FOUS30 KWBC 250826
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
426 AM EDT Fri Apr 25 2025

Day 1
Valid 12Z Fri Apr 25 2025 - 12Z Sat Apr 26 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTH TEXAS AND THE TEXAS PANHANDLE INTO SOUTHWEST OKLAHOMA...

...Summary...
Minor adjustments were made to the Slight Risk from yesterday's Day
2 ERO area across western portions of the TX/OK Red River Valley 
into the Cap Rock and Rolling Plains. Meanwhile, the Marginal Risk
area farther east was extended to include more of the Lower MS Valley.

...North TX...Central OK...TX-OK Panhandles...Eastern NM 
Slight Risk area which was hoisted yesterday in the Day 2 ERO, in 
coordination with affected TX/OK offices, continues into today's
Day 1 ERO with minor modifications. Upper level flow gradually becomes
more difluent today and tonight as the upper ridge axis pushes east
across the Southern Plains. Convection early this morning on the 
leading edge of the outflow boundary south of the MCS will continue
to diminish south of the Red River, resulting in not much rainfall
from an areal-average standpoint. However, late this afternoon and
into the evening with the daytime heating and more favorable
forcing (increasing difluence aloft), additional storms will 
develop along the dryline, likely growing upscale into an MCS that
will track slowly across the TX Panhandle and Red River Valley 
into the overnight hours. The activity will parallel a W-E oriented
surface frontal boundary that will become quasi-stationary 
overnight. The 00Z high-res guidance again indicated stronger
multicellular clusters and supercells, some of which become slow
moving per the simulated reflectivity guidance. Therefore there is
still concern that stronger storms/supercells within the MCS may 
move more slowly or even stall, causing a more enhanced flash 
flood risk, while the MCS as a whole remains over an area hard hit
with heavy rain in recent days. Within the Slight Risk area, the
latest (00Z) HREF shows highest probabilities of >5" within 12hrs
(40-50+ percent). Meanwhile, the Slight Risk area is also supported
by the latest UFVS-verified CSU ERO First Guess field.

...Lower-Mid MS Valley and TN Valley to OH Valley and Lower Great 
Lakes, Southern-Central Appalachians...
The Marginal Risk inherited from yesterday's Day 2 ERO was expanded
to include more of the Lower MS Valley and a little more of the Mid
Atlantic Region. Over northern areas, deep-layer instability ahead
of the cold front is largely elevated and rather meager (less than
1000 J/Kg). However given the higher relative soil moisture and
lower FFGs over these areas, embedded convective elements may cause
isolated instances of flash flooding. 

Farther south across the TN Valley and towards the Gulf Coast, 
instability will be more plentiful (Mixed-layer CAPEs of 1000-1500 
J/Kg), while TPW values are a bit more favorable as well (between 
1.5-1.7". Despite the more favorable thermodynamic profiles, the
more transient easterly progression of the shortwave energy will
limit the duration of the heaviest rainfall. Still, more elevated
1-2"+/hr probabilities in these areas may lead to localized short-
term runoff issues.

Hurley


Day 2
Valid 12Z Sat Apr 26 2025 - 12Z Sun Apr27 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHERN PLAINS INTO THE SOUTHERN OZARKS AND JUST WEST OF THE
MISSISSIPPI DELTA...

...Southern Plains...
The MCS that is expected to traverse the Texas Panhandle and along
the Red River Valley Friday evening and overnight will persist 
into Saturday across the Marginal Risk area. Guidance continues to
show more limited areal coverage of rain on Saturday as compared 
with Friday with any additional convection that develops along the
dryline and the quasi-stationary west-to-east front. This 
stationary front will eventually lift northeast as a warm front 
Saturday night, taking the more organized/widespread convection 
with it. Uncertainty regarding MCS behavior on Saturday remains 
very high, and further updates and the move of this time frame into
CAMs range may require a later upgrade to a Slight, particularly 
if heavy rain remains further west towards the Panhandle for 
longer. Regardless, additional convection is likely to form again 
Saturday afternoon along the dryline near the New Mexico border, 
which may further worsen flooding concerns in the areas where the 
storms develop.

Hurley/Wegman


Day 3
Valid 12Z Sun Apr 27 2025 - 12Z Mon Apr 28 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF
THE GREAT BASIN AND FROM PORTIONS OF THE NORTHERN ROCKIES INTO THE
NORTHERN PLAINS AND UPPER MISSISSIPPI VALLEY...

Mid-upper trough across the southern Great Basin early Sunday will
lift across the central Rockies by 12Z Monday. This in-turn will
nudge the upper ridge axis east of the Northern Plains and Upper MS
Valley late Sunday-Sunday night, with a quick onset of upper level
divergence in its wake. Along with the more favorable dynamical
forcing (upper divergence and deep-layer ascent), deepening
southerly flow will tap the western Gulf and lead to an uptick in
low-mid layer moisture transport, with TPW values climbing to 
1.25+ inches. Deep-layer instability meanwhile will also be ramping
up, to the tune of 1000-2000+ J/Kg MUCAPE per the 00Z GFS. Some of
the guidance (particularly the ECMWF) shows an MCS developing along
the western periphery of the upper ridge, then tracking east to
eventually southeast along the 850-300 mb thickness. At this point
(day 3 forecast) the forward-propagating Corfidi vectors would
support a more progressive MCS, though much will depend on the
strength of the LLJ. For now, have maintained a fairly broad
Marginal Risk area given the potential of intense short-term
(sub-3hr) rainfall rates.

Hurley

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