FOUS30 KWBC 142004
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
404 PM EDT Thu Aug 14 2025
Day 1
Valid 16Z Thu Aug 14 2025 - 12Z Fri Aug 15 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-ATLANTIC STATES...
...16Z Update...
...Mid-Atlantic...
Based on some of the latest CAMs guidance, featuring notable
agreement with the development of training thunderstorms across
portions of southern and eastern North Carolina, a higher end
Slight is in effect from the Hampton Roads area southwest through
far northeastern South Carolina. Confidence has therefore
increased on the potential for flooding, especially considering
recent heavy rainfall has saturated the soils in the area. Abundant
moisture with PWATs near 2.25 inches will support highly efficient
warm rain processes which will allow any storms, and especially
training storms to produce multiple inch per hour rainfall rates
with the heaviest cores. One caveat will be somewhat reduced
visibility from cloud cover from the ongoing MCS, which may help
portions of SC avoid the greatest impacts from flooding in favor of
areas further northeast into NC.
Confidence decreases just a bit across far northeastern North
Carolina and southeast Virginia, where much of the convective
activity expected will develop along the James River and the storms
may then progress a bit south of due east. There remains some
potential that those storms' cold pools help fuel additional
convective development over the more sensitive urban Hampton Roads
area.
...Northeast...
The Marginal Risk area was split around the NYC metro and Long
Island, noting a stationary boundary persisting across eastern MA
and RI, and more moisture from central NJ southward, resulting in a
relative minimum of rainfall over the NYC metro.
...MS/AL...
A Slight Risk upgrade was considered for this area for this
afternoon's expected convection across central MS and extending
into central AL. Plentiful atmospheric moisture and instability
will be available, however the lack of a strong enough forcing
mechanism should limit any flash flooding impacts to isolated
areas. Any slow moving convection along the immediate Gulf Coast
could result in isolated flash flooding in any urban areas, so the
Marginal Risk was expanded to include the Gulf Coast.
...Upper Midwest...
The Marginal Risk remains in place across much of Minnesota and
adjacent counties with this update. Expect some limited convection
to develop across the state within the broader low level jet this
afternoon, but any storms that develop should be fast moving.
Additional storms develop across northern Minnesota into tonight,
but these too should also be fast moving. Regardless, expected
coverage over the coming days will be increasing, along with the
flash flooding threat.
...Southwest...
No major changes were made, but there is some potential for
convective development over the Phoenix metro, or from outflows
from likely more numerous showers and storms along the Mogollon Rim
as those outflows move southwestward into the Valley. The Marginal
Risk was expanded to include these lower elevation areas with this
update.
Wegman
...Previous Discussion...
...Southern New England through the Lower Mississippi Valley...
Surface cold front draped from New England through the Ozarks will
drift gradually southeast but generally stall across the region
Thursday as the parent trough begins to fill, leaving gradually
more zonal flow over the eastern CONUS. Although heights will
gradually rise, weak shortwave impulses embedded within the flow
will maintain periodic ascent atop the front to create waves of low
pressure rippling northeast to produce locally enhanced lift, and
scattered to numerous thunderstorms are again expected across the
east on Thursday, primarily from southern New England through the
Gulf Coast.
Thunderstorms that develop will all have the potential to produce
pockets of heavy rainfall as thermodynamics remain impressive. PWs
of more than 2 inches will remain in a plume arcing within return
flow around offshore high pressure from the Florida Panhandle
through the northern Mid-Atlantic states. This PW will overlap with
elevated MUCAPE of 1000-2000 J/kg to create the favorable
thermodynamics for heavy rainfall, and the HREF indicates scattered neighborhood probabilities for 2"/hr rain rates across much of the
area supporting the broad inherited MRGL risk.
However, there appears to be at least a signal for some more
organized convection with a greater threat for excessive rainfall
across the eastern Carolinas and into Southeast Virginia. This
region will benefit from additionally focused ascent through the
presence of the cold front and a surface trough, upon which a more
pronounced shortwave is progged by the consensus of the models to
lift northeast during peak heating. This will promote at least
modest bulk shear to support multi-cell clusters across this area,
while most of the rest of the east should generally feature pulse
type thunderstorms. Additionally, mean 0-6km winds and propagation
vectors are aligned and parallel to the boundaries, suggesting an
enhanced training risk as well, which additionally supports the
elevated HREF and REFS probabilities for 3"/24hrs (and lower but
still notable 5"/24 hrs). For this reason the inherited SLGT risk
was adjusted to spread farther south into the eastern Carolinas.
...Southwest...
Mid-level ridge centered over NM to start Thursday morning will
weaken in response to height falls approaching from CA as a trough
digs down the Pacific coast. This will have the two- pronged affect
of lowering heights across the Desert Southwest/Four Corners
region, while also leading to increased cyclonic flow emerging
from near Baja to push a potent shortwave northward across AZ and
into UT. Additionally, this increasingly south flow, especially
between 850-700mb, will transport elevated PWs northward, and a
plume above 1" is progged to surge almost to UT by the end of the
forecast period. Where this combines with MUCAPE above 500 J/kg,
slow moving storms (0-6km mean winds of just around 5 kts to the
north) could become more organized leading to a longer duration of
heavy rainfall rates of 0.5-1"/hr. The total coverage is still
somewhat uncertain so the inherited MRGL risk still appears
appropriate, but this was expanded north to match recent guidance,
and convection is likely to be more widespread Thursday than it was
on Wednesday.
..Upper Mississippi Valley...
A warm front will lift northeast towards Minnesota today,
accompanied by a surge of 850mb winds to 20-30 kts advecting
anomalous PWs of 1.5 to 1.75 inches northeastward (+1 to +1.5
standard deviations above the climo mean). This front will be
driven eastward by a potent but flat and fast- moving shortwave
embedded within the nearly zonal flow aloft, leading to a period
of enhanced ascent aided by the warm front and accompanying
isentropic lift of the 850mb wind. This will result in scattered
thunderstorms across the area, with bulk shear of 25-35 kts
potentially supporting some organization into clusters. The
guidance is highly variable in both the placement and intensity of
convection today, and the QPF footprint and ensemble probabilities
suffer as a result. Still, with the impressive thermodynamics in
place, thunderstorms that develop could support rainfall rates of
1-2"/hr with at least short-term training possible, and the
inherited MRGL risk was curtailed for latest guidance but
continued.
...Pacific Northwest...
A rare August atmospheric river (AR) is progged to make landfall
this evening north of 45N into WA state with IVT above 500 kg/m/s
as reflected by both GEFS and ECENS probabilities above 90%. A weak
shortwave embedded within otherwise zonal flow aloft will help
produce ascent into this plume of IVT, but instability will be
negligible and jet level ascent appears to be displaced north of
the core of the AR. While deterministic model QPF output and
accompanying ensembles support a threat for heavy rainfall,
especially in the Olympics, this will be occurring atop extremely
dry soils and should generally be beneficial rainfall.
Additionally, in the AR/ERO verification data set since 2019, there
has never been an impactful (MRGL risk or higher) AR in the month
of August. For this reason, despite some heavy rain spreading
onshore this evening, no excessive rainfall impacts are expected.
Weiss
Day 2
Valid 12Z Fri Aug 15 2025 - 12Z Sat Aug 16 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
UPPER MISSISSIPPI VALLEY, THE SOUTHERN APPALACHIANS, DEEP SOUTH=20
TEXAS, AND FAR SOUTHERN ARIZONA...
...20Z Update...
...Upper Mississippi Valley...
Guidance has shifted a bit south and east with the band of heaviest
rainfall expected across portions of Minnesota and Wisconsin on
Friday. There is reasonable agreement that an MCS will form across
Minnesota and Wisconsin, then push south, following the
instability, towards Iowa and Illinois. It is really in the
formation stage of the MCS during the evening hours when the
flooding threat will be maximized, while after that, the MCS should
be a fast mover towards the south overnight. There has been some=20
time for the soils to dry in this area, though some rain expected
today, D1, could saturate soils in some areas ahead of this new
breakout of convection. Overall this area is on the lower end side
of the Slight Risk category, but any storms that get hung up on
either side of the MCS could make for locally higher impacts,
albeit isolated.
...Southern Appalachians...
In coordination with GSP/Greer, SC forecast office, a Slight Risk
area was added to portions of the Western Carolinas with this
update. HREF probabilities for FFG exceedance have increased to 30%
across this region, and guidance is in better agreement that
clusters of storms will form in this area, and the storms will
likely move slowly, chaotically, and be highly influenced by each
others cold pools, resulting in cell mergers. On the whole, the
storms will generally move south and east with time, so portions of
upstate South Carolina may get into the flooding threat later than
western North Carolina on Friday. A significant portion of the
certainty with the Slight Risk for this area consists of the soils
being saturated from recent, and in some cases, ongoing heavy
rainfall, which will have local streams, creeks, and rivers already
above normal, such that additional rainfall will result in widely
scattered instances of flash flooding. PWATs ahead of a southward
moving cold front will be between 1.5 and 1.8 inches, allowing for
sufficient moisture to be present to support storms capable of
locally heavy rainfall, especially when adding any upslope
component and forcing tied to the mountains.
...Deep South Texas...
In coordination with BRO/Brownsville, TX forecast office, a Slight
Risk area was introduced with this update. A healthy looking Invest
98L in the western Gulf is being very poorly handled in the
guidance thus far. However, there is good agreement that it will
remain on a northwestward trajectory, making landfall somewhere
near Brownsville on Friday. Any further intensification of this
system will increase the rainfall threat across Deep South Texas.
The greatest vulnerability for heavy rainfall will be in the urban
areas such as Brownsville and McAllen.
There remains much higher than normal uncertainty with the expected
rainfall footprint of Invest 98L. Since the guidance has poor
handling on this system, most of the guidance is dissipating the
rainfall very soon upon landfall, which pattern recognition would
suggest does not often happens with a tropical system on the move.
Thus, in contrast to much of the guidance, the Slight Risk was
hoisted acknowledging the potential for organized convection to
impact somewhere along the Texas Gulf Coast, most likely on Friday
morning and in Deep South Texas. Outflow and sea breezes may make
for slightly elevated chances of heavy rain and resultant flash
flooding along all of the Texas Gulf Coast and into Louisiana,
where the Marginal was expanded, but remains in place. Due to this
high uncertainty, large changes to the ERO risk areas are likely
with subsequent updates as the guidance gets a better handle on
Invest 98L. Should 98L intensify, additional upgrades may be
needed. Please see the National Hurricane Center website at
hurricanes.gov for more details about Invest 98L.
...Southern Arizona...
Monsoonal moisture in place across Arizona will combine with an
upper level vort max to result in greater than normal organization
of expected convection across far southern Arizona on Friday
afternoon. Given the higher than normal PWATs and potential for
organization; in coordination with TWC/Tucson, AZ forecast office,
a Slight Risk area was introduced with this update, mainly along=20
the International Border. Convection is likely to break out well=20
into Arizona, mostly along the Mogollon Rim, but guidance has been=20
less than reliable about the coverage of storms, particularly in=20
southern Arizona. Thus, despite some of the guidance being much=20
better, and suggesting a higher end threat, the Slight really=20
focuses where the vort max will be the primary forcing mechanism=20
for convection.
...Pacific Northwest...
After extensive coordination with SEW/Seattle, WA forecast office,
a Marginal Risk was introduced for all of western Washington with
this update. An extremely anomalous moisture plume associated with
a robust atmospheric river is forecast to move into western
Washington on Friday. PWATs will be as high as 6 sigma above
climatological normals for this time of year, which if verified,
will set both monthly records for August and threaten all time
records, as they near 1.75 inches. This incredible abundance of
moisture will translate to a 24-36 hour period of highly efficient
rainfall, streaming primarily into the west facing slopes of the
Olympics and Cascades Mountains. By far the heaviest rainfall
totals will be seen in those areas. For the more populated interior
valleys, including the Seattle Metro, the usual downslope shadowing
will occur. However, the combination of urbanization and the
unusually high amount of moisture is likely to overcome the
downsloping, resulting in far lower, but still significant amounts
of rainfall into those areas.=20
Contrasting the anomalous atmospheric moisture and rainfall, nearly
all of western Washington is in some amount of drought, ranging
from Minor to Extreme, as the area is in the midst of its dry
season. That dry season will abruptly end tonight through into
Saturday morning, as rivers approach record territory with all the
rainfall. The drought conditions should certainly dampen any
flooding impacts quite a bit, pun intended. Normally with this
unusual an amount of atmospheric moisture, the ERO would start at
Slight or Moderate and only go up from there, but given a good
amount of this rainfall will be beneficial and welcome for filling
local reservoirs, the Marginal was the decided happy medium to
account for localized impacts from flooding small streams and
creeks, as well as urban impacts, but flooding impacts should
remain relatively isolated since the atmospheric river has such a
large volume to fill before the rainfall becomes impactful.
Wegman
Day 3
Valid 12Z Sat Aug 16 2025 - 12Z Sun Aug 17 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE UPPER
MIDWEST INTO THE GREAT LAKES...
...20Z Update...
...Upper Midwest...
Relatively few changes were made to the risk areas on Day
3/Saturday. A higher end Slight is in effect for portions of
central Wisconsin. Additional heavy rainfall from MCS development
again on Saturday is likely to overlap with areas hit from heavy
rains on D2/Friday. This overlap, in addition to prior heavy rains
in the area, will keep soils saturated, and the rivers and streams
full. Thus, the additional rainfall expected Saturday will fall on
top of these saturated soils and full streams. Magnitude-wise, the
rainfall on Saturday will also be heavier across Wisconsin than
previous days, only increasing the potential impact. For these
reasons, the higher end Slight is in effect, and considerations may
need to be given, depending on how much rain falls in the interim,
as to whether a Moderate Risk may be needed in this area.=20
...Gulf Coast and Southeast...
The inherited Marginal Risk was greatly expanded across portions of
central Texas with this update. This is all due to uncertainty with
the track and eventual impacts associated with the moisture plume
with Invest 98L over the western Gulf. All indications are that the
associated moisture will push northward from Deep South Texas on
D2/Friday into central Texas on Saturday, but how organized the
storms become, their coverage, and heavy rainfall potential remain
highly uncertain. Thus, there is higher than normal probability
that future upgrades will be needed, there is just no certainty as
to where. The guidance is in decent agreement that any upper level
disturbance that 98L is able to form prior to its landfall and
subsequent dissipation over Deep South Texas will continue up the
Rio Grande towards the Big Bend Area. This has the potential to
bring heavy rainfall to areas that got the devastating flooding way
back on July 4. However, it's very important to note that the
guidance is far from agreed on this scenario.=20
...Southwest...
Monsoonal moisture will begin to push east on Saturday, expanding
the potential for isolated flash flooding across much of southern
New Mexico. However, there remains little to help focus that
moisture and those storms into any kind of organization, so while
the slopes of the mountains could act as a localized focus, and
where the flash flooding threat is highest, this too remains
uncertain. Most likely the storms will behave as they've done in
past events, remaining widely scattered, reliant on cold pools and
cell mergers, and not persist for very long. The inherited Marginal
was left unchanged with this update, as any heavy rainfall
footprint also remains unchanged.
...Pacific Northwest...
The Atmospheric River (A.R.) aimed at Washington State on=20
D2/Friday will begin to shift southward across Oregon on=20
D3/Saturday. Unlike Washington however, the A.R. will be on the=20
move towards the south, greatly limiting the time any one area is
under the plume of moisture tracking northeastward into the
northern Rockies. Since much of western Oregon is also in a
drought, the much lesser amounts of rainfall expected into the
Oregon Cascades and coastal ranges compared to areas further north
into Washington State should limit any flash flooding to sub-
Marginal levels. Considerations for a future Marginal Risk will be
needed should more rain impact northern Oregon on D2/Friday than
currently forecast.
Wegman
...Previous Discussion...
...Central Plains through the Great Lakes...
Pinched mid-level flow will become increasingly squeezed between a
trough over Ontario and a ridge expanding from the Gulf Coast.
Between these two features, nearly zonal flow will force a cold
front to generally align west to east and stall, providing a
convergent impetus for convection. Along this front, spokes of
vorticity will track west to east leading to locally enhanced
ascent to support waves of low pressure, while 850mb inflow returns
around the high pressure ridge and reaches 25-35 kts to draw
elevated thermodynamics characterized by PWs above 1.75 inches and
MUCAPE of 4000 J/kg into the boundary. This will support multiple
rounds of thunderstorms through the day, peaking during the
evening/Saturday night, with rainfall rates of 1-2"/hr or more at
times, leading to scattered instances of flash flooding. A SLGT
risk has been added for portions of this area where probabilities
from the SREF/GEFS/ECENS align for more than 3" of rain.
...Southwest...
Continued S/SW mid-level flow around the periphery of a ridge
positioned over the Mississippi River Valley will maintain elevated
moisture into the Southwest on Saturday. Guidance continues to
suggest that weak shortwaves/vorticity maxima will lift northeast
embedded within the flow, although the intensity of these features
is generally modest, and weaker than on Friday. Additionally,
although 850mb flow will remain southerly over NM, it is progged to
shift a bit more to the west over AZ, lowering the overall PW
anomalies and indicating the greater risk for any isolated flash
flooding will shift east from prior days. MUCAPE of 500-1000 J/kg
will support rainfall rates that may approach 1"/hr at times, and
if these fall across more sensitive features like burn scars or
urban areas, isolated instances of flash flooding could result.
...Southeast...
A wave of low pressure along a weakening cold front will drift
southeast away from the NC coast on Saturday. At the same time, a
building ridge over the Mississippi River Valley will lead to
anti-cyclonic mid-level flow out of the NW into the Southeast,
setting up a situation with moist confluent flow intensifying over
the region. PWs will likely remain elevated above 2" along the Gulf
Coast, but be lower elsewhere as drier air tries to advect from the
NE around the offshore low, and this will result in less widespread
coverage of thunderstorms than prior days. While coverage of
convection is expected to generally be scattered, some locally
enhanced rainfall along any convergence axes is possible due to
rainfall rates that will still reach 1-2"/hr at times. Where these
fall across vulnerable soils from widespread convection on prior
days, impacts could result, and a MRGL risk continues for portions
of the area.
Weiss
Day 1 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-J6ER26Z-mZMnPJ0Tip67XoXGVeXej1Pu-Cs-bqO9qER= aYVlNrJ_J0KtNaG67iREI0GvIxH-yxhDp-Oxw49PjFv4fd8$=20
Day 2 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-J6ER26Z-mZMnPJ0Tip67XoXGVeXej1Pu-Cs-bqO9qER= aYVlNrJ_J0KtNaG67iREI0GvIxH-yxhDp-Oxw49PPdpVC-I$=20
Day 3 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-J6ER26Z-mZMnPJ0Tip67XoXGVeXej1Pu-Cs-bqO9qER= aYVlNrJ_J0KtNaG67iREI0GvIxH-yxhDp-Oxw49PBnCJj6o$=20
=3D =3D =3D
To unsubscribe from WX-STORM and you already have a login, go to
https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at
cnovy@cox.net and ask to be removed from WX-STORM.
--- SBBSecho 3.28-Linux
* Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105)