FOUS30 KWBC 280813
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
413 AM EDT Mon Jul 28 2025

Day 1
Valid 12Z Mon Jul 28 2025 - 12Z Tue Jul 29 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE NORTHERN PLAINS AND UPPER MISSISSIPPI VALLEY..

...Northern Plains to Upper MS Valley...
Convection will move across portions of eastern MT and into the
western Dakotas this morning. Heavy rainfall rates with this
activity will pose a localized flash flood risk, however generally
quick cell motions should limit the extent of the threat. As we
head into the afternoon and evening hours this convection will 
grow upscale into an MCS as it moves across central SD into 
southern MN and northern IA. A forward propagating derecho is 
likely given the environmental ingredients in place and per SPC. 
This fast forward motion will likely cap the extent of any flash 
flood risk at the Slight level. 

The impressive CAPE and PWs around 2" near and just downstream of 
the MCS will support heavy rainfall rates. As the MCS matures the 
heaviest rainfall totals may end up near the developing comma 
head/bookend vortex where rainfall duration will be locally longer.
The 00z HREF supports total rainfall over 2", with a narrow axis 
of over 3" probable as well. Portions of eastern SD into southwest
MN received heavy rainfall this past night...and so if an 
additional 2-3" falls over these areas then isolated to scattered 
flash flooding is likely. Elsewhere, the threat is likely an urban 
risk driven by hourly rainfall locally as high as 2" as the MCS 
moves through.

...Southwest..
Another day of scattered afternoon convection expected across much
of NM into far southeast AZ. Both PWs and CAPE are forecast to be
higher today compared to yesterday, and so do anticipate a bit 
more convective coverage today. Activity will initially develop 
near areas of terrain, such as the Sacramento Mountains and Sangre
De Cristos. Given the moisture and instability in place...at least
an isolated flash flood threat is likely on/near these higher 
terrain areas, especially over the more sensitive burn scars. Cells
may tend to be transient today...either moving off to the west 
and/or dissipating fairly quickly, which may keep the flash flood 
coverage isolated in nature. Convection could actually persist into
the overnight hours over portions of southern NM...but the extent 
of instability by this time is unclear, and so rainfall rates 
should be tempered by this time. Overall this is a solid Marginal 
risk with isolated flash flooding likely, but at the moment think 
the risk falls just shy of Slight risk level coverage.

...OH Valley into the Carolinas...
Isolated to scattered thunderstorms are expected today across a
broad area stretching from KY/TN to southern MI and all the way to
the coastal Carolinas. Generally not expecting much organization 
to convection...but PWs of around 2" (or higher) will continue to 
support heavy rainfall rates. Localized rainfall of 2-3" are 
possible within the Marginal risk area...and while the coverage of
these amounts should be small...where it does fall some flash 
flood impacts are possible within areas of lowered FFG. The most 
concentrated probabilities in the HREF are actually over the 
coastal Carolinas, where a convergence axis should drive more 
convective coverage and some totals locally over 3". However with 
higher FFG over these areas the flash flood risk is still expected 
to stay isolated.

Chenard


Day 2
Valid 12Z Tue Jul 29 2025 - 12Z Wed Jul 30 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL AND NORTHERN PLAINS...

...Central and Northern Plains...
A stationary front will likely extend across southern IA/NE and
into the High Plains on Tuesday afternoon/evening. Easterly flow to
the north of this front will likely help trigger convective
development over the High Plains as shortwave energy moves out of
the Rockies and upper level divergence increases in the right
entrance region of an upper jet. This activity will likely push
east into the overnight hours across NE and eventually IA 
along/north of the stationary front. Upscale development into an 
MCS appears probable driving a heavy rainfall threat. Any eventual 
MCS will probably propagate off to the east fairly quickly, however
additional development downstream over NE/IA is possible given the
convergence near the front and what should be CAPE around 4000 
j/kg. PWs should be plenty high enough for heavy rainfall, ranging 
from 1.5" in the High Plains to locally over 2" from eastern NE into IA.

There remains some latitudinal uncertainty with the axis of 
convection during this period...although the general trend has 
been for a slightly farther south axis. There is also some question
as to how far east the risk exists, with the deterministic and 
ensemble EC and the 00z RRFS/REFS favoring NE, but the GFS and 
AIFS extending the threat into IA. Given a favorable track record 
for the AIFS and the presence of the boundary/instability/PWs over 
IA, we did want to make sure that the Slight risk at least covered 
into central IA. 

...Southwest...
Another day of scattered afternoon convection expected Tuesday.
Probabilities from both the 00z HREF and REFS show a bit more
persistence of high 1"/hr exceedance over the Sacramento Mountains
and vicinity. Thus the flash flood risk over these areas, and the 
susceptible burn scars, may very well be higher Tuesday than 
Monday. However there are some questions regarding the coverage of
heavier rainfall amounts away from these terrain areas, as the 
overall coverage could be a bit less than Monday. Given the small 
extent of the higher risk at the moment and the uncertainty on the
broader coverage, we will leave the ERO at a Marginal level 
pending what happens Monday. Although locally significant impacts
are possible within/near the Sacramento Mountain burn scars.

...Southeast...
Expecting greater convective coverage on Tuesday from the Southeast
into the southern Appalachians. Not much in the way of forcing to
organize convection, and generally expecting pulse activity that 
does not last all that long at any one location. This will act as 
a limiting factor for flash flooding...however there should be 
enough convective coverage that some cell mergers along outflows 
will be a possibility. PWs in the region are forecast between 2" 
and 2.3", with CAPE over 2000 j/kg...so the thermodynamic 
environment will support heavy rainfall rates. Thus even with a 
relatively short duration of rain at any one location, rainfall 
totals locally over 3" appear probable driving a localized flash flood risk.

Chenard


Day 3
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...

...Mid MS Valley...
At 12z Wednesday it seems likely that we will have some degree of 
organized convection ongoing in the vicinity of IA, although at 
this lead time getting these specifics right is tricky. 
Nonetheless, it appears probable that a better defined 
shortwave/MCV will be associated with this convection and slowly 
move east across IA/MO/IL and vicinity Wednesday into Wednesday 
night. Meanwhile we will have a slowly southward shifting cold 
front helping focus convergence, and the right entrance region of 
the upper jet adding some divergence aloft. With plentiful CAPE 
near the front and PWs locally over 2", the ingredients for areas 
of excessive rainfall are in place. Some uncertainty remains on the
details...but 00z models are coming into a better consensus on the
favored axis for heavy rainfall..generally stretching across 
portions of IA, northern MO and into central/northern IL and 
possibly portions of IN. 

...Southwest and High Plains...
Isolated to scattered convection will likely develop near areas of
terrain from NM into the Rockies. Generally not looking like the
greatest forcing, so the degree of convective organization remains
unclear. At the moment most models indicate mostly
isolated/scattered development, and generally not enough coverage
for anything more than a Marginal risk. However there is some
potential for a few organized clusters to move off the terrain 
into the High Plains, and so can not rule a targeted Slight risk or
two as we get closer in time and have more high res guidance to interrogate.

...Elsewhere...
A broad Marginal risk extends from the central Gulf Coast all the 
way into portions of New England. 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 the 
OH valley into New England. Generally quick cell motions and the 
lack of a significant QPF footprint in the 00z models keep this 
risk at a Marginal level.

Chenard
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