate this to be
more of a soaking rain with soils likely able to tolerate the
rates. Therefore, a risk area was not introduced with the
expectation that localized areal flooding is possible.

Pagano


Day 2
Valid 12Z Sun Apr 11 2021 - 12Z Mon Apr 12 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE FLORIDA PENINSULA ON SUNDAY...

Introduced a Marginal Risk area for areas of the Florida peninsula
along and ahead of a cold front making its way south during the
period. Thinking is that on-going convection at 12Z Sunday should
be making its ways southward towards the central Florida
peninsula. The area immediately south of the front on Sunday
morning should be most-primed to produce some downpours given an
atmosphere with precipitable water values approaching 1.7 inches,
a broad synoptic-scale confluent flow in the low- and mid-levels,
and the front which helps focus convective activity. This is
where some of the higher resolution CAMs have generated local 2 to
4 inch maximum rainfall amounts mainly during the day on
Sunday...with the NAM-NEST and HRRR generating isolated 1- and
2-inch per hour rates within the broader precipitation area.
Working against the potential for excessive rainfall is the fact
that the convection should be fairly progressive which limits the
amount of total rainfall. In addition, the Florida peninsula area
has generally been dry over the past few weeks (or more)...with
only a few exceptions over the southern peninsula...resulting in
1- and 3-hour Flash Flood Guidance in excess of 4 inches. As a
result, the area looks to be in better position to handle
downpours than if antecedent conditions had been wet. The areas
most prone to excessive rainfall should be in the immediate
vicinity of the front due to the potential rainfall rates, in
urbanized areas due to ponding of water and poor drainage of
runoff, and regions that experience multiple rounds of convection.

Bann


Day 3
Valid 12Z Mon Apr 12 2021 - 12Z Tue Apr 13 2021

The probability of rainfall amounts exceeding flash flood guidance
is less than 5 percent.

Bann

$$
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FOUS30 KWBC 102030
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
429 PM EDT Sat Apr 10 2021

Day 1
Valid 1741Z Sat Apr 10 2021 - 12Z Sun Apr 11 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL THROUGH THE
SOUTHERN TO CENTRAL APPALACHIANS...


1800 UTC update

Given the continued weakening of the convective complex over land
areas of the central to eastern Gulf coast, the marginal risk area
was removed. No changes made to the marginal risk area over the
Southern to Central Appalachians.

Oravec

1600 UTC update

Along the Gulf coast...the slight risk area was removed and the
marginal risk area was suppressed southward to along the immediate
Gulf coast from southern LA into North FL. Satellite imagery
continues to show warming cloud tops as the MCC weakens. Heaviest
rains on the leading progressive front end of the squall line are
moving out of the areas that saw heavy rains over the past 24
hours. This and the progressive nature of this portion of the
complex warrants a lowered threat level.

Across the Southern to Central Appalachians, the southern end of
the previous marginal risk area was extended southwestward by
approximately 80 nm into the far Upstate of South Carolina and
adjacent western North Carolina. The northern portion of the
marginal risk area was extended approximately 60-70 nm northward
into far eastern West Virginia. These changes were to cover the
spread of the 1200 UTC hi res guidance.

Oravec


0900 UTC discussion

A closed low will become stacked with the surface low deepening
across the mid-MS Valley, slowly lifting northeast into the
Midwest through the forecast period. A trailing cold front will
advance organized thunderstorms across portions of the Deep
South/northern Gulf Coast region through morning, with evening
convection into the southern/central Appalachians and moderate
rain within a deformation axis across portions of MO/IA/IL.

...Deep South/Northern Gulf Coast Region...
An organized line of convection ahead of a cold front will
continue to propagate southeast across portions of the Deep South
and along the northern Gulf Coast region this morning. This
activity will be moving rather quickly with most of the
precipitation offshore by early afternoon. With the potential for
very heavy rain associated with this line of thunderstorms, brief
training and pockets of wet antecedent conditions, flash flooding
is possible closer to the coast. This main line will advance
quickly east across portions of the Southeast through the
afternoon with very little concern for flash flooding beyond
southern GA.

Precipitable water values will surge to around 1.5 to 1.8 inches
this morning (which is supported by the latest TPW) across the
Deep South/Gulf Coast with ample instability over 1000 J/kg of
MUCAPE. In addition, divergence aloft will provide sufficient
large scale lift to keep this convection ongoing. While the
convection will be progressive, rainfall rates of 1-2 in/hr are
expected along with brief training of heavier embedded cells. This
is also supported by the latest HREF neighborhood probabilities.
In addition, some overlap of these higher rain rates may occur
atop Friday's rainfall, increasing the flash flood threat locally
over portions of the Lower Mississippi Valley into southern
Alabama and the Florida Panhandle. Therefore, the Slight Risk area
was retained and modified based on the latest observations and
model trends.

...Southern/Central Appalachians...
The warm front associated with the deepening surface low in the
central U.S. will advance northward through into the Mid-Atlantic
bringing much of the Southeast within the warm sector. Ahead of
the approaching cold front, moisture will surge with instability
also climbing. The mid-level impulse advancing the convection
across the Deep South/Gulf Coast this morning will lift toward the
Appalachians within the southwest flow aloft helping to sharpen
the trough with it becoming negatively tilted. Therefore,
anticipate thunderstorms to develop across the western Carolinas
moving north and east ahead of the approaching cold front this
evening.

Precipitable water values will increase to around 1.25-1.5 inches
(which is 2+ standard deviation above the mean) aided by the
nocturnal low level jet. While instability will not be robust,
MUCAPE values will climb close to 1000 J/kg. Given these factors
and the aforementioned strong forcing for ascent, rain rates may
approach 1 inch per hour. While the line of thunderstorms should
advance north and east fairly quickly, there will be brief
training and also orographic enhancement along the southern facing
slopes of the terrain. Areal average precipitation across the
higher terrain will range between 1-2+ inches.

Given Fridays convection (wet antecedent conditions) and lower
flash flood guidance values, there is enough potential for heavy
rain to lead to localized runoff concerns/flash flooding.
Therefore, a Marginal Risk was introduced at this update.

...Midwest...
As the aforementioned surface low deepens through the morning, the
deformation on the northwest flank will provide continual moderate
rain across northeast MO, southeast IA into northwestern IL. As
the upper level closed low and associated trough becomes
negatively tilted, Gulf moisture will surge north wrapping around
the eventual stacked low. The surface low will become wound up,
slow to progress northeast through the forecast period. This will
result in an axis of higher QPF under the deformation zone/pivot
point. So, while the areal average precipitation over the next 24
hours looks quite robust (around 2-4 inches), this will occur
throughout the day/overnight. Rain rates will likely be at the
highest this morning (0.50-0.75 inches/hour) as elevated
instability briefly increases. Otherwise, anticipate this to be
more of a soaking rain with soils likely able to tolerate the
rates. Therefore, a risk area was not introduced with the
expectation that localized areal flooding is possible.

Pagano


Day 2
Valid 12Z Sun Apr 11 2021 - 12Z Mon Apr 12 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE FLORIDA PENINSULA ON SUNDAY...

As a stacked closed low over the Midwest drifts slowly
northeastward from Sunday to Monday morning, a corresponding cold
front will move south and east from the Florida Panhandle into the
central Peninsula by 12Z Monday. As this occurs, a returning surge
of deeper moisture is forecast to be lifting northward from the
southern Gulf of Mexico with PWATs as high as 1.8 inches advecting
into the western Florida Peninsula.

Thunderstorms are expected to be ongoing at the start of the
period, 12Z Sunday, across the eastern Gulf, ahead of the cold
front. Periods of heavy rain will shift eastward and southward
during the day with mean deep-layer westerly supporting repeating
cells and brief training with moisture/instability supporting 2-4
inches of rain in 2-3 hours. Given dry antecedent conditions, any
flash flooding that occurs may remain limited to urban areas. The
Marginal Risk was kept for this update and adjusted slightly north
and south to account for the latest 12Z hi-res QPF after
coordination with affected WFOs.

Otto


Day 3
Valid 12Z Mon Apr 12 2021 - 12Z Tue Apr 13 2021

The probability of rainfall amounts exceeding flash flood guidance
is less than 5 percent.

Otto

$$
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FOUS30 KWBC 110804
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
403 AM EDT Sun Apr 11 2021

Day 1
Valid 12Z Sun Apr 11 2021 - 12Z Mon Apr 12 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE FLORIDA PENINSULA...

A stacked closed low will continue to track northeast toward the
Eastern Great Lakes Region through the forecast period. Mid-level
energy rounding the broad trough axis over the Southeast will move
atop a trailing cold front across the Florida panhandle and
Eastern Gulf of Mexico. Return flow ahead of this cold front and
shortwave will help instability and moisture surge north. Given
enough instability pooling in the Gulf, high bulk shear and plenty
of divergence aloft, the aforementioned shortwave will fuel
convective development through the early morning hours, becoming
organized with an MCS likely in the Eastern Gulf. This activity
will quickly advance south and east through the Florida peninsula
bringing the potential for heavy rain and isolated flash flooding.


Precipitable water values are expected to approach 2 inches aided
by 30+ knot low level southwesterly flow, which is around 2
standard deviations above the mean. Instability across central
Florida will increase through the morning/afternoon to around 4000
J/kg MUCAPE ahead of the main bowing line segment. Given these
ingredients alone, rain rates will be fairly high with hourly
totals exceeding 2 inches in some locations. This is supported by
fairly decent HREF probabilities through the afternoon. While the
MCS will be fairly progressive, there is some concern for
backbuilding along the southwestern flank of the outflow boundary
toward better instability in the Gulf. This could enhance hourly
totals, especially across the west coast of FL. Some of the high
resolution guidance holds convection back with some additional
development into the evening/overnight as the main surface cold
front approaches. So, there is a bit uncertainty with the overall
evolution of the convection. But at this point in the forecast it
is safe to say that linear convection will advance through central
FL during the morning/afternoon hours with some activity lingering
across the west/southern coast through the overnight. Areal
average precipitation will range between 2-4+ inches with a lot of
this falling within a couple of hours.

While much of this region has received below normal precipitation
over the past week and soils are very dry, there is still a small
potential for flash flooding to occur if the higher rain rates
fall atop an urban corridor and/or backbuilding occurs along the
line increasing hourly totals. Therefore, the Marginal Risk was
maintained with little adjustments made at this update.

Pagano



Day 2
Valid 12Z Mon Apr 12 2021 - 12Z Tue Apr 13 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann


Day 3
Valid 12Z Tue Apr 13 2021 - 12Z Wed Apr 14 2021

The probability of rainfall amounts exceeding flash flood guidance
is less than 5 percent.

Bann

$$
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FOUS30 KWBC 112028
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 PM EDT Sun Apr 11 2021

Day 1
Valid 1723Z Sun Apr 11 2021 - 12Z Mon Apr 12 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE FLORIDA PENINSULA...

1800 UTC update

The organized convection over central Florida continues to press
quickly to the southeast. Much of the 1200 UTC CAMS are slow with
this convection, which is a typical bias. The latest hrrr at 1600
UTC does have a better handle on the position. With this
progression, the northern portion of the previous marginal risk
was trimmed southward by approximately 100 nm. Very heavy
rainfall will continue with this squall line with hourly totals of
1-2" possible, with the biggest runoff threat likely in urbanized
areas.

Oravec

0900 UTC discussion

A stacked closed low will continue to track northeast toward the
Eastern Great Lakes Region through the forecast period. Mid-level
energy rounding the broad trough axis over the Southeast will move
atop a trailing cold front across the Florida panhandle and
Eastern Gulf of Mexico. Return flow ahead of this cold front and
shortwave will help instability and moisture surge north. Given
enough instability pooling in the Gulf, high bulk shear and plenty
of divergence aloft, the aforementioned shortwave will fuel
convective development through the early morning hours, becoming
organized with an MCS likely in the Eastern Gulf. This activity
will quickly advance south and east through the Florida peninsula
bringing the potential for heavy rain and isolated flash flooding.


Precipitable water values are expected to approach 2 inches aided
by 30+ knot low level southwesterly flow, which is around 2
standard deviations above the mean. Instability across central
Florida will increase through the morning/afternoon to around 4000
J/kg MUCAPE ahead of the main bowing line segment. Given these
ingredients alone, rain rates will be fairly high with hourly
totals exceeding 2 inches in some locations. This is supported by
fairly decent HREF probabilities through the afternoon. While the
MCS will be fairly progressive, there is some concern for
backbuilding along the southwestern flank of the outflow boundary
toward better instability in the Gulf. This could enhance hourly
totals, especially across the west coast of FL. Some of the high
resolution guidance holds convection back with some additional
development into the evening/overnight as the main surface cold
front approaches. So, there is a bit uncertainty with the overall
evolution of the convection. But at this point in the forecast it
is safe to say that linear convection will advance through central
FL during the morning/afternoon hours with some activity lingering
across the west/southern coast through the overnight. Areal
average precipitation will range between 2-4+ inches with a lot of
this falling within a couple of hours.

While much of this region has received below normal precipitation
over the past week and soils are very dry, there is still a small
potential for flash flooding to occur if the higher rain rates
fall atop an urban corridor and/or backbuilding occurs along the
line increasing hourly totals. Therefore, the Marginal Risk was
maintained with little adjustments made at this update.

Pagano



Day 2
Valid 12Z Mon Apr 12 2021 - 12Z Tue Apr 13 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Otto


Day 3
Valid 12Z Tue Apr 13 2021 - 12Z Wed Apr 14 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL GULF COAST...

A low to mid-level ridge is forecast to be situated over the
southern Gulf of Mexico Tuesday into Wednesday morning as weak
southern stream impulses attempt to infringe upon the northern
edge of the ridge in the vicinity of the central Gulf Coast. In
the 925-850 mb layer, a well defined boundary (oriented northwest
to southeast) will separate 1.5+ inch PWATs to its south and west
from less than 0.75 inch PWATs to its north and east. The low
level boundary is forecast to reach Louisiana near 12Z Tuesday
along with increasing instability and the developing of warm
advection driven convection. Cell movement should generally be
from southwest to northeast, but any organized clusters should
tend to follow along the axis of the low-level boundary,
potentially setting up a situation for training heavy rain as
inflow will be from the south-southwest. The thermodynamic
environment would be supportive of rainfall rates of at least 1-2
in/hr.

After the initial round of expected convection early in the day on
Tuesday, a second round of storms is possible late Tuesday
night/early Wednesday morning as increasing low level flow from
the Gulf meets with advancing convection moving eastward out of
eastern Texas...in advance of a southern stream shortwave.
Multiple rounds of convection could lead to 2-4 inches of rain
(locally higher) over the 24 hour period ending 12Z Wednesday for
portions of southern Louisiana/Mississippi. While there is still a
fair degree of uncertainty with shortwave and QPF placement, most
12Z models show the signal for heavy rain across portions of the
central Gulf Coast. With many areas in that part of the U.S.
having received over 300 percent of normal rainfall over the past
week, additional heavy rain could lead to a few areas of flash
flooding. A Marginal Risk was issued with this update to cover the
threat.

Otto

$$
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FOUS30 KWBC 131600
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1200 PM EDT Tue Apr 13 2021

Day 1
Valid 16Z Tue Apr 13 2021 - 12Z Wed Apr 14 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL GULF COAST...

...Central Gulf Coast...
A shortwave moving out of OK into AR will continue to track east
and continue to help convection grow upscale across portions of LA
and MS. Ahead of this shortwave, low level moisture will increase
with precipitable water values climbing over 1.75 inches aided by
fairly weak 850mb southerly flow; this is around 2 standard
deviations above the mean; the 30-40 knots of effective bulk shear
will be the factor in organizing the convection. With a bit of
diurnal heating and steep lapse rates in place, especially in the
vicinity of a portion of the polar front near the Upper TX and LA
coasts, expect MLCAPE values to potentially exceed 3000 J/kg.
This activity will become linear (the early signs of that are seen
in current observations with the convective cold pool leading to
increasingly north and northwest winds to the north of the polar
front) and sink southeast along the instability gradient. While
the forward propagation should be progressive, the hourly rain
totals will climb over 2 inches which are supported by the HREF
neighborhood probabilities. Local amounts of 5" are most possible
in the vicinity of the Teche Region/sugar country of southern LA,
per the 12z HREF probabilities of such through 12z (nearly 50%
chance).

Given the wet antecedent conditions in place across this region
over the past week (based on much of southeast LA observing 300+
percent of normal precipitation and the region being within the
95th percentile of 0-40cm soil moisture), an additional bout of
heavy rain, especially across urban locations will likely result
in scattered flash flooding and runoff. Therefore, the Slight Risk
area across southeast LA remains in place. While there was a very
brief consideration for a Moderate Risk, the organized convection
should be progressive enough to preclude more significant
widespread flash flooding. However, it should be noted that back
building convection along the outflow boundary across the
southwestern flank across southwestern LA and into eastern LA may
be possible through the afternoon/early evening. Therefore,
extended the Marginal Risk to account for this.


...Southern Plains/Lower MS Valley...
Under weak zonal mid-level flow, another subtle mid-level
shortwave is forecast to approach the Southern Plains tonight,
interacting with deep level moisture and lingering instability
along the stationary boundary draped across the region. There is
a bit more uncertainty with the evolution of this activity and the
overall coverage. However, various high resolution models are
starting to show the signal for convection and thus high rain
rates moving from eastern OK/TX into portions of AR/LA.

Precipitable water values across this region will climb to around
1.5 inches aided by weak low level southerly flow. The
instability gradient will build north through the
afternoon/evening with 2500 J/kg MUCAPE still lingering into the
overnight hours. With enough mid-level forcing for ascent,
convection could blossom with rain totals approaching 1.5
inches/hour. With multiple rounds of heavy rain in any given
location, this could result in around 1-2+ inches of rain within a
6 hour period. Though conditions have been dry, felt enough was
there to loop eastern TX and portions of the Red River of the
South into the pre-existing Marginal Risk area.

Roth/Pagano




Day 2
Valid 12Z Wed Apr 14 2021 - 12Z Thu Apr 15 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL GULF COAST...

Maintained the Marginal Risk area with a westward extension that
was introduced on Monday afternoon due to 00Z guidance continuing
to show trends started at 11/12Z. Those trends included some
heavier rainfall amounts late in the Day 1 period that approach
south-central and southeast Louisiana Abundant moisture will be
in place at the beginning of the period with 00Z GFS/ECMWF runs
showing PWATs exceeding 2.5 standard deviations above climatology
mainly over parts of Louisiana on Wednesday. There is a signal in
the guidance that another area of convection could develop near
the Central Gulf Coast very late in the period. Given the overlap
of heavy rainfall at the end of Day 1 and the beginning of Day 2,
some 4 inch amounts in southeast Louisiana have prompted the
inclusion of a Slight Risk area. The question remains with regard
how far east to bring the Slight Risk area. While WPC QPF does
taper off over far southern Mississippi and Alabama, several of
the operational models and their respective ensembles did show
some potential for isolated amounts in excess of 3 inches. Given
rainfall over the past week, the amount of rain forecast on Day 1
and some potential for isolated heavier amounts persisting into
the first half of Wednesday...carried the Slight Risk area towards
Mobile Bay.

Bann


Day 3
Valid 12Z Thu Apr 15 2021 - 12Z Fri Apr 16 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG THE
IMMEDIATE CENTRAL GULF COAST PRIOR TO THE PASSAGE OF A COLD
FRONT...

Locally heavy rainfall will be possible along the immediate coast
of the Central Gulf states early Thursday prior to the passage of
a cold front. There should still be abundant moisture in
place...between 1.5 and 2.5 standardized anomalies greater than climatology...that could result in briefly heavy rainfall rates
over an area that has had above normal rainfall. But most of the
guidance tended to show generally modest rainfall amounts,
suggesting that any excessive rainfall will be localized in
nature. In addition, it is possible that the front may have
already passed through the area prior to the start of the
forecast. Given the uncertainty, will not issue an area greater
than a Marginal risk and re-evaluate in upcoming outlooks.

Bann

$$
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FOUS30 KWBC 141558
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1157 AM EDT Wed Apr 14 2021

Day 1
Valid 12Z Wed Apr 14 2021 - 12Z Thu Apr 15 2021

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
LOUISIANA AND SOUTHWEST MISSISSIPPI...

...Lower MS Valley/North-Central Gulf Coast...
Mid-level shortwaves continue to ride along the fairly zonal flow
atop a surface boundary draped near the Gulf Coast. The latest
shortwave triggered convection which has moved into north-central
LA and southern MS/southeast LA, mostly elevated over a boundary
which is now partially offshore the LA coast. Uncertainty on the
evolution of this precipitation activity continues even within the
near term, with the 12z guidance and recent radar trends not
providing much clarity. Precipitable water values have risen to
1.6-1.8" ahead of a shortwave moving into AR, northwest LA, and
East TX; this is over 2 standard deviations above the mean. As a
shortwave is nearby, elevated convection has continued near and
within a MU CAPE pool of 1000-3000 J/kg remaining across LA and
southwest MS. While the convection should be fairly progressive
as it moves east, the boundary should continue to sink slowly to
the south. East-west training of convection is still possible
near and downwind of the MU CAPE pool until the shortwave passes
into MS. This could lead to hourly rain totals to 2". As a
result, areal average precipitation of 3-5" remain in the cards
across central/southern LA into portions of southern MS. While it
is quite possible that the MDT risk area has received much of the
rain it will get, enough pieces of guidance indicate heavy rain
potential as late as 21z, and along with the upstream convection
in northern LA, the Moderate Risk area has been maintained.
Southwest LA/the Golden Triangle of Southeast Texas has the
greatest potential at the short term as the surface boundary is in
their vicinity and the area is near the edge of the ML CAPE
gradient. Some of the convection in this area has been surface
based and has produced pockets of 3-4" of rain per radar
estimates. The greatest potential for 5"+ overall through 12z is
between Lake Charles and Sabine Pass per the 12z HREF
probabilities of 5"+, with probabilities approaching 30%.

There is another shortwave in the queue that will approach the
Southern Plains tonight triggering another round of convection
across eastern TX/OK. This activity will becoming better organized
along the aforementioned front over central LA into MS. Given
recent rainfall yesterday and today and what is expected this
afternoon, this region could see another round of heavy rain and
flash flooding by late tonight/early Thursday. This led to
significant continuity in the risk areas depicted.


...Southern Mid-Atlantic...
A deepening mid/upper level low will gradually meander east over
the Great Lakes region with ridging along the east coast shifting
offshore. Impulses riding the base of the trough axis will
traverse the region and interact with a lee trough and approaching
front. As a result, a weak surface low will develop by late
afternoon. In response, precipitable water values will climb to
around 1.25 inches (which is over 1.5 standard deviations above
the mean) aided by strengthening low level southwesterly flow.
With some diurnal heating this afternoon, this should help sharpen
the lee trough and strengthen low-level wind circulations to
create additional surface convergence. A lot of uncertainty
surround the amount of instability with MUCAPE values struggling
to get to 1000 J/kg. Regardless, there should be enough moisture
and boundary layer lift to promote convection that could drop 1-2
inches of rain per hour. This activity should be fairly
progressive, but backbuilding/training is possible along the front
as propagation vectors align with the mean wind. Thus, areal
average precipitation could exceed 2-3+ inches in some locations
within a few hours. While much of this region has observed around
normal precipitation as of late, there are some locations that
have seen above 200% of normal with recent convection. A Marginal
Risk was maintained with the potential for FFG being exceeded or
heavy rain occurring over an urban corridor. Most of the
precipitation activity should move offshore by late Wednesday
night allowing the potential for flash flooding to diminish after
that point.

Roth/Pagano



Day 2
Valid 12Z Thu Apr 15 2021 - 12Z Fri Apr 16 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER A SMALL
AREA ALONG THE CENTRAL GULF COAST...

Highlighted a small area, mainly confined to parts of Louisiana,
where there appears to be a marginal risk of excessive rainfall on
Thursday. In the broad scale picture, deepest
moisture/instability and the potential for heaviest rainfall rates
should be moving out to sea early on Thursday and there should be
a relative lull in activity. However, the models keep some
rainfall lingering over a region that already had more than 300
percent of normal rainfall in the previous week before localized 2
or 3 inch rainfall amounts fell on Tuesday...with the potential
for more heavy rain in the short-term again today. This area would
be particularly sensitive to flooding from even modest rainfall
amounts on Day 2. As a result, placed a Marginal Risk area that
was roughly co-located with the placement of the Moderate Risk
area on Day 1 despite rainfall amounts that would generally not
prompt a Marginal Risk area. Once there is less uncertainty on how
quickly rain shuts down following the passage of a cold front, it
is possible that the area can be removed in subsequent outlooks.

Bann


Day 3
Valid 12Z Fri Apr 16 2021 - 12Z Sat Apr 17 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
CENTRAL GULF COAST REGION FROM FRIDAY INTO EARLY SATURDAY
MORNING...

...Louisiana to Southern Mississippi/Alabama...
After a relative lull in convection capable of producing heavy
rainfall on Thursday, the return of a boundary from the Gulf of
Mexico combined with the approach of more shortwave energy from
the west should result in increasing risk of excessive rainfall on
Friday across an area already susceptible to flooding.

Low level flow should begin to back and strengthen across eastern
Texas and western/southern Louisiana on Friday in response to
shortwave energy approaching from the north and west. As it does
so, the low level flow will be riding up and over a
quasi-stationary boundary across the region. This should foster a
growing precipitation shield in the broad isentropic lift/warm
advection pattern...with convection becoming increasingly able to
produce heavier rainfall rates Friday night and into the pre-dawn
hours on Saturday as precipitable water values increase to between
1.6 and 1.8 inches. The orientation of the boundary and the
southwesterly low-level flow suggest that cell training is
possible.

The numerical guidance is in generally good agreement with this
scenario. One area of concern was with the 14/00Z GFS Friday
evening over southern Louisiana. The very large magnitude of
vertical velocities over a large part of southwest Louisiana is a
signature of convectively-induced grid-scale feedback. The real
concern is not so much that the GFS formed convection where it did
but how far downstream the low level thermal and wind fields
impact the model QPF. Thus, was more inclined to follow a non-GFS
solution for placement of the Slight Risk area at this time.

Bann

$$
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FOUS30 KWBC 161946
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
346 PM EDT Fri Apr 16 2021

Day 1
Valid 1914Z Fri Apr 16 2021 - 12Z Sat Apr 17 2021

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PARTS OF
THE CENTRAL GULF COAST REGION...


...North Texas-ArkLaTex into Louisiana to Southern
Mississippi/Alabama and the western Florida Panhandle...
The southern stream of the Westerlies will remain active, with the
northern stream troughs over the central Rockies-central Plains
and the other over the Great Lakes-Northeast-mid Atlantic
maintaining strong upper level confluence and coast-to-coast
deep-layer zonal flow between 30-35N over the CONUS. Meanwhile,
the southern stream continues to have tropical and sub-tropical
origins well southwest of Baja. The quasi-stationary, elongated
110-130 kt jet streak ~35N will maintain broad-scale frontogenesis
across the outlook areas into Friday night with sufficient
elevated instability (500-1000 j/kg) along the Gulf Coast and into
the Moderate and Slight risk areas. One round of convection
occurred earlier, which has since ejected into North FL. Some
intermediate convection is active across southeast Louisiana at
this time, while another batch is moving from East TX into central
LA. The greatest risk area for heavy, potentially excessive
rainfall will be from portions east-central and southeast
Louisiana, southern Mississippi, and southern Alabama. Within a
broad area of deep-layer lift and bolstered low-mid layer
frontogenesis with the passage of an upper-level shortwave trough
to the north, low-level flow will return northward steadily during
the period and overrun the surface boundary in place. Warm/moist
air will surge northward with the latest guidance suggesting
precipitable water values exceeding 1.7 inches will be common
while MUCAPE values approaching 1000 J/kg will nose onshore.

Heavy rain is still anticipated from portions of east-central LA
into southeast LA, southern MS, and into portions of southern AL.
Much of this area has seen well above normal precipitation in the
last 2 weeks. The departures from normal are between 300-600
percent and soil saturation is maxed out between 95-100 percent
for the top 200 cm. Many rivers are in minor to moderate flood
stage already and trending higher. The latest flash flood guidance
as a result is low. Three-hour FFGs are in the 2-2.5 inch range
(or less), while the 6-hour values are in the 2.5-3 inch range
(locally less). The expected rainfall should easily meet or exceed
these thresholds. As a result, between the expected heavy rainfall
and very anomalously wet soil conditions, the Moderate Risk was
maintained. Other than some trimming to the northwest side per
recent radar reflectivity trends, few changes were made in this
update.

Roth/Oravec/Hurley/Taylor


Day 2
Valid 12Z Sat Apr 17 2021 - 12Z Sun Apr 18 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL GULF COAST...

The approach of a weak mid/upper level shortwave from Texas and a
jet streak over the Tennessee Valley will provide the necessary
forcing for ascent for another round rainfall across portions of
the central Gulf Coast Saturday. At the surface, the effective
boundary is likely to displaced south/offshore with limited
surface based instability. However, with the approach of the
shortwave during peak heating, marginal MUCAPE is forecast to
develop (upwards of 500 J/kg) over portions of southern LA,
southern MS, and southern AL. This is within an area where
anomalously high precipitable water values and sufficient onshore
flow should remain in place (1.5 to 1.7 inches; around 2 std
deviations above normal).

While the model guidance has come together with the idea of
another overrunning precip shield for the area, there remains some
considerable differences in the potential amounts. The 12Z GFS was
the most aggressive with areal averages of near 2-3 inches. Lesser
amounts were seen in the other deterministic guidance but the
trend was higher compared to the previous cycle. The lack of
surface-based instability will limit hourly totals to 1" or less
(at most) but over the course of the day, another 1-2", locally 3"
will be possible. This is expected to fall over the most saturated
soils and ground conditions with little recovery from a very wet
week. As a result, a Marginal Risk was introduced for the
potential of localized flash flooding.

Taylor

Day 3
Valid 12Z Sun Apr 18 2021 - 12Z Mon Apr 19 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Taylor

$$
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FOUS30 KWBC 170757
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
357 AM EDT Sat Apr 17 2021

Day 1
Valid 12Z Sat Apr 17 2021 - 12Z Sun Apr 18 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL-EASTERN GULF COAST AND SOUTHEAST...

The approach of a weak mid/upper level shortwave from Texas and a
jet streak over the Tennessee Valley will provide the necessary
forcing for ascent for another round rainfall across portions of
the central-eastern Gulf Coast and Southeast Saturday-Saturday
night. At the surface, low pressure will track along a
quasi-stationary front just offshore the Gulf coast Saturday, then
across northern FL/southern GA Saturday night. Instability north
of these features will remain rather marginal (upwards of 500
j/kg) over portions of southern LA, southern MS, and southern AL.
This is within an area where anomalously high precipitable water
values and sufficient onshore flow should remain in place (1.6 to
1.6 inches; around 2 standard deviations above normal). Farther
east across North FL, the low track will lift the warm front
through much of the area, allowing this area to tap more robust
instability within the warm sector (mixed layer CAPEs 1000-2000
j/kg) ahead of flat upper shortwave and surface cold front.

The expansive Marginal Risk area accounts for the potential of
isolated or localized flash flooding, based on the varying
thermodynamic and antecedent environments across the outlook area.
Farther west (across southern LA/MS/AL), the lack of surface-based
instability will limit hourly totals to 1" or less over most
areas. However, these are areas where soils remain quite saturated
per the latest NASA SPoRT analysis (0-100 cm soil moisture
percentile 98+ percent over eastern LA/southern MS/far southern
AL), with little recovery from a very wet week. So it wouldn't
take as much rainfall to cause potential runoff issues. Farther
east (FL Panhandle/North FL/southern GA), stronger deep-layer
instability should allow for heavier short-term rainfall rates, as
the high-res guidance (including the 00Z HRRR) indicates spotty
clusters with 1.5-2.0"/hr rates later this afternoon and evening.
Soils across these areas aren't nearly as saturated, nevertheless
much of North FL has observed 200-400+ percent of normal rainfall
over the past week.

Hurley/Taylor


Day 2
Valid 12Z Sun Apr 18 2021 - 12Z Mon Apr 19 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE EASTERN GULF COAST AND NORTHERN FLORIDA...

The overrunning precipitation from the day 1 period along the
central Gulf and points north will have likely brought soils to
near saturation across parts of the Gulf Coast. Additional
scattered to widespread thunderstorms are expected to traverse
northern Florida and into southern Georgia as low-level winds
transport ~1.5 inches of PW (about +2 st. dev) along the surface
front. MUCAPE within this region will be adequate to sustain
development and produce moderate, possibly heavy rainfall. The
latest guidance suggests the highest QPF will be near the
Gainesville/Ocala area, with areal average of 1 to 2+ inches with
local maxes nearing 3 inches possible. This is expected to fall
over the most saturated soils and ground conditions with little
recovery from a very wet week. As a result, a Marginal Risk was
introduced for the potential of localized flash flooding.

Campbell


$$
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FOUS30 KWBC 171945
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
344 PM EDT Sat Apr 17 2021

Day 1
Valid 16Z Sat Apr 17 2021 - 12Z Sun Apr 18 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL-EASTERN GULF COAST AND SOUTHEAST...

The approach of a weak mid/upper level shortwave from Texas and a
jet streak over the Tennessee Valley will provide the necessary
forcing for ascent for another round rainfall across portions of
the central-eastern Gulf Coast and Southeast Saturday-Saturday
night. At the surface, low pressure will track along a
quasi-stationary front just offshore the Gulf coast Saturday, then
across northern FL/southern GA Saturday night. Instability north
of these features will remain rather marginal (upwards of 500
j/kg) over portions of southern LA, southern MS, and southern AL.
This is within an area where anomalously high precipitable water
values and sufficient onshore flow should remain in place (1.6 to
1.6 inches; around 2 standard deviations above normal). Farther
east across North FL, the low track will lift the warm front
through much of the area, allowing this area to tap more robust
instability within the warm sector (mixed layer CAPEs 1000-2000
j/kg) ahead of flat upper shortwave and surface cold front.

The expansive Marginal Risk area accounts for the potential of
isolated or localized flash flooding, based on the varying
thermodynamic and antecedent environments across the outlook area.
Farther west (across southern LA/MS/AL), the lack of surface-based
instability will limit hourly totals to 1" or less over most
areas. However, these are areas where soils remain quite saturated
per the latest NASA SPoRT analysis (0-100 cm soil moisture
percentile 98+ percent over eastern LA/southern MS/far southern
AL), with little recovery from a very wet week. So it wouldn't
take as much rainfall to cause potential runoff issues. Farther
east (FL Panhandle/North FL/southern GA), stronger deep-layer
instability should allow for heavier short-term rainfall rates, as
the high-res guidance (including the 00Z HRRR) indicates spotty
clusters with 1.5-2.0"/hr rates later this afternoon and evening.
Soils across these areas aren't nearly as saturated, nevertheless
much of North FL has observed 200-400+ percent of normal rainfall
over the past week.

Hurley/Taylor


Day 2
Valid 12Z Sun Apr 18 2021 - 12Z Mon Apr 19 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NORTHERN FLORIDA...

Favorable mid/upper level jet dynamics will support broad large
scale forcing for ascent across portions of northern Florida
peninsula. Gulf moisture lifting northward into the low level
boundary will be characterized by precipitable water values 1.5+
inches (between 1.5-2.0 standard deviations above normal) with
anywhere between 25-35 kts of inflow. With sufficient instability
expected to develop, a couple rounds of moderate to heavy rainfall
will be possible. 2-3 inches will be possible. This rainfall is
likely to fall on areas that have seen much above normal
precipitation over the last 2 weeks. The 14-day anomalies are
between 300-400 percent for portions of northern Florida
peninsula. Minimal changes were made to the existing Marginal
Risk, which looks to be in good position with the latest model
guidance.

Taylor

Day 3
Valid 12Z Mon Apr 19 2021 - 12Z Tue Apr 20 2021

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF CENTRAL FLORIDA...

The slow-moving surface front over Florida will continue to
provide focus for thunderstorm development for the day 3 period.
This multi-day event will have lowered FFG substantially further
increasing the risk for urban ponding and flooding conditions.
Strong moisture convergence near the coast and trailing front will
likely result in enhanced rainfall amounts along the coastline of
Waccasassa Bay and points inland. Some models are depicting QPF up
to 2 inches for this area, with a broad 1 to 1.5 inches coast to
coast. The 3-day forecast footprint for portions of Florida will
be 3 to 6 inches. A Marginal risk for excessive rainfall was
hoisted for this period for a large portion of central Florida.
Some consideration to a Slight Risk upgrade was made but the
heaviest rainfall is likely to fall just beyond the current Day 3
period.

Campbell/Taylor


$$
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FOUS30 KWBC 180806
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
405 AM EDT Sun Apr 18 2021

Day 1
Valid 12Z Sun Apr 18 2021 - 12Z Mon Apr 19 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM MUCH OF
NORTH FLORIDA INTO PORTIONS OF CENTRAL FLORIDA...

Favorable mid/upper level jet dynamics will support broad large
scale forcing for ascent across portions of northern Florida
peninsula. Gulf moisture lifting northward into the low level
boundary will be characterized by precipitable water values ~1.75
inches with anywhere between 25-35 kts of low-level inflow.
Normally the spring dry season, the moisture parameters are rather
anomalous for the latter half of April -- including PW and 850 mb
moisture flux (around 2 and 2-3 standard deviations above normal
respectively). With sufficient instability expected to develop
(MUCAPEs 1000-2000 j/kg), a couple rounds of moderate to heavy
rainfall will be possible. 2-4+ inches are likely in spots per the
high-res CAMs, including the 06Z HRRR. This rainfall is likely to
fall on areas that have seen much above normal precipitation over
the last 2 weeks, as the 14-day anomalies are between 300-400
percent over many locations in North and Central Florida. Given
the mesoanalysis and guidance trends, a slight southward
adjustment was made to yesterday's Day 2 Marginal Risk area.

Hurley/Taylor


Day 2
Valid 12Z Mon Apr 19 2021 - 12Z Tue Apr 20 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NORTHERN FLORIDA...

Favorable mid/upper level jet dynamics will support broad large
scale forcing for ascent across portions of northern and central
Florida. Meanwhile, Gulf moisture will be advecting into the
region, at times nearly parallel to the surface frontal boundary,
with 25-35 kts inflow. PW across much of the region will be 1.5+
inches (between 1.5-2.0 standard deviations above normal). With
sufficient instability expected to develop, a couple rounds of
moderate to heavy rainfall yielding 2 to 3 inches will be
possible. This rainfall is likely to fall on areas that have seen
much above normal precipitation over the last 2 weeks. The 14-day
anomalies are between 300-400 percent for portions of northern
Florida peninsula. The southern bound of the Marginal was expanded
south. Portions of northern/central Florida will likely have
isolated to scattered flash flooding impacts during this period.
The local forecast offices felt a Marginal Risk was sufficient for
this period, noting that a few locations had d1 drought conditions
prior to this multi-day heavy rain event. A Slight Risk may be
needed in the next forecast cycle and will continue to be
monitored.

Campbell/Taylor

Day 3
Valid 12Z Tue Apr 20 2021 - 12Z Wed Apr 21 2021

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF CENTRAL FLORIDA...

Additional rounds of thunderstorms are expected to fire along the
slow-moving surface front over Florida will continue to provide
focus for thunderstorm development for the day 3 period. By this
point, several locations across this region will likely have
already observed 2 to 5 inches with another 1 to 2+ inches
expected during this period. Large portions of central and
northern Florida have been 300 to 600% above normal for the past 2
weeks and local FFG will have greatly reduced by the day 3 period.
The sensitivity of the soils and additional rain may quickly
result in river rises, urban ponding and other flooding related
impacts. A Marginal Risk for excessive rainfall was hoisted for
this period and will likely require an upgrade to a Slight Risk as
it comes close to the near-term period. In coordination with the
local forecast offices across central Florida, the consensus was
to wait to see the amounts received in the day 1 period and to
reassess the need for a Slight Risk in further forecast cycles.

Campbell


$$
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FOUS30 KWBC 181945
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
345 PM EDT Sun Apr 18 2021

Day 1
Valid 16Z Sun Apr 18 2021 - 12Z Mon Apr 19 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM MUCH OF
NORTH FLORIDA INTO PORTIONS OF CENTRAL FLORIDA...

1600 UTC update

While there are some typical run to run variability with qpf
details in the new 1200 UTC hi res runs, their output fits into
the previous marginal risk area across north central Florida. No
changes made to the previous issuance.

Oravec

0900 UTC discussion

Favorable mid/upper level jet dynamics will support broad large
scale forcing for ascent across portions of northern Florida
peninsula. Gulf moisture lifting northward into the low level
boundary will be characterized by precipitable water values ~1.75
inches with anywhere between 25-35 kts of low-level inflow.
Normally the spring dry season, the moisture parameters are rather
anomalous for the latter half of April -- including PW and 850 mb
moisture flux (around 2 and 2-3 standard deviations above normal
respectively). With sufficient instability expected to develop
(MUCAPEs 1000-2000 j/kg), a couple rounds of moderate to heavy
rainfall will be possible. 2-4+ inches are likely in spots per the
high-res CAMs, including the 06Z HRRR. This rainfall is likely to
fall on areas that have seen much above normal precipitation over
the last 2 weeks, as the 14-day anomalies are between 300-400
percent over many locations in North and Central Florida. Given
the mesoanalysis and guidance trends, a slight southward
adjustment was made to yesterday's Day 2 Marginal Risk area.

Hurley/Taylor


Day 2
Valid 12Z Mon Apr 19 2021 - 12Z Tue Apr 20 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL FLORIDA PENINSULA...

A stalled cold front draped across the FL Peninsula will serve as
a focus for rounds of showers and thunderstorms with heavy
rainfall Monday and Monday night. As this front crawls slowly
southeast across the state, ascent through low-level convergence
will be aided by persistent upper level diffluence as a jet streak
remains anchored to the north, and weak PVA/height falls as
periodic shortwaves traverse the flow from west to east. Broad
cyclonic flow across much of the eastern CONUS will maintain
unidirectional westerly flow across the Gulf of Mexico and into
FL, advecting onshore PWs in excess of 1.75", above the 90th
percentile for the date according to the SPC sounding climatology.
Persistent warm and moist advection through D2 will also push warm
cloud depths towards 4000m as MUCape above 1000 J/kg as the
sounding becomes deeply saturated. While individual storm motions
may reach 30 kts to the northeast using 0-6km mean wind as proxy,
training of echoes is likely on the unidirectional, and boundary
parallel, flow. Additionally, regeneration of cells over the Gulf
of Mexico is likely where instability maximizes, and this will
enhance the rainfall potential across the Peninsula.

Recent rainfall across parts of the Peninsula has been more than
300% of normal the last 7-day, and parts of the area have 0-40cm
soil moisture now exceeding the 98th percentile. However, there
continues to be uncertainty into exactly where the heaviest rain
may align, and how often rain rates, which could exceed 1"/hr
according to the HREF probabilities, will occur. For these reasons
the MRGL risk was left unchanged from the previous forecast, with
just a subtle placement adjustment to account for model trends.

Weiss

Day 3
Valid 12Z Tue Apr 20 2021 - 12Z Wed Apr 21 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE
CENTRAL FLORIDA PENINSULA...

Rinse and repeat seems to be the theme for D3 as a slow moving
front, upper jet level diffluence, and weak shortwave impulses
embedded within the mid-level flow combine to produce heavy
rainfall across Florida. Although the front will try to progress
southeastward D3, there is some indication that increasing
instability Tuesday will stall the front and may even cause it to
waver back to the north, and the MRGL risk has been adjusted to
account for this potential. PWs still surging from the west at
1.75", or above the 90th percentile for mid-April, will maintain a
very moist column with 4000m of warm cloud depth and MUCape to
1000 J/kg or higher. With persistent westerly flow parallel to the
front, training of showers and thunderstorms with rain rates in
excess of 1"/hr are again possible across some of the same areas
that will receive heavy rainfall on Monday.

The main difference Tuesday which may enhance rainfall is a subtle
tightening of the mid-level heights as the main longwave trough
amplifies to the northwest in response to a neutral tilting
shortwave over the Great Lakes. This will drive some enhanced
mid-level flow and stronger moist advection, in conjunction with
more robust RRQ diffluence as the tail of the jet streak shifts
eastward. Guidance indicates a mid-level omega maximum developing
Tuesday, and both ECENS/GEFS probabilities and WSE plumes suggest
heavy rainfall across much of central FL adding up to more than 3
inches in some areas. Considered raising a SLGT for a narrow
corridor of the central Florida Peninsula, but after coordination
with TBW and MLB, opted to hold on to the MRGL at least one more
cycle due to uncertainty in rainfall expected on Monday (D2) which
will impact the soil moisture and flash flood potential on Tuesday.

Weiss

$$
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FOUS30 KWBC 191933
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
333 PM EDT Mon Apr 19 2021

Day 1
Valid 16Z Mon Apr 19 2021 - 12Z Tue Apr 20 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL FLORIDA PENINSULA...

1600 UTC update

Only some minor changes made to the previous Excessive Rainfall
Outlook. The marginal risk area was extended about 40 nm farther
south to cover the qpf spread from the new 1200 UTC hi res
guidance.

Oravec

0900 UTC discussion

A stalled cold front draped across the FL Peninsula will serve as
a focus for rounds of showers and thunderstorms with heavy
rainfall Monday and Monday night. As this front crawls slowly
southeast across the state, ascent through low-level convergence
will be aided by persistent, albeit weak upper level diffluence as
a jet streak remains anchored to the north, with periodic and weak
DPVA/height falls as periodic shortwaves traverse the flow from
west to east. Broad cyclonic flow across much of the eastern CONUS
will maintain unidirectional westerly flow across the Gulf of
Mexico and into FL, advecting onshore PWs in excess of 1.75",
above the 90th percentile for the date according to the SPC
sounding climatology. Persistent warm and moist advection through
the period will also push warm cloud depths towards 4000m as
MUCape above 1000 J/kg as the sounding becomes deeply saturated.
While individual storm motions may reach 30 kts to the northeast
using 0-6km mean wind as proxy, some cell training is likely on
the unidirectional, and boundary parallel, flow. Additionally,
regeneration of cells over the Gulf of Mexico is likely where
instability maximizes, and this will enhance the rainfall
potential across the Peninsula.

Recent rainfall across parts of North and Central FL has been more
than 300% of normal the last 7-day, while portions of North FL
have 0-40cm soil moisture now exceeding the 98th percentile.
However, the growing model consensus is that the bulk of QPF
Monday-Monday night will align farther south of where the heaviest
rain had fallen over the past 24-30 hours -- which makes since
considering the mesoanalysis trends (surface frontal location and
pooling of greatest instability).
For this reason the ERO was again capped at a Marginal Risk,
denoting the potential of at most localized short-term runoff
issues from isolated stronger cells capable of producing 2-3+
inches within a couple of hours.

Hurley/Weiss


Day 2
Valid 12Z Tue Apr 20 2021 - 12Z Wed Apr 21 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE CENTRAL
FLORIDA PENINSULA...

Forcing for ascent provided by upper level jet dynamics and weak
shortwave trough will work with a slow moving front sagging
southward across the Florida Peninsula. Precipitable water values
approaching 1.7-1.8" combined with instability 2000-2500 J/kg to
produce widespread/numerous thunderstorms capable of producing
heavy rainfall. The mean flow oriented along the storm motions
will favor repeating/backbuilding storms. The latest HREF
probabilities suggest 2"/hr totals will be possible during the
afternoon hours across the central Florida peninsula.

An additional 1-3" is expected with some localized 4-5" totals
likely. There will be some overlap with what has fallen in the
last 2 days and therefore some of the soils will be increasingly
saturated. Soil moisture is starting to increase in the top 40 cm
layer and recent anomalies are 200-400 percent of normal. The 12z
HREF probabilities of exceeding the 6-hour flash flood guidance
rises to 30 percent or so during the afternoon hours. Overall, not
much change to the Slight Risk area, just some minor adjustments
based on the latest model guidance trends.

Taylor

Day 3
Valid 12Z Wed Apr 21 2021 - 12Z Thu Apr 22 2021

The probability of excessive rainfall exceeding flash flood
guidance is less than 5 percent.

Taylor

$$
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FOUS30 KWBC 202007
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
406 PM EDT Tue Apr 20 2021

Day 1
Valid 16Z Tue Apr 20 2021 - 12Z Wed Apr 21 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
CENTRAL FLORIDA...

16z Update: Will maintain the Slight risk across portions of
central FL. We did trim the west coast of FL out of the Slight, as
seems like the best focus for backbuilding convection will be
across central FL to the east coast of FL. Erosion of instability
may be a limiting factor as the day progresses...but the HRRR does
show a continued max of 850mb moisture transport into the evening
hours over central FL. This would support some continued
convective development on the western flank of ongoing
activity...supporting some training potential into the afternoon
if instability is able to persist. Tend to think flash flooding
will remain rather localized, and mainly confined to any more
susceptible urban areas...but the threat of 2-3" in an hour will
be there given the PWs and moisture transport into the region.

Chenard

...Previous Discussion...

Between 1.5 and 3+ inches of rain fell yesterday across much of
the Slight Risk area. Tuesday and Tuesday night, as the surface
front lifts slowly northward while the upper jet remains north of
the outlook area, an uptick in frontogenesis and moisture
transport will occur in the low-mid levels. 850-700 mb moisture
flux anomalies in fact remain high for the latter half of April --
with an area of +3 standard deviations per the GEFS and SREF
lifting across Central and northern portions of South FL Tuesday.
Deep >0C cloud layers (WBZ levels climbing to near 13kft) with PW
values averaging 2.5 standard deviations above normal will favor
highly efficient warm rain processes, which along with the subtle
boost in synoptic scale forcing, could offset the relatively lack
of robust instability (MUCAPEs averaging 1000-1500 j/kg) in
producing heavy to potentially excessive short term rainfall
rates. Moreover, the uptick in southwest to west-southwest
low-level flow (25-30 kts at 850 mb), aligning nearly parallel to
the mean 850-300 mb flow and surface front orientation, will favor repeating/back-building storms, which would also help to boost 1-3
hourly rainfall rates. An additional 1-3" is expected with some
localized 4-5" totals likely. There will be some overlap with what
has fallen in the last 2 days and therefore some of the soils will
be increasingly saturated. Soil moisture is starting to increase
in the top 40 cm layer and recent anomalies are 200-400 percent of
normal. Overall, not much change to the Slight Risk area, just
some minor adjustments based on the latest model guidance trends.

Hurley/Taylor


Day 2
Valid 12Z Wed Apr 21 2021 - 12Z Thu Apr 22 2021

The probability of excessive rainfall exceeding flash flood
guidance is less than 5 percent.

Bann

Day 3
Valid 12Z Thu Apr 22 2021 - 12Z Fri Apr 23 2021

The probability of excessive rainfall exceeding flash flood
guidance is less than 5 percent.

Bann


$$
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FOUS30 KWBC 212004
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
403 PM EDT Wed Apr 21 2021

Day 1
Valid 16Z Wed Apr 21 2021 - 12Z Thu Apr 22 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Chenard


Day 2
Valid 12Z Thu Apr 22 2021 - 12Z Fri Apr 23 2021

The probability of excessive rainfall exceeding flash flood
guidance is less than 5 percent.

Pereira


$$
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FOUS30 KWBC 221922
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
321 PM EDT Thu Apr 22 2021

Day 1
Valid 16Z Thu Apr 22 2021 - 12Z Fri Apr 23 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Chenard


Day 2
Valid 12Z Fri Apr 23 2021 - 12Z Sat Apr 24 2021

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS THE LOWER
MISSISSIPPI VALLEY AND THE GULF STATES...

There continues to be a strong signal for a heavy rainfall event
developing over eastern Texas and Oklahoma that spreads east
across the Lower Mississippi Valley and the central Gulf Coast
states. PW values build to 1.5"+ (nearing 2 standard deviations
above normal for late April) along the Gulf Coast while continuing
to transport northward into the central U.S.
A low pressure system exiting the Rockies will track across the
southern tier states, lifting a warm front up the Mississippi
Valley while a cold front sweeps in a few hours later. The upper
level jet will be passing overhead and provide divergence
aloft/enhanced vertical motion in the low/mid levels of the
atmosphere. Low-level inflow rises to 40-60 knots (with effective
bulk shear to match), close to the value of the mean 850-400 hPa
wind. Instability will be increasing across the Southern Plains
ahead of the cold front with CAPE values of 1500+ J/Kg supporting
periods of heavy rainfall.

There is some degree of spread on where the maximum values will
occur, but the overall consensus suggests a fairly large area
where 2-4 inches will fall with very isolated maximums of 5-6
inches possible. Although some locations have had several days to
recover from recent soaking rains, rain of this magnitude may
quickly become problematic, thus increasing the threat for flash
flooding. The Slight Risk area was slightly reshaped for this
update, but remains about the same. The Marginal Risk area spans
from eastern Texas to Alabama and from central Oklahoma into the
Midwest. The LCH/Lake Charles LA, SHV/Shreveport LA, and
LIX/Slidell LA forecast offices were consulted in this update.

Roth/Campbell

$$
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FOUS30 KWBC 231959
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
357 PM EDT Fri Apr 23 2021

Day 1
Valid 1910Z Fri Apr 23 2021 - 12Z Sat Apr 24 2021

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
CENTRAL LOUISIANA AND SOUTHWEST MISSISSIPPI...

19z Update: Same overall thinking as in the 16z discussion below.
However confidence in the axis has increased enough over the past
few hours to go ahead with the MDT risk upgrade over portions of
LA and southwest MS. The last five HRRR runs all depict localized
rainfall of 6-8" within the MDT risk, and the environmental setup
does seem to support this potential. Still some latitudinal
spread...so can not completely rule out the highest rainfall axis
being just south or north of the MDT risk...but the most likely
location is a narrow axis with the MDT from central LA into
southwest MS. More numerous flash flooding, some of which could be
locally significant, is possible in this region.

Chenard

16z Update: Based on the latest guidance we did shift the Slight
risk a bit south over LA/MS. Guidance is coming into increasing
agreement on an axis of 3-6" of rain later this afternoon into
tonight somewhere across LA into MS. While the system as a whole
is progressive, we note a rather broad region of strong 925-850mb
moisture convergence over the region. This would support an
elongated west to east axis of convection...thus resulting in some
training as everything pushes eastward with time. FFG is initially
quite high over the region, which should initially limit the flash
flood risk. However continued rain should help lower FFG, and the
better instability may actually advect into the area on the back
end of the convective complex...which could result in higher rates
on top of saturated soil conditions. Also of note, is the latest
0-100 cm NASA SPoRT soil moisture percentiles are still over the
80th percentile across this region...potentially suggesting a
quicker than usual lowering of FFG as heavy rain moves in. Long
story short, this setup does seem to support the potential for a
more organized flash flood risk over portions of LS/MS.

Given that, we closely considered a MDT risk upgrade. In the end
we opted to hold off and maintain the Slight. Main reason for this
was continued latitudinal spread in the axis of heaviest rain.
This axis will likely be pretty narrow, and highly dependent on
how quickly convection forms and organizes over east TX and
western LA. The slower the organization the more northward any
training will set up. The quicker we see development/organization
then the further south the axis will be...as organized activity
will more quickly deviate from the deep layer mean flow and train
west to east along the convergence axis. Overall, we did not want
to go with a broad MDT, so the best option was to maintain the
Slight, and handle any uptick in expected impacts through our MPDs
as the event evolves later today.

Chenard

...Previous Discussion...

There continues to be a strong model signal for an axis of heavy
to locally excessive rainfall from northeast Texas, eastward into
portions of the Lower Mississippi Valley and the Central Gulf
Coastal states ahead of strong southern stream height falls moving
east from the Southern Plains into the Lower Mississippi Valley
late Friday into early Saturday. The low level flow is expected
to strengthen today off the western Gulf, advecting anomalous PW
values, 1.5 to 2+ standard deviations above the mean, into the
strengthening west to east oriented frontal boundary stretching
across this area. Aloft, a very favorable coupled jet will
accentuate lift in the vicinity of this front from late Friday
into the overnight hours of early Saturday morning. Convection
likely to enhance quickly this afternoon over northeast Texas,
spreading downstream in the vicinity of the west to east oriented
front. Training of cells along and just north of the front is
likely late afternoon into Friday night/early Saturday with
increasing downstream progression possible toward the end of the
day 1 time period. The western end of the previous slight risk
area was expanded to the northwest to cover the area where the
latest href probabilities are high for 2"+ and 3"+ totals this
period. At the hourly time scale, probabilities are greatest for
1"+ hourly totals beginning in the 1800-2100 UTC time period over
northeast Texas and pushing downstream after 0000 utc Saturday
into the Lower Mississippi Valley region. The 2"+ hourly
probabilities max out over northeast Texas and northwest Louisiana
in the 2000 UTC Fri to 0400 UTC Sat time frame, decreasing after
that for areas farther downstream given the expected increasing
downstream movement. While the 2"+/hr probabilities decrease with
time, the potential for training of cells will still pose the risk
of very heavy hourly totals. There continues to be some
differences with the latitude of the max axis, with the day 1 qpf
favoring the slightly farther south axis of the arw, arw2 and
fv3cam. Areal average 1-3" amounts depicted on the latest day 1
qpf with potential for areas of 4-6" totals where training
maximizes.

Oravec


Day 2
Valid 12Z Sat Apr 24 2021 - 12Z Sun Apr 25 2021

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL IN AND NEAR SOUTH
GEORGIA...

By 24/12z, a potent shortwave will have developed a convective
complex across the Lower Mississippi Valley and progressed into
the Eastern Gulf States. A very favorable upper level jet pattern
and split in the Polar Jet and Subtropical jet will continue to
support very strong low level response and surface cyclogenesis
with the MCS. Deep moist, unstable air (1500-3000 J/kg ML/MU
CAPEs) will continue to be available from the West Central Gulf
angling to the Northeast as the wave/favorable upper-level
dynamics rapidly progress into the Upper TN valley and Southern
Appalachians by midday. As as result, the low level moisture
plume while still strong and supportive of 1.75-2" total PWats
will be more obliquely oriented to any outflow boundaries/linear
convective structures. The moisture plume wavers in the vicinity
of South GA for 12-18 hours, depending which model you believe.
Strong 850 hPa inflow from the instability source should support
backbuilding (propagation vectors less than 5 knots), keep
instability fields from progressing too far to the northeast, and
potentially aid in longer duration/training of convection. Given
very high moisture flux convergence, hourly rain rates in excess
of 2-2.5"/hr are likely with much higher sub-hourly rates/totals.
This may result in local 3-5" rainfall totals in the vicinity of
South GA...reducing slightly eastward as it moves further from the richer/deeper Gulf moisture. This area has been very dry over the
last few months with soil saturation values well below normal in
the range of 10-20%, which suggests high FFG values in the area
are fairly accurate. Given the hourly rates and potential for
training, cell mergers from storms in organized and pulse
convective modes, and mesocyclone formation, flash flooding
appears possible. The preferred guidance QPF-wise was on the
southern side of the guidance spread, due to a lack of mid-level
capping as 700 hPa temperatures are modest (near or below 6C).

There remains the potential for interaction with frictional
convergence and convective development near/along the Gulf Coast,
which would reduce instability closer to the best dynamic forcing.
The 12z ECMWF is supportive of convection right along the FL
Panhandle/Big Bend sections of the Gulf coast. Slightly reduced
convective influx (due to proximity to best forcing aloft) should
reduce convective rainfall efficiency slightly, but the contrary
to further north, grounds remain saturated from recent storms over
the last month, with AHPS 2-week anomalies over 300% of normal.
While the recent dry trend has helped, NASA SPoRT 40cm soil
saturation values remain at or above 85%, suggesting local totals
of 3-5" may pose renewed flash flooding concerns.

Roth/Gallina

Day 3
Valid 12Z Sun Apr 25 2021 - 12Z Mon Apr 26 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Roth/Gallina

$$
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FOUS30 KWBC 240720
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
319 AM EDT Sat Apr 24 2021

Day 1
Valid 12Z Sat Apr 24 2021 - 12Z Sun Apr 25 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHEAST...

The strong southern stream height falls supporting the organized
convection early Saturday morning across the Lower Mississippi
Valley and Central Gulf Coast will continue to push eastward
Saturday into the southeastern quarter of the nation. The
simulated radars from the 0000 UTC hi res guidance suggests an
increasingly progressive squall line will form with these height
falls early this morning and push out of Alabama into Georgia at
the beginning of the period. While this squall line will likely
produce heavy rains with hourly totals of 1"+, the greatest
concerns for potential flash flooding will be on the trailing
southwest portion of the squall line that will not be as
progressive and has the potential for training of cells in a more
west to east direction. The low level flow is expected to attain
an increasing westerly component after 1200 UTC from southern
Alabama into the Florida Panhandle/far north Florida, southern
Georgia into far southern South Carolina. These areas will likely
see the best potential for west to east training of cells in an
axis of pw values 2+ standard deviations above the mean. HREF
probabilities are 60-90% for 2"+ totals ,50-80% for 3"+ totals and
40-70% for 5"+ totals across this area where training may occur.
Hourly precipitation totals of 2"+ and event totals of 5-7"
possible in areas of training.

Oravec



Day 2
Valid 12Z Sun Apr 25 2021 - 12Z Mon Apr 26 2021

The probability of rainfall exceeding flash flood guidance is less
than 10 percent.

Petersen



Day 3
Valid 12Z Mon Apr 26 2021 - 12Z Tue Apr 27 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Petersen


$$
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FOUS30 KWBC 242346
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
745 PM EDT Sat Apr 24 2021

Day 1
Valid 01Z Sun Apr 25 2021 - 12Z Sun Apr 25 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHEAST...

The strong southern stream height falls continue to push eastward
through the Southeast. Recent radar trends have shown training of
cells in a more west to east direction with occasional activity
feeding it from the southwest. From the southeast AL eastward
across south GA/adjacent northernmost FL will see the best
potential for continued west to east training of cells in an axis
of pw values 2+ standard deviations above the mean over the next
several hours, with convection farther north across northeast AL,
northern GA, and eastern TN either fading or becoming more
progressive with time. Hourly precipitation totals of 2"+ and a
broadening area of event totals of 4-8" appears likely within
areas of ongoing and expected training.

Roth/Chenard/Oravec


Day 2
Valid 12Z Sun Apr 25 2021 - 12Z Mon Apr 26 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Taylor



Day 3
Valid 12Z Mon Apr 26 2021 - 12Z Tue Apr 27 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Taylor


$$
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FOUS30 KWBC 250756
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
356 AM EDT Sun Apr 25 2021

Day 1
Valid 12Z Sun Apr 25 2021 - 12Z Mon Apr 26 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent

Oravec


Day 2
Valid 12Z Mon Apr 26 2021 - 12Z Tue Apr 27 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Weiss



Day 3
Valid 12Z Tue Apr 27 2021 - 12Z Wed Apr 28 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PARTS OF
TEXAS AND OKLAHOMA...

A positively tilted but amplifying full-latitude trough will move
out of the western CONUS Tuesday, with its axis becoming draped
from the Central Plains to the Four Corners by Wednesday morning.
Downstream of this trough, a subtropical jet streak will
intensify, advecting Pacific moisture into the region while also
producing prolonged upper ascent within its diffluent quadrants.
Beneath this trough, a surface cold front will advect eastward,
but its forward progression is likely to be impeded by increasing
southerly flow, which combined with the amplified mid-level
pattern will lead to a very slow moving boundary.

Along this front, moisture advection will intensify as 850mb flow
approaching 40 kts lifts out of the Gulf of Mexico, pushing PWs
towards 1.75", above the 90th percentile for the date and nearing
+2 standard deviations above the climo mean. Deep layer ascent
through surface convergence and upper diffluence will provide the
impetus for rounds of showers and thunderstorms, especially the
latter half of D3 as MUCape rises towards 1000 J/kg, fostering
rain rates which will likely exceed 1"/hr at times.

Guidance still features quite a bit of longitudinal spread in the
axis of heaviest rainfall, but the trend this morning has been for
a slower and further west solution. This is reflected by an
increase in the WSE mean QPF for individual stations near the Red
River Valley along the TX/OK border, but also a shift in the ECENS
and NBM probabilities for 24-hr rainfall which now indicate a low
chance for 3 inches. Much of the region contained within the MRGL
risk has been dry recently, noted by 7-day rainfall that is
primarily less than 50% of normal with scattered exceptions,
leading to 0-40cm soil moisture that is near the median for late
April according to NASA sport measurements. Despite this, after
coordination with the affected WFOs, the MRGL risk was raised due
to the likelihood of training echoes from storm motions parallel
to the front which could cause localized flash flooding Tuesday
evening/night.

Weiss

$$
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FOUS30 KWBC 252250
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
650 PM EDT Sun Apr 25 2021

Day 1
Valid 01Z Mon Apr 26 2021 - 12Z Mon Apr 26 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent

Roth/Chenard


Day 2
Valid 12Z Mon Apr 26 2021 - 12Z Tue Apr 27 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Taylor



Day 3
Valid 12Z Tue Apr 27 2021 - 12Z Wed Apr 28 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PARTS OF
TEXAS AND OKLAHOMA...

A positively tilted but amplifying full-latitude trough will move
out of the western CONUS Tuesday, with its axis becoming draped
from the Central Plains to the Four Corners by Wednesday morning.
Downstream of this trough, a subtropical jet streak will
intensify, advecting Pacific moisture into the region while also
producing prolonged upper ascent within its diffluent quadrants.
Beneath this trough, a surface cold front will advect eastward,
but its forward progression is likely to be impeded by increasing
southerly flow, which combined with the amplified mid-level
pattern will lead to a very slow moving boundary.

Along this front, moisture advection will intensify as 850mb flow
approaching 40 kts lifts out of the Gulf of Mexico, pushing PWs
towards 1.75", above the 90th percentile for the date and nearing
+2 standard deviations above the climo mean. Deep layer ascent
through surface convergence and upper diffluence will provide the
impetus for rounds of showers and thunderstorms, especially the
latter half of D3 as MUCAPE rises towards 1000 J/kg, fostering
rain rates which will likely exceed 1"/hr at times. The mean flow
orientation suggests some potential for training or repeating
rounds of thunderstorms.

While the 12Z guidance still has some west/east spread in the axis
of the heaviest rainfall - the trend this cycle was for some
clustering toward a slower and slightly west solution. The 12Z GFS
was a fast/progressive solution which resulted in its heaviest
rainfall further east than many of the other solutions. The
consensus among the 12Z CMC/UKMET/ECMWF was fairly good and
generally was the basis for the WPC QPF blend. This led to
slightly higher amounts forecast across portions of OK/KS.

Much of the region contained within the Marginal Risk has been dry
recently, noted by 7-day rainfall that is primarily less than 50%
of normal with scattered exceptions, leading to 0-40cm soil
moisture that is near the median for late April according to NASA
sport measurements. With higher predictability and confidence in
the swath of heaviest rainfall, a Slight Risk may be warranted
across far north Texas and across central to northeast Oklahoma.

Weiss/Taylor

$$
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FOUS30 KWBC 262023
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
422 PM EDT Mon Apr 26 2021

Day 1
Valid 16Z Mon Apr 26 2021 - 12Z Tue Apr 27 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent

Chenard


Day 2
Valid 12Z Tue Apr 27 2021 - 12Z Wed Apr 28 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PARTS OF
TEXAS AND OKLAHOMA...

...2030Z Excessive Rainfall Discussion...
No upgrade to a Slight Risk occurred in this outlook package. The
12Z suite of global models and their associated ensembles
maintained a similar placement as their 26/00Z counterparts and
were similar in rainfall amounts. As mentioned previously, the
area from northeast Texas into eastern Oklahoma has had much in
the way of organized precipitation in the previous 14 or 30 days
which would be the only place of concern for excessive rainfall
and the focus remains generally to the west of there with only
limited overlap.

Bann


...0830Z Excessive Rainfall Discussion...

A positively tilted but amplifying full-latitude trough will move
out of the western CONUS Tuesday, with its axis becoming draped
from the Central Plains to the Four Corners by Wednesday morning.
Downstream of this trough, a subtropical jet streak will
intensify, advecting Pacific moisture into the region along what
will likely be a very slow moving surface boundary.

Along this front, moisture advection will intensify as 850mb flow
approaching 40 kts lifts out of the Gulf of Mexico, pushing PWs
towards 1.75", which is nearing +2 standard deviations above the
climo mean. Several rounds of showers and thunderstorms,
especially the latter half of day 2 are likely as MUCAPE rises
towards 1000 J/kg, with rain rates likely to exceed 1"/hr at
times. Mean flow orientation suggests some potential for training
or repeating rounds of thunderstorms.

The 00z guidance continues to exhibit some west/east spread in the
axis of heaviest rainfall, with the GFS still the fastest outlier,
resulting in its heaviest rainfall farther east than the better
consensus. The rest of the guidance favors an axis west of here,
from roughly north central TX into central OK, and a non-GFS blend
generally formed the basis for the WPC QPF.

Much of the region contained within the Marginal Risk has been dry
recently, noted by 7-day rainfall that is primarily less than 50%
of normal with scattered exceptions. As the axis of heaviest
rainfall comes into better focus, a Slight Risk may be warranted
across far north Texas and across central to northeast Oklahoma.
However, at this time, opted to keep just a marginal risk.

Santorelli



Day 3
Valid 12Z Wed Apr 28 2021 - 12Z Thu Apr 29 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PARTS OF THE
SOUTHERN PLAINS TO THE WESTERN OHIO VALLEY...

...2030Z Excessive Rainfall Discussion...
Maintained the Slight Risk area from the Southern Plains into the
Ohio Valley...with only minor modifications of the northern
periphery of the Slight Risk area in the western Ohio Valley. The
12Z suite of models still depicts an amplified upper trough
makings its way slowly eastward...with broad south to southwest
flow drawing deep moisture northward between the trough axis and a
building ridge axis over the eastern U.S.. While the large-scale
picture is similar between the computer models, there is a camp
(including the GFS) which allows the eastern U.S. ridge axis to
break-down enough to allow for a more eastern/southern QPF
solution while a second camp (including the NAM...and to a lesser
extent the ECMWF) maintain the strength of the ridge which keeps
the axis of heaviest rainfall farther west. Given that the model
QPF remains in the 2 to 4 inch range with isolated maximum
rainfall amounts in excess of 5 inches, did not see any clear
signal to make significant changes to the Slight Risk area nor
upgrade the risk area. The northern periphery of the Slight Risk
area was trimmed a bit in areas that have had little rainfall in
the previous couple of weeks...suggesting that those areas were in
a better position to handle the QPF.

Bann


...0830Z Excessive Rainfall Discussion...

An amplified upper level trough will continue to move only slowly
eastward during the day 3 period as a strong subtropical jet
intensifies and advects significant moisture from central/eastern
Texas into the Middle Mississippi Valley and Ohio Valley. PW
values ranging from 1.5-2" are likely, which is as much as +2 (or
more) standard deviations above normal. Numerous showers and
thunderstorms from the day 2 period will expand along and ahead of
the very slow moving or stationary frontal boundary and with mean
flow oriented parallel to the front, there is potential for
training of thunderstorms and flash flooding.

The 00z suite of guidance does show some east/west spread across
parts of the southern Plains, with more north/south oriented
spread to the north from the Mid-Mississippi Valley into the Ohio
Valley. Just like on day 2, the 00z GFS is the farthest east/south
with its axis of heaviest rainfall, with the majority of the
guidance favoring a position a little west/north of the GFS. As
such, the GFS was excluded from the WPC blend. WPC generally
favored a middle ground approach at this point, consistent best in
line with the CMC and the in house ensemble bias corrected model.
Yesterday's 12z ECMWF seems too far north across the Ohio Valley
with a secondary QPF axis, however the 00z run did shift just
slightly south. The resulting WPC forecast shows a broad region of
2-4" QPF from north central Texas into southwest Missouri.

It has been the case for a few days now that there is fairly high
confidence in a heavy to possibly significant rainfall event
across this region but plenty of uncertainty in the details of
exactly where the heaviest rain may fall. It is quite likely that
an upgrade to a moderate risk will be needed in a future issuance
of the ERO, but given the current spread in the guidance, there is
not enough confidence to place one now. At this point, the best
approach was for rather broad marginal and slight risk areas
stretching from parts of central/eastern Texas, through the Middle
Mississippi Valley, and into parts of the Ohio Valley.

Santorelli

$$
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FOUS30 KWBC 041626
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1214 PM EDT Tue May 04 2021

Day 1
Valid 16Z Tue May 04 2021 - 12Z Wed May 05 2021

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM EAST
CENTRAL MISSISSIPPI, ACROSS CENTRAL ALABAMA INTO NORTHWEST
GEORGIA...

1600 UTC update

The primary change to the previous Day 1 Excessive Rainfall
Outlook was to extend the moderate risk area farther to the
northeast across central Alabama into northwest Georgia.
Convection continues to enhance along and to the southeast of the
outflow boundary currently stretching across northern Alabama into
northern Mississippi. Concerns are for training of convection
over the next several hours along this outflow as the low level
flow become increasingly parallel to this boundary. This
convection may move over areas of northern Georgia into east
central Alabama that received heavy rains over the past 24 hours
and have lower ffg values. in areas of training hourly rainfall
totals of 1-3" possible. See WPC's mesoscale precipitation
discussion #0150 valid until approximately 2030 UTC for additional
information across this area.

A small slight risk area was also added across portions of south
central to southwestern Kentucky after collaboration with WFO LMK.
This area has received very heavy rainfall amounts over the past
24 hours. There is potential for additional scattered convection
this afternoon into this evening which may fall across areas that
have received the heavy precipitation, resulting in potential
runoff issues.

Elsewhere, the slight risk area was trimmed to the southeast
across western to central Tennessee by approximately 60 to 150
miles based on current radar trends.

Oravec


0900 UTC discussion

...Deep South into the Tennessee Valley...
The thermodynamic setup ahead of the approaching upper shortwave
trough remains favorable for heavy rain according to the latest
guidance, including low level moisture flux standardized anomalies
(3+ sigmas above the mean) and IVT values (at over above 1000
kg/(m*s)). Area of low pressure at the surface traversing the
northern TN and southern OH Valleys will likely result in multiple
heavy rainfall axes -- one farther north across much of TN into
southern KY associated with the frontal wave (deep-layer synoptic
support), where the instability will be sufficient (ML CAPE
1000-2000 J/kg) to support 1-1.5+ inch/hr rainfall rates. Lower
FFG values farther north also enhance the potential for flash
flooding, as depicted with the latest HREF exceedance
probabilities.

Farther south (Deep South/just inland of the central Gulf coast),
2000-4000 J/kg of ML CAPE partially induced by an eroding
mid-level capping inversion along with precipitable water values
~2" and convergent 850 hPa flow of 35-45 knots will allow for
hourly totals to 2.5" within the stronger, more organized
convection. Strong southwesterly mean flow should keep activity
fairly progressive until the front lays a bit more toward the
coast Tuesday night which will orient it more toward the mean flow
and could allow some repeating cells, which is shown most vividly
on the 00z FV3CAM. In terms of pinpointing an area of greatest
risk, while spread continues across MS and AL, the best guess as
to the area of higher impact currently is across southern MS and
southern AL in the middle of the expected instability pool, so per
coordination with the JAN/Jackson MS, BMX/Birmingham AL, and
MOB/Mobile AL, a Moderate Risk was added. More than a few pieces
of guidance (though usually high biased) indicate local amounts of 5-7"...believe local 3-6" amounts are most plausible considering
the frontal progression for much of the period. Higher potential
is theoretically possible closer to the Gulf Coast, depending upon
how mesoscale forcing/outflow boundary movement evolves Tuesday
night into Wednesday morning. If convective trends or the
guidance follows suit and trends southward later today, a
southward shift or expansion of the MDT risk is possible.


...Upper Ohio Valley...
The broad poleward flow of Gulf moisture brings 1.5 inch PWs
across the Midwest/Ohio Valley into PA and southeast NY which is 2
to 2.5 standard deviations above normal. Some instability,
occasionally over 1000 J/kg, will be enhanced by right entrance
jet forcing to allow localized heavy rain over what has been near
normal rainfall over the past two weeks. Therefore, a Marginal
Risk is maintained from KY through western PA into far western NY,
and expanded eastward to encompass heavy rain signals seen in the
guidance across northeast PA and in the vicinity of New York City.


Roth


Day 2
Valid 12Z Wed May 05 2021 - 12Z Thu May 06 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM SOUTHEAST
LOUISIANA TO SOUTHERN GEORGIA...

The cold front becomes more west to east oriented over the central
Gulf Coast Wednesday, allowing low-level inflow to be come nearly
parallel to the front and mean 850-500 mb flow. This brings an
enhanced potential for cell training within a favorable
thermodynamic environment (PWs 1.75-2.00 inches...mixed-layer
CAPES 1500-2500+ j/kg). With greater confidence of rainfall along
the central Gulf Coast from NOLA east along the FL/GA line, shrank
the Marginal Risk south a bit from central GA and removed from SC
which have been dry over the past week.

Jackson

Day 3
Valid 12Z Thu May 06 2021 - 12Z Fri May 07 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Jackson

$$
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FOUS30 KWBC 051543
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1142 AM EDT Wed May 05 2021

Day 1
Valid 12Z Wed May 05 2021 - 12Z Thu May 06 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG PORTIONS
OF THE CENTRAL GULF COAST...

1600 UTC update

Latest radar trends are showing the heaviest rainfall rates moving
offshore of eastern Louisiana, far southern Mississippi and far
southern Alabama. The slight risk was removed given these trends
and the marginal risk was suppressed southward by about 30-50
miles. The eastern end of the marginal risk area was trimmed back
to the west, taking it out of far southern Georgia and adjacent
north Florida as per 1200 UTC hi res guidance.

Oravec

0900 UTC discussion


...Central Gulf Coast...
A pre-frontal outflow boundary is laying near the coast currently
as the main upper low has ejected out, though a trailing
positively-tilted upper level shortwave is seen as significant
darkening on water vapor imagery across southeast TX and northwest
LA which may take some hours until it bypasses the region. There
have been attempts to shift the boundary offshore the MS and AL
coasts overnight, but the southerly movement in southeast LA
halted and recently reversed, raising concerns that the new
guidance may be correct in keeping/backing up the heavy rain
threat back to portions of southeast LA, southernmost MS,
southernmost AL and western FL panhandle coasts through the
morning into the early afternoon hours before the cold front
finally sweeps through the area. This is also when the guidance
shows a slight uptick in 850 hPa inflow, near or slightly
exceeding the mean 850-400 hPa wind in the area. ML CAPE of
1000-2500 J/kg remain to the south of the boundary (which should
continue to be the case), precipitable water values of 1.75-2"
remain along it, and effectively bulk shear should remain
sufficient for some organization. The concern would be increasing
saturation due to the ongoing and expected rainfall, so a Slight
Risk was hoisted in coordination with MOB/the Mobile AL forecast
office earlier and recently expanded west to New Orleans per more
recent coordination with LIX/the Slidell LA forecast office.

...Maryland/Virginia...
Showers and thunderstorms are expected to occur in two batches
today...one in the morning across northern MD and one in the late
morning/early afternoon from roughly DC south and eastward with a
frontal passage. Rainfall remains generally low over the past
couple weeks for the area, but the last couple days have seen two
distinct streaks of heavy rain near I-70 and I-66. Since the two
batches of rain/convection today do not appear to overlap, no
threat area is depicted, but if another concentrated corridor of
heavy rain intersects those of the past couple days or the urban
area, isolated issues can't be ruled out. The threat for
excessive rainfall is non-zero, but not quite up to to 5%
threshold.

Roth


Day 2
Valid 12Z Thu May 06 2021 - 12Z Fri May 07 2021

The remnant cold front stalls over the northern FL Peninsula
Thursday in deep layer westerly flow. Moisture advection off the
Gulf continues with PWs rising to around 1.75 inches which is two
standard deviations above the norm. With the surface front
parallel to the deep layer flow, instability present, and some
right entrance region upper level divergence from a jet streak
over GA there is potential for repeating cells with 1 to 2"/hr
rain rates on the northern Peninsula. This area has been dry
lately, so flash flood guidance is up around its normal 4"/hr in
this area. Therefore the excessive rain is low, but nonzero.
Uncertainty with placement of the front (the ECMWF remains farther
south than most other guidance) made for no Marginal Risk to be
drawn, but should frontal position confidence increase a need may
arise.

Jackson

Day 3
Valid 12Z Fri May 07 2021 - 12Z Sat May 08 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Jackson

$$
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FOUS30 KWBC 071530
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1130 AM EDT Fri May 07 2021

Day 1
Valid 16Z Fri May 07 2021 - 12Z Sat May 08 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Oravec


Day 2
Valid 12Z Sat May 08 2021 - 12Z Sun May 09 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL GREAT PLAINS AND ADJACENT MISSOURI VALLEY...

Heavy to potentially excessive rainfall is possible late Saturday
into early Sunday across parts of the Central Plains as a
deepening area of low pressures moves eastward. The 06/00Z NCEP
guidance has been consistently suggesting that rainfall rates will
be increasing during the evening in a region of low- to mid-level frontogenesis...especially once southerly winds tap enough
moisture to help increase instability (on the order of 1000 to
1500 J per kg)...and once the upper level flow becomes
increasingly difluent in response to a h25 mb low begins to take
of a neutral to modestly negative tilt over the Rockies. As a
result, storms should be increasingly capable of producing
downpours across parts of the Plains. While the anomalies
increase to between 1.5 and 2 standard deviations above
climatology...the actual precipitable water values remain in the
range of 1.25 to 1.5 inches and maximum equivalent potential
temperatures peak a little more than 330K. That should help limit
maximum rainfall rates...as will the fact that the NCEP guidance
and ECMWF have trended towards a more progressive solution
(although the GFS may be a bit too progressive). Finally, much of
the area has had below average rainfall over the past two weeks to
a month. As a result of collaboration/coordination with offices
in the area, the Slight Risk was downgraded to a Marginal Risk
area.

Bann


Day 3
Valid 12Z Sun May 09 2021 - 12Z Mon May 10 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
LOWER MISSISSIPPI REGION...

...Lower Mississippi Valley...

A cold front attached with the system moving into the Great Lakes
region on Day 2 should be pushing southward into the Lower
Mississippi Valley. That front will provide the focus for some
convection. Precipitable water values in excess of 1.5 inches will
be in place (which is in the 90th percentile for this time of
year) should be in place by the time the front arrives in parts of
Louisiana and Mississippi on Sunday afternoon. ARW-core ensemble
members are most aggressive in generating 2+ inches of rainfall in
24 hours while only two or three NMM members met that
threshold...with amounts apparently held in check by modest
instability. However, the area has experienced several rounds of
excessive rainfall in the previous few weeks which has made the
area more susceptible to excessive problems. Considering that the
best precipitable water values and mesoscale forcing only
partially overlaps the axis of greatest rainfall in the previous
30 days...felt no more than a Slight Risk was warranted at this
point. Later shifts will be evaluating and shifts in placement or
changes in the amount of rainfall forecast for further adjustments
to the outlook.

...Ohio Valley...
As the surface low pressure continues eastward from the Great
Lakes on Sunday, it will be spreading areas of moderate to locally
heavy rainfall into the Ohio Valley. Moisture influx will not be
as strong as it was farther west on Day 2 and the low level
isentropic left will also be weakening with time. As a result,
there amount of rainfall should not be as much as areas farther to
the west. While there was some heavy rainfall in parts of the
Ohio Valley during the past week or so, the areal coverage of the
heaviest rainfall was widely-scattered...meaning that much of the
area can still handle the amounts of rainfall expected without too
many problems. The main concern would be if there is enough
instability to result in sufficiently high rainfall rates or in
areas where terrain can be a factor.

Bann

$$
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FOUS30 KWBC 091550
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1149 AM EDT Sun May 09 2021

Day 1
Valid 16Z Sun May 09 2021 - 12Z Mon May 10 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE LOWER MISSISSIPPI VALLEY...

1600 UTC Update...

Made slight modifications to the Day 1 ERO, based on the convective/mesoanalysis trends as well as the 12Z high-res
guidance (including the HREF exceedance probabilities). Extended
the Slight Risk a little farther south across more of east TX and
central LA, based on the latest ARW/ARW2 progs (including parallel
runs), while also encompassing the enhanced 2+ inch/hr
probabilities per the 12Z HREF.

Hurley

The Slight risk of excessive rainfall for later today into tonight
stretches from northeast TX into central and northern LA, far
southern AR and central MS. Convection will break out this
afternoon along/ahead of a southeastward moving cold front. Plenty
of instability will be in place to support deep/strong
convection...and PWs are forecast to increase over the
climatological 90th percentile. Thus likely to have some efficient
rainfall producers from cells that develop. The stronger forcing
and core of 850mb moisture transport will generally be focused
northeast of this region. However, often on the tail of these
features we do end up with an enhanced corridor of low level
confluence along with a slowing of the front. This stronger
sfc-925mb convergence combined with a 300mb jet streak overhead
should be enough for organized convective development...with the
slowing off the front allowing for some training/backbuilding. The
general consensus within the Slight risk area is for a swath of
2-3" areal averaged rainfall...with localized totals of 4-6". FFG
across this region is pretty high...although soil saturation is
running above average over the area. This suggests that we may see
flooding impacts develop pretty quickly once FFG is
approached/exceeded. Thus scattered flash flooding is anticipated
over the Slight risk area. This Slight risk was shifted a bit
southeast from our previous issuance...taking into account 00z
global and HREFv3 model trends.

Convection is expected to eventually forward propagate or
redevelop towards the Gulf Coast overnight. Thus while some
training of cells over the Slight risk is expected...the duration
of this training will be somewhat minimized by this eventual
forward propagation. This will eventually bring the heavy rain
potential further south into southern MS and southern LA. There is
a bit more uncertainty with regards to the intensity/organization
of activity by this time...but at least an isolated flash flood
threat may evolve...warranting a southward expansion of the
Marginal risk into these areas.

Convection over northern MS, AL, TN and southern KY will also pose
a heavy rainfall threat today. Activity here should generally stay progressive...and the ingredients for flash flooding here are not
as good as what is forecast further southwest over the
aforementioned Slight risk area. Nonetheless, there has been an
uptick in forecast QPF over these areas, and isolated pockets of
flash flooding are certainly possible. Probably not enough
coverage for a Slight risk, but enough of a threat to warrant a
northward and eastward expansion of the Marginal risk to encompass
this entire area.

We did opt to remove the Marginal risk over IN, OH and PA. While
1-2" of rain is forecast here...this will be primarily stratiform
in nature...with rates not expected to get high enough for a flash
flood threat. Localized areas of ponding water within urban and/or
low lying areas are possible...but not enough to warrant a
Marginal risk.

Chenard


Day 2
Valid 12Z Mon May 10 2021 - 12Z Tue May 11 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS THE
LOWER MISSISSIPPI VALLEY...

...Northeast Texas to Central or Southern Mississippi/Alabama...
Made relatively few changes to the Excessive Rainfall Outlook.
Convection may be on-going at the start of the period on Monday
morning as a front approaching the Gulf Coast states from the
north. Thinking is that most of that activity will shift from
Louisiana into Mississippi or Alabama early in the day...followed
by another round of showers and thunderstorms which form later in
the afternoon across parts of eastern Texas as low level moisture
begins to move back northward ahead of mid-level shortwave energy.
Operational deterministic model runs still showed two QPF
maxima...with heaviest amounts just either side of 1 inch in each
area. Spaghetti plots of QPF from the ensembles still showed
enough of a signal for 1 to 2 inch amounts anywhere between the
areas to warrant keeping one risk area considering antecedent
conditions across the area. With precipitable water values
remaining above 1.5 inches...some briefly enhanced rainfall rates
seem possible even with the modest model QPF values.

Bann





Day 3
Valid 12Z Tue May 11 2021 - 12Z Wed May 12 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MIDDLE GULF COAST REGION TO EASTERN AND SOUTH TEXAS...

The front which moved into the Gulf coast region on Day 2 will
advance deeper into Texas on Tuesday...with east to southeast low
level flow becoming established over South Texas ahead of the
front. Aloft, mid- and upper-level height falls associated with a low-amplitude trough coming out of the southern Rockies will start
to draw deeper moisture northward...with the NCEP models showing
precipitable water values over 1.75 inches spreading northeastward
covering much of the Marginal Risk area while 2 inch precipitable
water values develop closer to the Gulf coast. The result should
be increasing coverage of cells capable of producing downpurs
whether driven by instability south of the front or by elevated
convection farther north.

Opted for a Slight Risk from parts of eastern Texas eastward into
Mississippi based on the anomalously wet period there combined
with anticipated rainfall amount in the Day 1 and 2 periods.
Thinking that a Marginal Risk area will be suffifcient for areas
south of the front given drier conditions.

Bann

$$
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FOUS30 KWBC 091933
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
333 PM EDT Sun May 09 2021

Day 1
Valid 16Z Sun May 09 2021 - 12Z Mon May 10 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE LOWER MISSISSIPPI VALLEY...

1600 UTC Update...

Made slight modifications to the Day 1 ERO, based on the convective/mesoanalysis trends as well as the 12Z high-res
guidance (including the HREF exceedance probabilities). Extended
the Slight Risk a little farther south across more of east TX and
central LA, based on the latest ARW/ARW2 progs (including parallel
runs), while also encompassing the enhanced 2+ inch/hr
probabilities per the 12Z HREF.

Hurley

The Slight risk of excessive rainfall for later today into tonight
stretches from northeast TX into central and northern LA, far
southern AR and central MS. Convection will break out this
afternoon along/ahead of a southeastward moving cold front. Plenty
of instability will be in place to support deep/strong
convection...and PWs are forecast to increase over the
climatological 90th percentile. Thus likely to have some efficient
rainfall producers from cells that develop. The stronger forcing
and core of 850mb moisture transport will generally be focused
northeast of this region. However, often on the tail of these
features we do end up with an enhanced corridor of low level
confluence along with a slowing of the front. This stronger
sfc-925mb convergence combined with a 300mb jet streak overhead
should be enough for organized convective development...with the
slowing off the front allowing for some training/backbuilding. The
general consensus within the Slight risk area is for a swath of
2-3" areal averaged rainfall...with localized totals of 4-6". FFG
across this region is pretty high...although soil saturation is
running above average over the area. This suggests that we may see
flooding impacts develop pretty quickly once FFG is
approached/exceeded. Thus scattered flash flooding is anticipated
over the Slight risk area. This Slight risk was shifted a bit
southeast from our previous issuance...taking into account 00z
global and HREFv3 model trends.

Convection is expected to eventually forward propagate or
redevelop towards the Gulf Coast overnight. Thus while some
training of cells over the Slight risk is expected...the duration
of this training will be somewhat minimized by this eventual
forward propagation. This will eventually bring the heavy rain
potential further south into southern MS and southern LA. There is
a bit more uncertainty with regards to the intensity/organization
of activity by this time...but at least an isolated flash flood
threat may evolve...warranting a southward expansion of the
Marginal risk into these areas.

Convection over northern MS, AL, TN and southern KY will also pose
a heavy rainfall threat today. Activity here should generally stay progressive...and the ingredients for flash flooding here are not
as good as what is forecast further southwest over the
aforementioned Slight risk area. Nonetheless, there has been an
uptick in forecast QPF over these areas, and isolated pockets of
flash flooding are certainly possible. Probably not enough
coverage for a Slight risk, but enough of a threat to warrant a
northward and eastward expansion of the Marginal risk to encompass
this entire area.

We did opt to remove the Marginal risk over IN, OH and PA. While
1-2" of rain is forecast here...this will be primarily stratiform
in nature...with rates not expected to get high enough for a flash
flood threat. Localized areas of ponding water within urban and/or
low lying areas are possible...but not enough to warrant a
Marginal risk.

Chenard


Day 2

The Day 2 outlook will be updated by 2030Z.


Day 3
Valid 12Z Tue May 11 2021 - 12Z Wed May 12 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MIDDLE GULF COAST REGION TO EASTERN AND SOUTH TEXAS...

21Z update... The Marginal Risk was extended further west over the
Texas Hill Country and into South Texas- areas southwest of Corpus
Christi, Texas. The Slight Risk area was extended more to the
west/southwest across east-central Texas. The southward moving
front over Texas and the central Gulf states will slow its
progression during this period, in a west-to-east orientated
boundary. With very moist air streaming in from the east/southeast
over boundary, convection will build westward and support training
cells along and north of this boundary. There is an increasing
signal for moderate to heavy rainfall to initialize over the Hill
Country and surrounding locations, with some guidance exceeding 2
inches. QPF was increased over this region, as well as, eastern
Texas to western Mississippi. There is an elevated threat for
flash flooding across a large portion of Texas during this period.

Campbell

Previous discussion... The front which moved into the Gulf coast
region on Day 2 will advance deeper into Texas on Tuesday...with
east to southeast low level flow becoming established over South
Texas ahead of the front. Aloft, mid- and upper-level height
falls associated with a low-amplitude trough coming out of the
southern Rockies will start to draw deeper moisture
northward...with the NCEP models showing precipitable water values
over 1.75 inches spreading northeastward covering much of the
Marginal Risk area while 2 inch precipitable water values develop
closer to the Gulf coast. The result should be increasing
coverage of cells capable of producing downpours whether driven by
instability south of the front or by elevated convection farther
north.

Opted for a Slight Risk from parts of eastern Texas eastward into
Mississippi based on the anomalously wet period there combined
with anticipated rainfall amount in the Day 1 and 2 periods.
Thinking that a Marginal Risk area will be sufficient for areas
south of the front given drier conditions.

Bann

$$
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FOUS30 KWBC 111532
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1131 AM EDT Tue May 11 2021

Day 1
Valid 16Z Tue May 11 2021 - 12Z Wed May 12 2021

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTHERN LOUISIANA...

1600 UTC update

A moderate risk area was added across portions of southern
Louisiana where the combination of current convection focusing
along the stationary frontal boundary, and potential for
additional convection in the 0000-1200 UTC Wednesday period pose
an increased risk of flash flooding. The 0000 utc hi res guidance
is not depicting the current activity along and to the north of
the stationary front well at all. The last several hrrr runs are
doing better here, although they are also underdone when comparing
their simulated radars to the observed. The hrrr runs, 1200 utc
especially, show this current activity continuing into early
afternoon, especially across eastern Louisiana. The 1200 utc hrrr
shows additional heavy rain potential in the 0000-1200 Wednesday
period along the stationary front. The h85 flow is expected to
attain a more westerly trajectory at this time which would support
potential for training of cells in a west to east direction. In
areas of training, runoff issues are increasingly likely,
especially in urban areas of New Orleans, Baton Rouge and
Lafayette and areas where soils are nearly saturated. Hourly
rainfall amounts of 1.5-2"+ possible in areas of training this morning/afternoon and again in the early morning hours of
Wednesday. Please see WPC's mesoscale precipitation discussions
#0169 and #0170 valid until approximately 1800 UTC for additional
information across southern to eastern Louisiana.

Oravec

0900 UTC discussion

Overall a rather tricky convective forecast today. Have a general
idea on where the heaviest rainfall today into tonight is most likely...northeast TX, southern AR, LA, and central to southern
MS...but confidence is lower on the timing and exact magnitudes.
The low amplitude nature of the approaching wave is leading to
this uncertainty in exact evolution. We have enough forcing to
likely organize convection...but not a strong enough vort or low
level boundary to dominate as the main driver of convection. Thus
what we get is a rather messy convective forecast...with enough
instability/PWs in place for heavy rain, and enough forcing to at
least get loosely organized convection at times through the period.

Our best guess is that ongoing convection over northeast TX and
northern LA continues to slowly drift southeastward, and
eventually grows upscale over over portions of LA/MS as daytime
heating allows the activity to become more rooted at the surface.
This convection will probably pose some flash flood risk as it
grows upscale and at least briefly backbuilds/trains this morning
into the afternoon hours. Then we should see additional
development over central TX by later this morning closer to the
better forcing and along an eastward moving front. This activity
should initially be progressive...however as it moves into
northeast TX, southern AR and northern LA we may see some upscale
development and cell mergers as it moves into a region of greater
instability and low level inflow/moisture transport. This activity
may very well organize into a squall line that pushes southeast
towards the Gulf Coast, moving over areas of increasingly
saturated conditions. There's also a multi model (although not
unanimous) signal for strong convection near the Rio Grande into
south TX this afternoon into tonight.

So summing this all up, we have a rather large geographic area
where at least localized flash flooding is possible...stretching
across much of central/east TX across the lower MS Valley.
Embedded within this we will have a Slight risk stretching from
northeast TX into southern AR, LA, central/southern MS and
southwest AL. Northeast TX, northern LA and southern AR will be
impacted by two rounds of convection...the elevated cells ongoing
this morning...and the afternoon activity described above. The
MS/AL and southeastern LA portion of the Slight risk is where the
best signal for 2"+ areal averaged rain exists. They may also see
two rounds, one this morning/afternoon out of the activity pushing
southeast as of 08z, and another as a potential squall line moves
across tonight. Again the details remain messy and of low
confidence but this region does appear to be within a favorable
region for heavy rainfall...positioned along the sfc based CAPE
gradient and low level boundary. So would seem inevitable that
portions of LA, MS and far southern AL see heavy rainfall at some
point today as activity upstream propagates into the region and
exhibits some backbuilding/training characteristics. But exactly
how organized activity is, and how widespread excessive rainfall
is, remains a question.

Would not be surprised if localized significant flash flooding
evolves at some point today or tonight. Antecedent conditions over
this region are becoming increasingly saturated, and multiple
convective rounds today could result in some higher end
impacts...especially if urban areas end up within any
backbuilding/training complex. Thus quite possible the event ends
up verifying as a MDT risk somewhere within the broad Slight.
However just not confident enough on the details at this
point...so any MDT would come with a decent bust potential at the
moment. So thought the best strategy was to stick with a broad
Slight, with the understanding that localized swaths of higher
impacts are possible depending on how exactly things evolve and
whether or not any of the more sensitive areas within the region
are impacted.

Chenard


Day 2
Valid 12Z Wed May 12 2021 - 12Z Thu May 13 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL LINGERING ALONG
THE GULF COAST REGION...

Overall, there is model agreement that a quasi-stationary front
draped across the Gulf Coast region will evolve into a cold front
in response to the approach and passage of a synoptic-scale
trough. The agreement in the model QPF placement was not as
strong...with the GFS being the quickest to sweep nearly all but
the lightest rainfall out to sea while the ECMWF maintained its
heaviest rainfall in a stripe of rainfall inland from the Gulf
Coast region into the Southeast U.S.. The most consistent signal
in the global models for locally heavy rainfall was over portions
of South Texas near the coast early on Day 2...although the
agreement was far from unanimous...apparently in response to some
persistent low level southeasterly flow.

Given the range of solutions, was initially tempted to break the previously-issued Marginal Risk area into two areas. The 11/00Z high-resolution guidance, including the ARW and NMM cores and the
NAM-NEST, still maintained some bands of locally heavy rainfall
from the Texas Upper Coast eastward across southern Louisiana into
parts of Mississippi or Alabama (especially in the first 12 hours
of Day 2). That placed the high-res QPF roughly between the QPF
areas in the GFS and the ECMWF...and in proximity to entrance
region of an upper level jet shown in each model and in an area
that the 00Z GEFS and 03Z SREF plots showed lingering potential
for 1 to 2+ inches. As a result, trimmed some of the more inland
portions of the Marginal Risk area but maintained one area rather
than twp areas. This also accounts for the possibility of
on-going problems across parts of southern
Louisiana/Mississippi/Alabama lingering from the end of the Day 1
period.

Bann


Day 3
Valid 12Z Thu May 13 2021 - 12Z Fri May 14 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann

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