FOUS30 KWBC 060827
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Tue Aug 6 2024

Day 1
Valid 12Z Tue Aug 06 2024 - 12Z Wed Aug 07 2024

...THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
GEORGIA, SOUTH CAROLINA AND NORTH CAROLINA...

...WIDESPREAD AND NUMEROUS INSTANCES OF FLASH FLOODING WITH
SIGNIFICANT AND POTENTIALLY CATASTROPHIC FLOODING LIKELY...

...Southeast...

Significant impacts are likely to continue across portions of
southeast Georgia into eastern South Carolina and southeast North
Carolina. Some uncertainty remains with regards to the structure
of Debby by this time. Portions of the High risk area could end up
within a dry slot at some point Tuesday...but even if this happens
the extreme antecedent rainfall and expectation of areas of heavy
convective banding near and north of the center more than supports
a continuation of the High risk. Also some uncertainty with regards
to how far north and inland the convective bands get into North
Carolina, so some adjustments to the Moderate Risk area may be
needed. There will likely be numerous instances of significant to
catastrophic flooding for both rural and urban areas along with
rising streams, particularly for the eastern South Carolina.

A few feeder bands into Debby will pass across Florida during this
period however both intensity and coverage along the west coast of
Florida will be greatly diminished. The Slight Risk that was
previously in place for portions of central and southern Florida
was trimmed back to stop near northeast Florida. A Marginal Risk
for that area reflects the level of threat as the impacts from
Debby wains.

...Mid-Atlantic/southern portions of the Northeast...

The Moderate Risk area was expanded to now include northern New
Jersey, far southeast New york and portions of Long Island. During
this period a stationary front draped across the region, and an
approaching shortwave and upper jet streak from the west, and some
connection to tropical moisture out ahead of Debby. Will likely
have an MCS ongoing at 12z Tuesday moving east southeast across
portions of Michigan and the Ohio Valley. This activity may end up
surviving across the Mid- Atlantic, and we may see another
convective cluster develop downstream of this MCS along/near the
stationary front. The location and magnitude of the flash flood
risk depends on a couple factors...does the MCS persist and merge
in with frontal convection resulting in a training axis, and where
exactly is the front by Tuesday afternoon. Overall this setup does
have the look of a potential locally high impact flash flood event.
There is enough forcing and instability, and a stationary front
that could act as a focus for training/backbuilding cells. The
exact location of where the greatest amounts will focus remain
uncertain however the most favorable area seems to span from far
northeast Maryland into central New Jersey, but this is dependent
on the eventual location of the front and also the propagation of
the upstream MCS.

...Southwest...

The monsoonal moisture and diurnal heating will keep showers and
thunderstorms developing during the afternoon and evening hours across
the Southwest during this period. Convection to develop along the
terrain, and then move westward into some of the lower elevations
resulting in an isolated to scattered flash flood threat. The well
defined shortwave currently over northern Mexico will approach
southern Arizona by later Tuesday, which should bolster rainfall
amounts and to some degree coverage of convection across southern
Arizona. A Slight Risk remains in effect for portions of southern
Arizona while a Marginal Risk remains in effect for much of the
Four Corners region.

...Northern Rockies into Northern High Plains...

Fast-moving thunderstorms are expected to pass over the Rockies and
adjacent High Plains during this period. There is potential for
training of cells over areas that have low 1-hour FFGs. Isolated
instances of flash flooding may occur during this period therefore
maintained the Marginal Risk area.

Campbell/Chenard


Day 2
Valid 12Z Wed Aug 07 2024 - 12Z Thu Aug 08 2024

...THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTH AND NORTH CAROLINA...

...WIDESPREAD AND NUMEROUS INSTANCES OF FLASH FLOODING WITH
SIGNIFICANT AND POTENTIALLY CATASTROPHIC FLOODING LIKELY...

...Southeast and Mid-Atlantic...

A High Risk for excessive rainfall remains in effect for eastern
South Carolina and southeast North Carolina as Debby continue to
dump widespread heavy rain along its very slow track; likely
resulting in continued significant rainfall and flooding over the
High risk area. There is at least some flash flood risk that will
extend up into the eastern Mid-Atlantic Wednesday as well...with
moisture ahead of Debby likely continuing to interact some with the
front to the north and right entrance region of the upper jet. A
Slight Risk area was raised for portions of northern Delaware, far
northeast Maryland, southeast Pennsylvania and southern New
Jersey.

...Southwest...

The increased moisture flux from the Gulf of California will make
its way further north/east across the Four Corners region, helping
maintain enhanced rainfall during the afternoon and evening. A
Marginal Risk area covers much of the Four Corners region and
extends into southeast Wyoming and western Nebraska.

Campbell


Day 3
Valid 12Z Thu Aug 08 2024 - 12Z Fri Aug 09 2024

...THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTH AND NORTH CAROLINA...

...WIDESPREAD AND NUMEROUS INSTANCES OF FLASH FLOODING WITH
SIGNIFICANT AND POTENTIALLY CATASTROPHIC FLOODING LIKELY...

...Eastern Seaboard...

Debby continues to be a soaker for the Carolinas as it tracks
inland once again while spreading heavy rain further north into the
Mid- Atlantic region. Several more inches of rain expected during
this period, bringing the multi-day totals into the 10 to 20 inch
range with locally higher maxes. There will likely be widespread
significant, possibly catastrophic flooding already ongoing by
this period and will persist. A High Risk is in effect from South
Carolina to north-central North Carolina. A Moderate spans from
southern South Carolina to central/southeast Virginia. Further
north, moisture from Debby will interact with a frontal system and
ongoing MCS activity to further enhance convection across the
northern Mid-Atlantic and Northeast. Although some of the stronger
rainfall will have already occurred, antecedent conditions along
with additional thunderstorms capable of producing heavy rainfall
with keep the threat for excessive rainfall and local flooding
concerns elevated. Much of the Mid-Atlantic and southern portion of
New England have a Marginal Risk in effect.

...Four Corners, Central Rockies and High Plains...

The Monsoonal pattern will persist through this period across the
Four Corners region while increasing across portions of the Rockies
and adjacent High Plains. Shortwave energy passing through the
Rockies will help provide additional forcing for convection to
focus along the higher terrain of Colorado and New Mexico. Model
guidance suggests a concentration of cells capable of producing 1
to 2 inches with very isolated maxes greater than 3 inches. A
Slight Risk was maintained for portions of central Colorado and
northeast New Mexico given the increased risk for isolated to
scattered flooding concerns.

Campbell
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