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FOUS30 KWBC 260051
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
851 PM EDT Fri Apr 25 2025

Day 1
Valid 01Z Sat Apr 26 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...

...01Z Update...
A majority of the changes made to the Day 1 Excessive Rainfall
Outlook were based on short-term trends in satellite and radar
imagery. Expanded the southern end of the Marginal risk area in
Louisiana along a northwest-southeast convergence zone...with the
idea that upscale growth will continue for a period in response to
strengthening low level flow from the Gulf. Farther
west...maintained the Marginal and Slight risk areas after
expanding and realigning the western side of each outlook category
to better match the satellite depiction.

...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 SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHERN OKLAHOMA AND FAR NORTH TEXAS...

...20Z Update...

In coordination with OUN/Norman, OK forecast office, a Slight Risk
upgrade was introduced with this update. The flooding rains
in the Slight Risk area are expected to be from ongoing storms as
part of an overnight MCS that will be impacting the area at the
start of the period at 12Z/7am CDT. FFGs in the area have been
lowered from recent heavy rains along the Red River, including for
some areas as recently in the last few hours. Guidance remains in
good agreement of the development of a new MCS from dry line
convection this afternoon and evening, with the MCS persisting
through the overnight and into the morning hours before
dissipating. The good agreement with tonight`s MCS stands in stark
contrast to the poor agreement of the various MCSs over the past
few days, so it lends to enough confidence between that and the
lowered FFGs along the Red River to upgrade the area to a Slight.
Elsewhere the surrounding Marginal remains largely unchanged.

Wegman

...Previous Discussion...

...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...

...20Z Update...

Very few changes were needed with the Marginal Risk areas on
D3/Sunday. Both Marginals are low-confidence, with the area of
Nevada struggling with both amounts and low snow levels, while most
of the northern Plains is relatively difficult to flood in general,
since the area is far enough from the Gulf to not have to contend
with heavy rain that often. That said, the LLJ from the Gulf will
extend that far north on Sunday, advecting in MUCAPE values
between 1,000 and 2,000 J/kg, so the atmospheric ingredients remain
present for storms to become strong enough to produce heavy
rainfall. It remains to be seen whether that rainfall will be heavy
enough to cause flooding. Given the relatively low FFGs, good
instability, and a constant moisture stream from the LLJ, the
Marginal in the area looks good.

Wegman

...Previous Discussion...

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