FOUS30 KWBC 281946
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
346 PM EDT Mon Jul 28 2025

Day 1
Valid 16Z 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...

16Z update... Some eastward nudge of the Slight Risk further into
Wisconsin and Iowa was made to prefect the latest guidance and WPC
trend. The best focus for higher accumulations and rainfall rates
continue to be across southern Minnesota and the surrounding areas.

Campbell

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...

16Z update... The Marginal Risk was expanded southeast toward the
Big Bend area of West Texas. Abundant mid-level moisture will be
present during this period and some of the CAMs are hinting at
thunderstorm development will take place this afternoon and
evening.

Campbell

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 AND FOR PORTIONS OF THE SOUTHWEST...

...Central and Northern Plains...

21Z update... The trend of shifting the QPF footprint further east
across the central U.S. persisted with the most recent run of
guidance. Portions of southern Iowa, northwest Missouri and
northern Kansas saw an uptick in coverage and amounts for this
period. As such the Slight Risk area was expanded on its eastern
boundary to cover this part of the region. Hourly rain rates of 
1-2 inches/hour still expected to pass through the Plains toward 
the Midwest.

Campbell

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...

21Z update... The signal for heavy rainfall increased for portions
of New Mexico and far southwest Arizona for this period. Much of
this part of the region is sensitive to higher rainfall
intensities/amounts. In coordination with the local forecast 
offices concern was raised for not only recent burn scars but also 
the Sangre de Cristo Mountains and possibly the Gila region of 
southwest NM. A Slight Risk was raised for this period to cover 
this increased threat for local flash flooding potential. 

Campbell

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
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