FOUS30 KWBC 300845
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
445 AM EDT Wed Jul 30 2025

Day 1
Valid 12Z 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...
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 SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID-ATLANTIC INTO SOUTHERN NEW ENGLAND...

...Mid-Atlantic into New England...
Heavy rainfall and at least some flash flooding appears likely
Thursday across portions of the Mid-atlantic and possibly into
southern New England. Convection is likely to focus along a slow
moving cold front, with broad mid level troughing and height falls
providing large scale ascent. The upper level jet is displaced
pretty far north, however still likely to get some enhanced upper
level divergence into the Mid-Atlantic from this feature.

Still quite a bit of uncertainty with the details of this 
convective risk. Some of this uncertainty is tied to the evolution
of the shortwave/MCV associated with the ongoing convection over
IA/IL this morning. The strength of that feature and how it
interacts with the front and developing longwave troughing will
likely impact the frontal timing and both the axis and magnitude 
of the flash flood risk. The front should become more progressive 
by Thursday evening, but how things evolve before then are 
unclear. There is a concern that we could see cell development 
Thursday afternoon just ahead of the front as southerly flow 
increases which could set the stage for a fair amount of cell 
merger activity driving up rainfall totals. Also it looks like an 
area of low pressure tries to spin up along the front Thursday 
night with an inverted trough axis extending northward. The ECMWF 
and 3km NAM keep this troughing inland, which could be a pretty 
ideal setup for low topped warm rain convection through the 
overnight. However, most other models push this troughing offshore 
and clear things out by the overnight. 

The environment (slow moving front, increasing large scale 
forcing, PWs over 2" and plentiful instability) supports excessive
rainfall...its just going to come down to location and duration,
both of which remain uncertain. The current consensus among the 
models still supports a corridor from DC to PHL to NYC (including 
much of eastern PA and NJ) as the areas most likely to be 
impacted. The 00z experimental REFS ensemble neighborhood 
probabilities of exceeding 5" are 40-70% across this corridor. The 
REFS can run high with its QPF output, and may be under 
dispersive...but nonetheless still a signal worth watching. We gave
some consideration to a MDT risk upgrade this cycle, but after 
collaboration with local WFOs we opted to maintain a Slight risk 
given some of the lingering questions on how the details of the 
event will evolve. As the event comes more into range of the HREF 
today and tonight we will hopefully gain more confidence on these 
details, and a MDT risk upgrade is still a possibility with later 
updates. For now this is still considered a higher end Slight risk 
from DC to eastern PA, NJ and NYC, and locally significant impacts 
could evolve.

...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...

...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
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