FOUS30 KWBC 220809
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
409 AM EDT Tue Jul 22 2025

Day 1
Valid 12Z Tue Jul 22 2025 - 12Z Wed Jul 23 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE UPPER MISSISSIPPI VALLEY, SOUTHWEST AND SOUTHEAST...

..Northern Plains/Upper Midwest...

Regional radar mosaic across the Northern Plains depicts a
migrating cluster of thunderstorms over the Dakotas with the proxy
of the convective complex along the ND/SD border. Mean storm
motions continue to point to the complex of thunderstorms migrating
east-northeast with heavy rainfall likely to be impacting portions
of eastern ND and far northeast SD into MN by the turn of the new
D1. This complex will likely draw some attention for flash flooding
by itself, but the environment will be somewhat favorable for the
first half of the D1, so the threat will likely be more isolated
with the best chance occurring in any urbanized settings. Majority
of this heavy precip will likely fall north of I-94 in MN with the
Minneapolis metro unlikely to see much of any impacts. The period
becomes more favorable for heavy rain prospects as we move into the
second half of the forecast as a stationary front currently 
analyzed over IA/IL into SD will lift northward through the
afternoon with an appreciable advection of deep layer moisture
priming the environment for the following evening. Surface low over
the High Plains will slowly advance through SD during the forecast
period with a cold front dropping steadily out of Canada with eyes
on the Upper Midwest by the end of the D1. The proxy of the warm
front and advancing cold front will lead to a tight window of
enhanced sfc-850mb convergence within a formidable pool of moist,
unstable air positioned across the eastern Dakotas through much of
MN/WI. PWATs between +2 and +3 deviations will be present over the
aforementioned areas with values >2" likely as we move beyond 00z
Wed. LLJ initiation across the Upper Mississippi Valley will help
trigger an area of heavy convection across the Dakotas which will
eventually migrate eastward through the Red River basin into the
northern half of MN. 

As of this juncture, the greatest threat for heavy rainfall >2" is
likely over southeast ND, far northeast SD, and adjacent areas of
northwestern MN with an eastward expansion over the central portion
of MN (St. Cloud area). 00z HREF and REFS neighborhood
probabilities for >2" are between 80-90+% for these locations with
>3" (60-80%) and >5" (20-35%) well within a reasonable depiction
for a widespread flash flood threat in that area of the northern
CONUS. HREF EAS probabilities for >1" are also robust with 70-90%
for >1" and 40-60% for >2" situated over those same areas, a
testament to the growing consensus within the CAMs window as we
move closer to the event. This aligns very well with the EC AIFS
Ensemble mean QPF output of 1.25-1.5" in that corridor, a very
strong output when you take into account the traditional low bias
from the AIFS in general. 

The FFG's located across central MN are small deterrent for any
appreciable risk upgrades as the threat targets places where
1/3/6hr indices remain a little high compared to areas a bit
downstream and further south. Despite those indices, multiple flash
flood warnings are increasingly likely over southeast ND, northeast
SD, and central MN as we step through the evening, especially with
a growing threat of training and back-building as proposed by the
weakening Corfidi Upshear vectors in the CAMs as we move into the
evening. This makes sense as the mean flow becomes parallel to the
boundary with the approach of the front to the north. 

There is some chance this setup nudges north as the mid-level ridge
to the south will truly flex and the warm front progression should
migrate pretty rapidly to the north through the afternoon and
evening, especially with the added nose of LLJ structure positioned
to the south. Where this ends up is the ultimate delineation point
of where training could occur and have the highest prospects for
flash flooding. For now, a high end SLGT is forecast over that
corridor positioned over ND/SD/northwest MN with an eastward
extension through central MN prior to the Arrowhead. Pending
forward propagation speed of the nocturnal convection, the risk
could evolve further downstream prior to the end of the D1 period,
so it will be something we will monitor closely. In any case, the
SLGT risk still runs through portions of the Arrowhead into
northwestern WI back into those favorable areas to the west.

...Southeast...

Elevated PWATs and instability across the Southeastern CONUS will
maintain a posture of widely scattered thunderstorms capable of
locally enhanced rainfall rates between 2-4"/hr that could spur
flash flood prospects through the course of the daytime hrs. A 
weak surface reflection along the SC coast with a weak quasi-
stationary front aligned west to east across the Southeast will 
lead to enhanced regional convergence that could allow for slow-
moving heavy thunderstorms to train over any area in proxy to these
features for a few hrs. These are the types of setups that can 
provide a sneaky 4-7" maxima in any given location, especially 
near the low pressure center along the Carolina coastal plain. 
Surface trough extending off the southern flank of the low will  
bisect southeast GA down through northern FL with guidance 
consistently pinning a more defined heavy precip axis within the 
confines of this area. FFG's remain very high across that specific 
area, so the threat for widespread flash flooding is very low, 
especially when considering the probabilities for >5" are still 
running <25% for a majority of that region, a signal necessary for
more appreciable impacts to occur in these zones. 00z HREF
neighborhood probs for >5" are higher than previous forecasts for
the coastal Carolina's with the max probs (30-45%) located from
Morehead City, NC down through Charleston, SC. Multiple population
centers are encompassed in that zone, the most likely locations to
see any flash flood prospects due to urbanization factors. EAS
probabilities for >1" are also pretty elevated over the Upper SC
coast (>50%) with a modest output over the area referenced above.
The synoptic and thermodynamic evolution support a strong coastal
convergence pattern, especially with the afternoon sea breeze 
during the life cycle of the surface low, so the upgrade to SLGT 
from the last forecast update maintains merit. Considering the 
current signals, there was no reason to deviate from the previous 
forecast, so the SLGT risk remains due to continuity.

...Southwest...

Another period of monsoonal type convection will enhance localized
flash flood prospects across eastern AZ and much of NM/CO.
Persistent moisture advection along the western flank of a ridge
positioned across the Mississippi Valley will be the driver for the
threat as scattered to widespread showers and thunderstorms will 
litter the Southwestern CONUS up into the Central Rockies by 
this afternoon, carrying through the evening hrs. Any area within 
the confines of the above regions will have a threat for impacts 
due to convective cores likely producing rates >1"/hr with upwards 
of 2"/hr in the strongest cells. Areal QPF average is generally 
between 0.25-0.75", but neighborhood probabilities for >1" over 
eastern AZ into NM/CO are well above 70% with >2" probs running 
between 40-70% over the more terrain favored areas in both 
locations. The most prominent signal for rainfall eclipsing 1"
aligns within the Sangre de Cristos and the eastern side of the
Mogollon Rim near the AZ/NM line. These are the most likely
locations to see heavy rainfall and heightened flash flood
prospects this period with the best chances over any remnant burn
scars. Mean storm motions will be generally weak, so any convective
cores will run the chance of impacting the same areas for multiple
hours before collapsing or moving away from their initiation
points. A broad SLGT risk encompasses much of NM, southwest TX, and
southeast AZ with a bit of an extension into CO along the Sangre de Cristos.

Kleebauer
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