046
FOUS30 KWBC 301953
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
353 PM EDT Wed Jul 30 2025

Day 1
Valid 16Z Wed Jul 30 2025 - 12Z Thu Jul 31 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MIDDLE MISSISSIPPI VALLEY AND FOR PORTIONS OF THE FRONT RANGE
AND ADJACENT HIGH PLAINS...

...Mid MS Valley...

As anticipated from overnight, the MCS held together and has
remained progressive as it tracked across the Mississippi River
this morning. With the MCS and cold front having tracked through
WI/IA, have trimmed the Slight Risk out of the region but extended
it a little farther east into southern Michigan based on 12Z HREF
guidance focusing low chance probabilities (20-30%) for localized
rainfall totals >5". Otherwise, have maintained the Marginal Risk
back through the Iowa and extending it as far west as southeast
SD. This is due to the approach of a remnant MCV that is catching
the eyes of some CAMs (HRRR/ARW) with locally heavy amounts this
evening and into tonight. It is possible this MCV works its way
east into Iowa by early tomorrow morning, posing a low chance for
additional heavy rainfall and flash flooding late tonight and into
early Thursday morning.

...Southern Appalachians...

A Slight Risk was introduced this cycle given the region is mired
in highly anomalous PWATs and visible satellite shows strong
surface based heating is underway. The 12Z RNK sounding, located
just north of the Slight Risk area, measured a 1.80" PWAT and
MLCAPE >600 J/kg with low-mid level RH values averaging close to
85%. There was also very little CIN present and the convective temp
was 80F, which is likely to be met before 16Z. PWATs are likely to
range between 1.7-1.9" this afternoon, which is above the 97.5
climatological percentile, and RAP forecasts suggest as much as
1,000 J/kg of MLCAPE could be available this afternoon. Steering
winds are light and vertical wind shear is weak, but torrential
downpours with maximum rainfall rates up to 2.5"/hr are possible in
complex terrain sporting saturated soils.

Outside of minor tweaks to the inherited Marginal and Slight Risks
areas, the overnight forecast discussion details largely remain on
track.

Mullinax


---Previous Discussion---

...Mid MS Valley...
An eastward moving squall line will be ongoing at 12z this morning
across portions of eastern IA into northern MO,and given the
downstream instability axis do expect this to generally survive
across much of central and northern IL into early this
afternoon. The progressive nature of this convection should limit
the extent and magnitude of flash flooding, however hourly rainfall
of 1-2" will remain likely. This will be enough to drive at least
an isolated flash flood risk as convection moves eastward. This
MCS/MCV will help drive a cold front south across MO and KS, and do
expect convection to expand over these areas by afternoon. A bit
unclear whether it takes the form of a progressive squall line
(limiting the flash flood risk), or if more discrete cell formation
allows for some training/cell merging.

The overall environment...a slowly southward shifting cold front
helping focus convergence, the right entrance region of the upper
jet adding some divergence aloft, plentiful CAPE near the front and
PWs locally over 2"...certainly supports areas of excessive
rainfall. At a minimum, isolated instances of flash flooding can
be expected across portions of KS/MO/IA/IL/IN. The magnitude and
coverage of this threat will come down to convective mode and
evolution today/tonight. As mentioned above, convective mode
generally favoring more progressive squall lines may end up
keeping the risk isolated. However if we are able to get any
recovery after this initial round of convection the MCV and
favorable synoptic and moisture ingredients will remain in place.
Thus, higher end impacts are still possible in this setup, but far
from guaranteed given the questions that remain regarding
convective mode/evolution and instability persistence.

...Southwest and High Plains...
Isolated to scattered convection will likely develop near areas of
terrain from NM into the Rockies. Easterly low level flow into the
Front Range of CO will likely result in slow cell motions and the
possibility of at least isolated flash flooding. Increased
convergence near the front could support a bit more convective
coverage and eventual organization from portions of eastern NM into the
OK and TX Panhandles as well. Only change to the inherited Slight
risk was to expand it northward into WY where both the HREF and
REFS show an uptick in 1" per hour probabilities by this evening as
convective coverage increases and a few cell mergers become
likely.

...Elsewhere...
A broad Marginal risk extends from the central Gulf Coast all the
way into portions of southern NY. High PWs and CAPE and convective
coverage will drive the risk from the Gulf Coast into the TN
valley. A lack of organization may keep FFG exceedance coverage
down, but high rainfall rates will likely locally approach or
exceed FFG. A cold front will drive the risk form portions of WV
into southern NY. Convective coverage and organization should be
lacking...however with the front slowing cells should be slow
moving and could drop a quick 1-2" of rain. Likely enough to drive
at least a localized urban flash flood risk.

Chenard

Day 2
Valid 12Z 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...

An exceptionally strong 250mb jet streak over southeast Canada is
placing its diffluent right-entrance region over the Northeast on
Thursday will provide act as an excellent source for thunderstorms
to not only develop but flourish in an environment that is loaded
with highly anomalous moisture content. A potent 500mb vorticity
max tracking east will instill its own source of strong lift,
prompting a surface low to develop along a frontal boundary that
sets up over the northern Mid-Atlantic. There are two areas of
concern regarding heavy rainfall--

Northern PA to Southern New England: The first area is from
northeast PA on east through the Lower Hudson Valley and into
southern New England. Here, there is likely to be a narrow corridor
of 850-500mb FGEN that supports 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 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 tomorrow and
into Thursday night.

...Other Risk Areas...

Aside from some minor adjustments to the Marginal Risk area based
on the latest WPC QPF forecast, the forecast remains on track. The
Southern Rockies do bear watching, but potential Slight Risk
upgrades will be also determined upon how sensitive soils are
following today`s thunderstorm activity.

Mullinax

---Previous Discussion---

...Gulf Coast into OH Valley...
Another day of slow moving pulse thunderstorms is expected over a
large swath of the region to the south of the cold front. Similar
to Wednesday, 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.

...Rockies into the Plains...
Scattered convection is likely from eastern AZ into NM northward
all the way into MT. Currently not seeing a strong indication of
organization that would lead to Slight risk level flash flood
coverage. However localized flash flooding will again be possible
over this broad region. There are some height falls beginning to
approach from the west, so that could perhaps end up driving a
more organized risk somewhere within this broad Marginal risk. But
at the moment not seeing enough of a signal in the global models
or CAMs to go with any Slight risk areas.

Chenard

Day 3
Valid 12Z Fri Aug 01 2025 - 12Z Sat Aug 02 2025

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

Minor tweaks to the Marginal and Slight Risks areas were made based
on new WPC QPF and 12Z guidance. Overall, no significant
adjustments were made and the forecast remains on track. Will keep
an eye on the northern Rockies and central Rockies in future
forecasts should any need for Slight Risk upgrades be necessary.

Mullinax

---Previous Discussion---

...Southeast and Mid-Atlantic...
A Slight risk was maintained across much of the carolinas. Likely
to have a well defined cold front dropping south over NC that will
help 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. 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.

There is still some degree of uncertainty with the details by
Friday. The 00z ECMWF is an outlier really hanging the front and
better upper level support well to the north. Thus it keeps a heavy
rainfall risk over NJ into NYC and less of a concentrated risk over
the Carolinas. This is not supported by the GFS/UKMET/CMC...and
the 00z ECMWF AIFS ensemble also looks more in line with the
GFS/GEFS. Thus we will continue to lean the EROs to be more in line
with the non ECMWF solution. However we will need to continue to
monitor trends going forward.

...Lower MS Valley and Vicinity...
A cold front should make it all the way into portions of TX/LA and
MS by Friday afternoon. This should act as a focus for convective
development, and with a large pool of PWs over 2", heavy rainfall
rates are likely. 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...
Convection is likely from NM into MT Friday. The best environment
for heavy rainfall will probably be across the northern end of
this risk area, centered across MT and vicinity. More in the way
of mid level troughing will be over this corridor, to go along
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 flash flooding.
However none of the 00z global models are all that aggressive with
QPF output, and the one CAM that goes this far (the 00z RRFS)
suggests a more progressive convective mode. Thus we will stick
with a Marginal risk and continue to monitor trends.

Chenard

Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt