FOUS30 KWBC 220809
QPFERD

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

Day 2
Valid 12Z Wed Jul 23 2025 - 12Z Thu Jul 24 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
MINNESOTA, NORTHERN WISCONSIN, AND THE WESTERN U.P. OF MICHIGAN...

...Upper Midwest...

The progression of convection from the prior D2 will bleed over
into the front half of D3 with the cold front slowly migrating
eastward allowing some advancement of the organized convective
pattern to spill over into the Arrowhead of MN and adjacent
northern WI and western U.P. There's a better consensus on this
occurring given the aligning mass fields from relevant
deterministic, as well as an agreement on the pattern progression
as the surface low over the northern plains finally lifts northeast
along the front and clears into the northern lakes by Wednesday
afternoon. The final 12hrs of the HREF blended mean QPF output was
pretty clear in a heavy precip presentation across northern WI
through the U.P with the focus likely along and south of the
southern lake shores of Superior. Deterministic depictions are
upwards of 2-4" in additional QPF spilling over from the previous
forecast period leading to some 24hr totals from 00z Wed to 00z Thu
running upwards of 4-5" over the western U.P and northern WI. This
would entice the threat for flash flooding due to the anticipated
rates between 1-2"/hr with higher intra-hour rates expected given
the favorable environment of PWATs >2".

The cold front will still be slowly pushing eastward through the 
second half of the period with guidance indicating another wave of 
low pressure potentially enhancing the convective pattern across 
the Upper Midwest as we move into Wednesday evening. There's a 
growing consensus on heavy precip redeveloping across the central 
and northern plains between NE/SD and migrating east-northeast 
along and ahead of the slow-moving cold front. With flow remaining 
parallel to the boundary across a majority of the cold front 
positioning, heavy convection with some back-building/training 
evolutions are favored which would undoubtedly lead to, at least 
some scattered flash flood concerns. The highest probabilities sit 
firmly over northeast NE up through the southern half of MN where
ensembles delineate the most favorable proxy of the cold front. QPF
means are running between 1-2" from Minneapolis down towards Omaha
with the prob fields off the latest ECENS and EC AIFS Ensemble
depicting the highest threat across southwest MN towards
Minneapolis with the second round of convection. This was enough to
extend the previous SLGT risk further to the southwest to cover for
the threat with even some potential for extend further back into NE
if the consensus grows and/or magnitudes of rainfall increase
further. This window (00-12z Thu) will need to be monitored closely.

...Southeast...

A slowly westward migrating upper trough off the Southeast coast
and attendant surface low over the northern Gulf will lead to
locally heavy rainfall prospects from the central Gulf coast over
into Florida. The signal for rainfall exceeding 3" remains very
spotty within the varying deterministic, however the threat remains
for isolated heavy convection anywhere along the South Carolina
coast down through the northern half of the FL peninsula over into
the central Gulf coast. Overall, the threat remains on the lower
end of the MRGL risk category, but environment and proximity of
favorable surface pattern maintains the threat for a few robust
cells that could easily overproduce with PWAT forecasts firmly
above 2.25" with some of the coastal plain likely to see PWATs
exceeding 2.5" at times. 

...Central and Southern Rockies...

Ample moisture, instability and weak mid-level perturbations in 
proximity to the Continental Divide will lead to slow-moving bouts
of convection that will amplify heavy rain prospects that could 
induce flash flooding. Terrain complexity and the slow-moving
nature suited by lackluster mean flow and cell anchoring over the
topography spell for a sneaky flash flood risk across the central
and southern Rockies with a general maxima positioned across CO,
west of I-25. A MRGL risk exists for the threat, but if signals
within the CAMs continue to grow as we move closer to the threat
window, a targeted upgrade is not out of the question. For now,
have maintained continuity with the MRGL and will await for the 12z
CAMs output as they will have the full D2 output/prob fields to
assess for the threat.

Kleebauer


Day 3
Valid 12Z Thu Jul 24 2025 - 12Z Fri Jul 25 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL PORTIONS OF THE
CENTRAL AND HIGH PLAINS, MIDWEST, AND SOUTHEASTERN U.S...

...Central Plains to Great Lakes...

Cold front progression and sheared mid-level shortwave ejecting out
of the Rockies will become an integral part in enhancing regional
convective output from the Central Plains up through the Great
Lakes. The environmental parameters such as favorable
thermodynamic properties (Elevated MUCAPE), weak mid-level forcing, 
and sfc-850mb convergence in correlation to the front will provide
enough of a mechanism to drive scattered to widespread areas of
thunderstorm activity capable of producing localized flash
flooding. Good news is the models have the threat in question. The
bad news is the consensus on where is pretty much up in the air.
The most favorable areas in the ensemble means are across the
central Midwest back into the Central Plains with a secondary
maxima focused across the Great Lakes. The prospects for a SLGT are
relatively high within the bounds of the broad MRGL risk, but the
threat for scattered flash flood concerns is very much warranted
with the signal. 

...High Plains...

Another closed low off CA will allow for a broad diffluent mid and
upper level regime with a multitude of smaller shortwaves migrating
northeast out of the Four Corners. Scattered convective pulses will
likely transpire over the Front Range down along the Continental
Divide. Any area within these bounds could be subject to heavy
precip with a potpourri of individual deterministic outputs
signaling the threat for 2-3+" in any location. Not enough of an
organized area to produce anything more than a broad MRGL risk
encompassing the above areas, so maintained continuity from previous forecast.

...Southeast...

Low pressure along the central Gulf coast will entice yet another
period of heavy convective potential from LA back into FL/GA. PWATs
remains very high (>2.25" for the period with ample instability
situated along the Gulf coast and areas just inland. This is a
prime setup for somewhere to see over 5" of rainfall in a spot, but
the question of exactly where is very much up in the air. Just
assessing the SLP positioning, the best threat is likely over
Southern LA, mainly along and south of I-10. This is a signal
within multiple global outputs, and coincides with the best
convergent winds on the northern side of any surface circulation.
In any case, still kept the previous MRGL inherited just due to the
lack of a discernible organized maxima, but don't be surprised to
see a targeted upgrade somewhere along the central Gulf coast as we
inch closer to the time frame of interest.

Kleebauer
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