FOUS30 KWBC 050839
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
439 AM EDT Mon Aug 5 2024

Day 1
Valid 12Z Mon Aug 05 2024 - 12Z Tue Aug 06 2024


...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHERN SOUTH CAROLINA, EASTERN GEORGIA AND NORTHEAST FLORIDA...

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

...Southeast...

Hurricane Debby will usher in a lot of moisture (+4 standard
deviation/over 2 inches PW values) as it makes landfall this
morning and tracks inland. The slowing of the forward speed of
Debby has also increased the likelihood that the storm will be
stronger at landfall and allowing more time for the storm to
gather Gulf moisture off the sea surface temperatures in the upper
80s to near 90. Further, any wind damage from debris may clog
drainage ditches and small creeks and streams, which too could
increase the local flooding threat.

Areal averages for this period range from 3 to 9 inches although a
couple of models suggest local maxes upwards of 15 inches will be
possible for this period, particularly near Georgia coastline and
far southern South Carolina.

A broad High Risk spans from north/northeast Florida, southeast
Georgia and southern South Carolina. A Moderate Risk remains in
effect from central Florida to central South Carolina. The rain
fall amounts have the potential to be historic for South Carolina
with several instances of significant flash flooding.

...Northern Plains to Upper Midwest...

A low over the Central Plains, high pressure to the north over
Canada, a passing shortwave, and somewhat higher than normal
moisture from Minnesota to Michigan will cause training
thunderstorms across this region, tracking East-Southeast along a
stalled front. Models show localized totals between 3-4", which
will likely fall within a 3-6 hour period per the 3-hourly QPF
output. Much of this region remains wet after a very wet Spring and
first half of summer. Training, if fast-moving storms are likely along
a stalled front and fast-moving upper level shortwave, which will
continue into Michigan and into Ohio. The primary area of focus
remains around the Twin Cities to southern Wisconsin, where recent
wet weather, the strongest storms happening at peak heating into
the evening, and urbanization have all increased the threat for
flooding into the higher-end Slight category.

...New England...

Convection will focus along the same front that is draped over the
Midwest; which will be stronger than typical for late summer. The area
has been hard hit in recent days including presently, resulting in
low FFGs. With soils near saturation the threat for excessive
rainfall and localized flooding concerns will be elevated. However,
a limiting factor may be that the storm motion is expected to be
fairly progressive.

...Southwest...

Monsoonal flow in this region will keep isolated to scattered
convection possible for this period. The latest guidance continued
to depict sparse storm coverage over parts of north-central
Colorado.

Campbell/Wegman


Day 2
Valid 12Z Tue Aug 06 2024 - 12Z Wed Aug 07 2024

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

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

...Southeast...

A very slow-moving Debby will cause a prolonged period of very
heavy rain across this area; in addition to rains from Monday will
likely result in widespread and numerous instance of flash
flooding with scattered areas of catastrophic flooding likely. The
areas most likely to be hit hardest by the rain will be highly
dependent on the track of the storm's center, which is uncertain
since modest changes in the steering currents result in just enough
of a shift in the center's location at any one time to impact
where the heaviest rain bands set up. Nonetheless expect a
widespread 4 to 8 inches of rain with locally higher amounts just
during this time period. There will likely be numerous instances
of significant to catastrophic flooding for both rural and urban
areas along with rising streams.

Feeder bands into Debby will continue...albeit greatly diminished
in both intensity and coverage along the west coast of Florida. Due
to the likely hard hit nature of this area by Tuesday, even
smaller amounts of rain are likely to result in outsized impacts
(or at least keep ongoing flooding longer, especially in the likely
event of 1-3 nearly stationary feeder bands of rain continuing
into the region from the Gulf. The Marginal and Slight Risk areas
were expanded further into central and southern Florida to account
for this.

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

A Moderate Risk area was raised for extreme northeast Maryland,
southeast Pennsylvania and central/southern New Jersey where there
is a growing signal for a PRE (predecessor rain event) to unfold
as Tropical Cyclone Debby impacts the Southeast/Mid- Atlantic
region. Training convection will refire with daytime heating
Tuesday afternoon, continuing into Tuesday night. From Baltimore
northeast up the I-95 corridor to the Boston metro, training
storms and urbanization may cause an outsized risk of flash
flooding. While there is some spread in the guidance in where the
front will stall, the training storms will be capable of dumping 2
to 4 inches, locally 5+ inches which would quickly surpass local
FFGs and lead to scattered to possibly widespread instances of
flooding.

...Southwest...

Monsoonal moisture and diurnal heating will maintain showers and
thunderstorms across the Southwest during this period. A Marginal
Risk area covers much of the Four Corners region. There is a
notable increase with Gulf of California moisture advecting
northward into southern Arizona which will further increase
potential rainfall efficiency and amounts. A Slight Risk covering
portions of southern Arizona was maintained for this period.

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

A renewed round of thunderstorms are expected to be fast-moving,
but there will be the potential for training and low 1-hour FFGs.
Isolated instances of flash flooding may occur.

Campbell/Wegman


Day 3
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...

Debby will continue its very slow track northward along the
Southeast coastline while dumping additional heavy rain over a very water-logged region. By this point, the multi-day storm
accumulation will likely be in the double digits with maxes in the
20 to 30 inch range near the Savannah metro and all along the
Carolina Coastal Plain. With rainfall of the caliber catastrophic
flooding would be likely/definite and compounded by coastal surge
and waves. A High Risk is in effect for much of Carolina Coastal
Plain with a Moderate spanning from southeast Georgia to southern
North Carolina.

...Southwest...

The increased moisture from the Day 2 period 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
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