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FOUS30 KWBC 291605
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1205 PM EDT Tue Jul 29 2025

Day 1
Valid 16Z 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...

...1600Z Update...

...Central and Northern Plains...

12Z surface analysis depicts a stationary front extending across
southern IA/NE and into the central High Plains which is forecast
to remain generally within the same vicinity this
afternoon/evening. Post-frontal upslope flow across northeastern
Colorado/southeastern Wyoming/and the Nebraska Panhandle should
encourage initial afternoon storm development before an increasing
low level jet into the early evening hours encourages and supports
more widespread development with hi-res guidance suggesting upscale
development into an MCS. Strong to extreme MLCAPE values upwards
of 4000 J/KG along the boundary from central Nebraska eastwards
into the Middle Missouri Valley as well as PWATs 1.5 to 2 standard
deviations above the mean will support intense thunderstorms
capable of heavy rainfall with 12Z hi-res guidance suggesting
rainfall rates of 2" per hour, possibly as high as 2.5-3" per hour,
are possible. Initial convection along the High Plains of Colorado
in the Palmer Divide region will also be supportive of heavy
rainfall given anomalously high dew points across the region.
Further to the northwest, hi- res guidance also supports a
secondary area of additional development with upslope flow along
the Rockies in the northern High Plains of northern
Wyoming/southeastern Montana, with the potential of upscale growth
into a second MCS traversing east- southeastward into South Dakota
and areas of the Nebraska Panhandle/northern Nebraska. Similarly
high PWATs will again support heavy rainfall with the latest hi-res
guidance supportive of locally higher totals of 2-3" and some
initially more scattered instances of flash flooding.

In addition to the expected MCSs, the noted low-level jet, high
CAPE, etc. extending eastward along the boundary looks to also
support additional thunderstorm development ahead of the
approaching MCSs, increasing the chances of heavy rainfall totals
overnight between these initial storms as well as the passage of
the MCS, especially for areas north/east of the Sand Hills of
central Nebraska into the Upper/Middle Missouri Valley. Updated hi-
res guidance supports the potential of totals of 2-4", locally as
high as 5", with moderate HREF probabilities (40-70%) of exceeding
3" and low-end probabilities (10-30%) of exceeding 5". This
continues to support higher-end Slight Risk probabilities for the
corridor.


Elsewhere for the Southwest and the Southeast, the previous
forecast reasoning remains on track. In New Mexico/far west Texas,
concerns remain with ongoing cloud cover and the lack of
destabilization later to support more potent thunderstorms/locally
heavy downpours. However, the latest hi-res guidance continues to
suggest the potential of totals of around 1-2.5", especially in
vicinity of the Sacramento Mountains where areal burn scars remain
particularly sensitive to additional rainfall. Therefore, the
Slight Risk was maintained for now.

Putnam

...Previous Discussion...

...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

Day 3
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.

There is quite a bit of model spread with the details of this
convective risk. The driver of most of this spread appears to be a
shortwave/MCV that intensifies associated with the NE/IA
convection Tuesday night into Wednesday. This feature then tracks
eastward and enhances lift over the Mid-Atlantic and Northeast.
Some models have a stronger more well defined feature, which then
helps keep the front a bit farther north resulting in a farther
north QPF axis. At the moment it does seem like the 00z NAM and 12z
ECMWF are a bit too amplified with this feature...thus find it
unlikely that the heaviest QPF swath will be as far north as they
depict. The 00z ECMWF came south a bit, but probably still too far
north. Both the 00z ECMWF AIFS and 18z NOAA EAGLE machine learning
ensembles show highest probabilities closer to the I95 corridor,
generally from DC to PHL to NYC and into southern New England. This
also seems to fit the expected placement of higher instability/PW
overlap. Also will note that the 00z RRFS has a very similar axis
as well. Thus the Slight risk will remain positioned across this
corridor, although confidence on this exact placement remains
average at best given some of the run to run differences we are
seeing.

Wherever the event does focus there is certainly a potential for
higher end impacts. PWs near the slow moving front will be over 2"
over a broad area, with embedded values likely getting towards
2.3". We should also have plenty of instability to work with, which
combined with the high PWs, should drive heavy rainfall rates.
Combine that with some training potential near the slow moving
front, and can not rule out the possibility of eventually needing a
Moderate risk...especially if the more sensitive Urban corridor
remains a focus going forward.

...Gulf Coast into OH Valley...
Another day of slow moving pulse thunderstorms is expected over a
large swath of the region. 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 enough confidence to go with any Slight risk
areas for this region.

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