FOUS30 KWBC 311343
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
943 AM EDT Thu Jul 31 2025

Day 1  Valid 1328Z Thu Jul 31 2025 - 12Z Fri Aug 01 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE NORTHERN MID-ATLANTIC...

...Mid-Atlantic into New England...

14Z special update...

Adjusted the Moderate Risk this morning to include New York City
and parts of southern CT given the location of the frontal boundary
this morning and CAM trends. 12z HRRR depicts a reasonable scenario
with a line of slow-moving convection near New York City and along
coastal CT where an area of low-level convergence could anchor
storms along this region. Where storms do set up, rainfall rates
exceeding 2"/hr is likely to overwhelm the urbanized terrain along
I-95. An additional adjustment was made to expand the SLGT Risk
southward across the Blue Ridge of central VA. More information 
will be available with the regular 16Z update.

Snell

A potent vorticity max paralleled with an exceptionally strong
upper level jet streak over southeast Canada and highly anomalous
moisture content will spin up a surface low along a frontal
boundary over the northern Mid- Atlantic and spread enhanced
rainfall to a vast portion of the East. There are two areas of
concern regarding heavy rainfall. The first area is from northeast
Pennsylvania on east through the Lower Hudson Valley and into
southern New England will have a band of heavy and efficient
rainfall. PWATs will rise to the 1.8-2.0" range and low-level WAA
ahead of the front will give rise to potential training. There is
uncertainty regarding the amount of available instability, which
could reach as high as 500 J/kg. Still, the modeled soundings
depict warm cloud layers up to 13,000ft deep, so while rates may
not be as high as farther to the south, warm rain processes could
still provide 1-1.5"/hr rates in these areas Thursday afternoon and
into Thursday night. A Slight Risk remains is in place for these
regions with locally significant flash flooding possible,
particularly for areas closer to the Poconos, higher elevations of
southern New England.

Northern Mid-Atlantic: Farther south, while the Mid-Atlantic
region is a little farther removed from the jet-streak dynamics to
the north, the region is closer to the deepening surface low with
strong surface-925mb southerly theta-e advection directed into the
surface front. From northern NJ and southern PA on south to
northern VA, a tropical air-mass (depicted by 2-2.25" PWATs and
low-mid 70s dew points) will coincide within a warm sector that
becomes increasingly unstable (MUCAPE between 1,000-2,000 J/kg). It is
worth noting that SST anomalies along the Mid-Atlantic and
Southeast coasts are well above normal too, acting as an abnormally
rich moisture source for southerly low-level flow to draw from. By
Thursday afternoon, low-level WAA into the front will support
storms enveloping the region with additional storms from the
Appalachians tracking east. Modest vertical wind shear aloft will
also support organized clusters of storms, some of which will have
the potential to backbuild and train over the I-95 corridor into
Thursday night. Latest 12Z HREF guidance shows 24-hr probabilities
for >3" of rainfall are high (>70%) from the DC/Baltimore area on
north through northeast MD, southeast PA, and into southern NJ.
Just as concerning is this same region sports 30-60% chances for
>5" of rainfall.

Impact potential for Moderate Risk Area: These probabilities
listed above serve as signals in ensemble guidance that highlight
the potential for significant flash flooding throughout the country,
let alone over a highly populated urban corridor. Given the
atmospheric parameters in place, it is possible that the Mid-
Atlantic I-95 corridor witnesses thunderstorms that produce
rainfall rates up to 3"/hr, with perhaps the most intense storms
potentially generating 2" rainfall totals in as little as 30
minutes. QPF may seem light at first glance given the hourly
rainfall rate potential, but this is largely due to lingering
uncertainty in the placement of the heaviest rainfall. Many
individual convective models show localized totals surpassing 5"
with even some localized maxima approaching 8". Residents in the
Mid-Atlantic should ensure they have reliable means of receiving
warnings issued by their local WFOs throughout the day and into
Thursday night.

...Gulf Coast into Ohio Valley...

Slow moving thunderstorms will persist today and tonight over a
vast area south of the cold front. High PWs and CAPE and
convective coverage will drive a localized flash flood risk. A
lack of organization may keep FFG exceedance coverage down, but
high rainfall rates will likely locally approach or exceed FFG.
This region is covered by a Marginal Risk for excessive rainfall
and isolated flash flooding.

...Rockies into the Plains...

Scattered thunderstorms expected to fire up from eastern
Arizona/New Mexico northward to Montana. In general, the storms
will likely lack organization for a more elevated threat for flash
flooding. A Marginal Risk is the appropriate threat level at this time.

Mullinax/Campbell


Day 2  Valid 12Z Fri Aug 01 2025 - 12Z Sat Aug 02 2025

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

...Southeast and Mid-Atlantic...

During this period a cold front will be dropping south through the
region which will trigger a more organized corridor of convection
Friday afternoon. Meanwhile both terrain driven and coastal
convergence driven convection are also likely by afternoon. This
combination will result in good convective coverage, which will
ultimately likely lead to a decent degree of cell merger activity.
Much of the area will have PW values nearing 2.5 inches along with
abundant instability. PWs are forecast as high as 2.3"-2.5", with
plentiful instability. Very heavy rainfall rates are expected in
this thermodynamic environment, which combined with the likelihood
of cell mergers, should drive an isolated to scattered flash flood risk.

...Lower MS Valley and Vicinity...

Convection will fire along the cold front as is continues to drop
south during this period. Pooled PW values exceeding 2 inches will
aid in the development and enhancement of heavy rainfall across the
region. Large scale forcing and moisture transport is weaker here,
so not expecting widespread heavy rainfall...but localized heavy
rainfall rates should be able to drive an isolated flash flood risk.

...Rockies and Plains...

Scattered convection is expected to span from Montana to New Mexico
into Friday. The most favorable environment for heavy rainfall will
likely setup from Montana into the Dakotas, which is where mid
level troughing will be paired with the highest PW anomalies in
the CONUS...near early August peak values. Combine this with CAPE
forecast around 2000 j/kg and the environment appears conducive to
areas of localized flash flooding. Hourly rainfall rates up to
1.25-1.5 inches/hour possible over the Northern Rockies/Montana and
up to 2 inches/hour across the Dakotas.

Campbell/Chenard


Day 3  Valid 12Z Sat Aug 02 2025 - 12Z Sun Aug 03 2025

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

...South and Southeast...

The strong front mentioned will continue pressing southward,
draped from the Southeast westward across the Deep South, Lower
Mississippi Valley and over Texas. High moisture content and
instability along and ahead of the front interacting with the right
entrance region of the upper jet will promote heavy to excessive
rainfall in thunderstorms that could cause localized flash
flooding. Very heavy rainfall rates are expected and should drive
an isolated to scattered flash flood risk. A Slight Risk remains in
effect for portions of South Carolina, southern Georgia and extreme
northeast Florida. The front stretching west across the Gulf Coast
states in a moist and unstable environment could promote scattered
diurnal thunderstorms into Saturday thus maintaining the Marginal Risk.

...Rockies and Plains...

Shortwaves rounding the eastern edge of the upper ridge in the
western U.S. is forecast to interact with the same front that bends
back across the Great Plains, where rounds of rain and
thunderstorms are likely with possibly multiple MCSs. High rain
rates should be supported at times with the above normal moisture
and instability with some areas possibly seeing repeated rounds of
storms as the front begins to lift as a warm front. The region is
covered by a Marginal Risk. Some lingering monsoonal moisture will
keep convection possible around southeastern New Mexico resulting
in isolated flooding concerns over sensitive burn scars.

Campbell
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