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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FOUS30 KWBC 290814
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
414 AM EDT Tue Jul 29 2025

Day 1
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...
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,
generally 1.5 to 2 SD above late July normals.

There remains some latitudinal uncertainty with the axis of
convection during this period. The trend amongst the 00z high res
guidance is for two convective clusters...one moving across
southern SD, and another from NE into IA. The Slight risk was made
broader to account for this potential. Still think the greatest
rainfall magnitudes are more likely to occur with the southern
swath across NE into IA, where instability and surface convergence
will be higher. EAS probabilities from both the HREF and REFS
indicate the best coverage of 2"+ rainfall to focus over portions
of eastern NE into western IA. Both of these ensemble suites also
indicate a 30-50% chance of exceeding the 3hr FFG across this same
corridor. Thus higher end Slight risk probabilities exist from
eastern NE into western IA where some training of convection may
result in a greater flash flood risk Tuesday evening/night.

...Southwest...
Scattered to numerous areas of thunderstorms have persisted Monday evening/night across central and southern NM. A well defined
shortwave/MCV will likely persist into the day today. While this
feature should act as a forcing mechanism today, abundant cloud
cover associated with this feature may help reduce convective coverage/intensity.

However, confidence remains higher on a flash flood risk today
over the Sacramento Mountains and vicinity. Probabilities from both
the 00z HREF and REFS show more persistence of high 1"/hr
exceedance today...with elevated probabilities at least from
19z-22z. There is some hope that enough cloud cover persists that
we are not able to destabilize enough for maintained deep
convection. However most indications suggest we do see a gradual destabilization take place by late morning into the early afternoon
over southeast NM. If this occurs then flash flooding near the
Sacramento Mountains appears likely...and significant impacts are
possible near area burn scars.

Confidence is lower over the rest of NM. The 00z CAMS really
struggle to build instability back after the widespread convection
of Monday night. Thus the coverage and intensity of convection is
muted compared to Monday. Since day shift just introduced the
Slight risk we will leave it for this cycle given the uncertainty.
It very well may not verify outside of the aforementioned
Sacramento Mountain region...however with the remnant MCV and PWs
similar or slightly higher than Monday, if we are able to
destabilize then Slight risk level coverage of flash flooding could
still materialize.

...Southeast...
Expecting greater convective coverage today 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 and chaotic cell motions (weak mean
winds) that some cell mergers along outflows will be a
possibility. PWs in the region are forecast between 2" and 2.4",
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. Both the
HREF and REFS have 3"+ neighborhood probabilities in the 40-70%
range over most of the Marginal risk area, however the spatial
coverage of these amounts is quite small. Thus while localized
areas of flash flooding are likely today, not currently seeing
enough coverage to justify a Slight risk at this point.

Chenard


Day 2
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...
At 12z Wednesday it seems likely that we will have some degree of
organized convection ongoing in the vicinity of IA. 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.

There is actually quite a bit of uncertainty with the convective
details Wednesday into Wednesday night. There has been a subtle
northward shift in the forecast MCV location, seemingly now
tracking across IA into northern IL. However the northern extent
of the excessive rainfall risk will likely be capped by a lack of
instability well north of the cold front. The 00z ECENS and AIFS
ensembles are focusing the best rainfall potential from eastern IA
into northern IL. This seems reasonable, although the magnitudes
remain in question and are dependent on the degree of instability
in place. The better instability may actually reside over portions
of central MO into central IL, and so if convection is able to
organize this far south then a greater flash flood risk could
evolve. All this to say that confidence is low, and thus a broad
Slight risk remains to encompass the various possible scenarios.
Higher end impacts are possible in this setup, but far from
guaranteed given the questions that remain regarding the MCV
track/intensity and instability in place.

...Southwest and High Plains...
Isolated to scattered convection will likely develop near areas of
terrain from NM into the Rockies. Day shift introduced a Slight
risk for this activity, and that still seems reasonable. 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. Guidance is coming into better agreement that some
upscale growth and eastward propagation into northeast NM,
southeast CO and the OK Panhandle is probable as well, focusing
near the cold front.

...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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