FOUS30 KWBC 271225
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
825 AM EDT Wed May 27 2026

Day 1 Valid 1223Z Wed May 27 2026 - 12Z Thu May 28 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTHEAST TEXAS, CENTRAL GULF COAST, AND AREAS OF THE OHIO VALLEY
TO CENTRAL APPALACHIANS...

12Z Special...
Minor adjustment was made to the southern extent of the Marginal
Risk across South Texas to reflect ongoing convection. Refer to MPD
#256 for further details regarding the near-term heavy rain/flash
flooding threat across this region.

Pereira

...Texas to Central Gulf Coast...

Radar this evening is pretty telling with the current evolution
over TX revealing quite the complex of thunderstorms migrating
eastward out Hill Country into the I-35 corridor. These storms will
continue to shove eastward with assistance from both a relatively
solid mean steering flow and outflow propagation stemming from the
organized convection. Aloft, two notable features are depicted via
WV satellite and UA analysis across the Southern Plains. The first
is a well-defined negatively tilted trough oriented over NM into
the TX Panhandle that will be slow to deviate from its current
positioning over the course of the D1, even some prospects of the
upper pattern closing off across the Panhandle area of TX/OK which
would allow for some general enhancement of organized convective
clusters under the primary circulation stemming from any upper
low (ULL). In any case, this general upper pattern across the
region will entice broad cyclonic flow across the south-central
portion of the CONUS leading to convective development around the
periphery of the trough/ULL with an extension of activity likely to
be situated from southeastern CO, across KS and OK, and eventually
swinging back into the Lower Mississippi Valley. Instability maxima
will still be located further to the south and east within closer
proxy to the Gulf, however the upper levels and generally above
normal PWAT anomalies over the entire scope of the Southern Plains
should still act as a favorable environment for locally heavy
rainfall leading to some flash flood prospects over the arcing area
referenced above.

That said, one of the wild cards at play in this setup is the
maturation/development of any MCV structures that would locally
enhance convective episodes and likely trigger a more beneficial
low-level convergence pattern capable of better flash flood
prospects. This has been an output via some of the CAMs this
evening, most notably the HRRR where MCV development across OK
would trigger some locally enhanced precip maxima located within
the broad cyclonic presence. These features are tough to forecast
in the short term with more of a nowcasting component likely to be
instituted before committing to anything more concrete in the
forecast, so this will bear watching over the course of the D1.

The more notable feature of interest for this evening and carrying
through the D1 will likely be the evolution of a trailing vorticity
maxima ejecting out of Coahuila this evening with the current
radar/sat composite indicating a cluster of thunderstorms currently
migrating east out of the Mexican state. The current MCS over Texas
has a defined southern edge arcing back into south TX with a strong
cell currently situated above 50 miles to the east of Laredo.
Recent 00z CAMs indicate the Mexican disturbance following the tail
end of the complex over TX and focusing on a heavy rain core
trailing across south TX with a due east push of the mid-level
disturbance. Consensus on heavy rainfall for the period will begin
across southeast TX with the highlight likely to start around
Corpus Christi and surrounds with moderate to heavy rain lingering
across the Upper TX coast. As the outflow from the current complex
shoves east, the environment will become stabilized behind the
complex leading to a weakening convective regime as the disturbance
to the south begins to enter into the region. There's some
discrepancy on the specifics of the convective placement as we step
through the morning, but the trends are for the immediate TX Gulf
coast into southern LA as being the most notable areas of interest
with the evolution of expected heavy rainfall during the D1. This
is reflected well within the neighborhood prob fields off the
latest HREF signaling >3" running between 40-70% across the entire
stretch from Port O'Connor all the way towards New Orleans.
Downstream prospects are more in due part to the continued
southerly flow continuing to advect deep moisture into the region
allowing for an environment capable of producing heavy convection
during peak afternoon heating. The migration of the outflow on the
northern edge of the current complex will likely skirt the southern
portion of LA with some models maintaining its presence as far east
as southeastern LA before it finally dwindles. In any case, the
environmental favorability in the period lies within that coastal
plain where elevated theta_E's lend to a heightened threat for
heavy rainfall in anything that crops up with hourly rates likely
to reach 2-3"/hr in the heavier cell cores.

Considering the complex evolutions referenced above for both areas
of interest, the highest probabilities aligning with the Upper TX
and Central Gulf coast allows for a continued SLGT risk positioning
in these locations with a broad MRGL encompassing the overall
cyclonic pattern oriented over the Southern Plains to Lower
Mississippi Valley. Will still need to monitor the evolution
closely as deviations in the outflow positioning, or the
introduction of either closed ULL reflections or MVC's over the
Southern Plains could very well aid in additional upgrades in any
areas encompassed by these features.

...Ohio Valley to Central Appalachians...

Latest surface analysis over the Ohio Valley to Mid Atlantic
indicates a defined quasi-stationary front bisecting northern KY
through southern WV into central VA with pockets of showers
currently migrating northeast within the mean flow around a broad
ridge extension off the Atlantic seaboard. This frontal alignment
and lingering mid-level vorticity analyzed over the Ohio Valley
will lead to scattered to widespread convective regime over the
course of today with a heightened threat for locally heavy rainfall
occurring by late-morning and afternoon as environmental
destabilization takes shape. The frontal placement will be
important with regards to eventual convective initiation as this
will become a focal point for any future development over the
course of the afternoon. Model consensus is the boundary wavering
slowly to the north during the morning as a weak disturbance begins
to ride along the boundary leading to some minor regional
amplification and a slow push north of the front to about the
latitude of I-70. A sweeping trough exiting out of Ontario will
also help tighten an axis of confluence across the Midwest and
Ohio Valley by later in the afternoon, enticing a squeeze of the
thermal gradient and regional instability axis that aligns with the
frontal placement.

Instability markers by the afternoon hours will run between
2000-3000 J/kg for MUCAPE with weak shear presence leading to
scattered to widespread thunderstorm initiation along and south of
the front with generally modest cell motions to the southeast after
development. PWAT anomalies will sit firmly within +2 standard
deviations above climo within the proposed area of impact, a factor
that will correlate to strong cell cores capable of heavier
downpours with hourly rates between 1-2"/hr. Previous periods of
rainfall in the last three to four days have really brought down
the areal FFG markers for all 1/3/6 hour thresholds with a vast
majority of places along the Ohio River basin and points north
running <1"/hr in the hourly exceedance indicator. This points to
any heavier convective impacts could cause issues with the remnant
soil moisture and hydrologic concerns that are lingering after the
last event. This is especially true for eastern OH, southwest PA,
and western WV where complex topography only exacerbate the
potential in these zones. The latest 00z HREF continues to paint a
favorable concept of heavier rain prospects along and south of I-70
with a corridor of heavier rainfall likely to occur over southeast
IN through southern OH into northwest WV. EAS probs for >1" are
actually pretty elevated (50-80%) in the zone along I-79 over
towards the Appalachian front in WV with modest probs of 25-40%
located along the aforementioned line extending through IN/OH. A
lot of the flash flood focus will likely correlate with the hourly
rates and overlap of heavy rain over areas still saturated from
previous periods. Considering the antecedent conditions and
expected widespread convective regime along the front, the SLGT
risk was maintained for an increasing threat for flash flooding
during the period.

...Northern Rockies...

Relatively buoyant environment within a deep, anomalous moisture
presence across the Northern Rockies along with regional ascent
provided by sheared vorticity maxima maneuvering through the area
will allow for another round of widely scattered convection to
materialize across the Lewis Range in northwest MT. Instability and
regional forcing will be a bit less than the previous period,
however sensitivities  remain from remnant burn scars present
within the mountainous terrain in the aforementioned area which is
a cause for concern for any cells that happen to occur over the
burn scars themselves. Hourly rates between 0.5-1"/hr in the
stronger cell cores would be capable of providing localized flash
flood impacts over those more sensitive grounds and this setup will
maintain the prospects given the anomalous moisture presence as
PWATs remain above the 90th percentile over the entire stretch of
the Northern Rockies. The previous MRGL risk inherited was
maintained given the relative continuity.

Kleebauer
$$
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 * Origin: Project Scorpio TEST (618:250/6)