Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1156 AM EDT Fri Apr 25 2025

Day 1
Valid 16Z 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...

...16Z Update...

No changes were made to the inherited ERO risk areas across the
country. The latest 12Z CAMs guidance just in have not shifted
significantly enough in any area to justify any changes, as an
ensemble of the guidance remains remarkably consistent with
previous runs, especially as compared with previous days.

This consistency may be due to the forcing, weak as it is, finally
shifting east into the Midwest and off the Plains, replacing the
previous troughing with some weak ridging. The ridging stabilizing
the upper levels through their warming is likely a contributing
factor to the lessening coverage of storms over most of the Plains.
There remains a notable convergence zone in the lower levels in the
Slight Risk area, which is why additional convection will develop
along the dry line this afternoon.

Further east, the large Marginal Risk from the Mississippi River
east is largely remnant convection with faster moving storms and
generally weaker forcing, making any stronger signals that would
favor a Slight largely nonexistent or too variable between
guidance to lock in on any one area.

Wegman

...Previous Discussion...

...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 diffluent 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 Apr 27 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


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