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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