FOUS30 KWBC 010756
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
356 AM EDT Thu May 1 2025

Day 1
Valid 12Z Thu May 01 2025 - 12Z Fri May 02 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN SOUTHERN
OKLAHOMA, FAR NORTH TEXAS, FAR WESTERN ARKANSAS, WESTERN
PENNSYLVANIA AND THE CENTRAL APPALACHIANS...

...Southern Plains...

Shortwave ejection out of the Southern Rockies will interact with a
cold front pressing south through the Central and Southern Plains
by the late-afternoon and evening hours Thursday through Friday
morning. A relevant theta_E differential will be aligned within 
the confines of the boundary providing a suitable thermodynamic 
alignment such that area convection anchors and pushes along the 
west to east oriented instability gradient across Southern OK.
Majority of CAMs are in consensus of a heavy rainfall footprint
along and south of I-40, generally configured to the timing of the
cold front, shortwave interaction, and LLJ convergence signal. This
is primarily right around 00z when the convective threat enhances
with the integration of the nocturnal LLJ, a timing historically
prevalent for this time of year and this setup is no exception.

00z HREF neighborhood probs for >1" are relatively high (50-80%) 
within the corridor extending from Southwest OK across the Red 
River Basin towards the AR border. >2" is highest just north of the
river from I-35 on east, a testament to the timing of the
shortwave arrival and upscale growth of the convective threat as it
maneuvers to the east-southeast. Rainfall rates between 1-2"/hr are
most likely when assessing the hourly rate probs from the both the
NBM and HREF output, a rate assessment that is more than suitable
considering the blanket low FFG intervals due to the antecedent
moist soils from the past 36-48 hrs of rainfall. The areal QPF 
footprint of >1" aligned within those lower FFG's pretty much
perfectly, a scenario that lends to a greater credence of a touch
higher risk category that what is normal the case for this setup.
The previous SLGT risk was relatively maintained with minor
adjustments on the edges pertaining to recent hi-res QPF and in
conjunction with the ML First Guess Fields. 

...Interior Mid Atlantic...

Shortwave trough currently positioned over the Southern Plains will
lift northeast through the Mississippi Valley with sights on the
Great Lakes region by the second half of the D1. Downstream, an
improved diffluent signature will align over the Ohio Valley and
points northeast allowing for regional ascent to boost convective
initiation in-of places within that diffluent core. The most
notable area for convective development lies within an established
theta_E tongue riding northward through the Appalachians Front 
with poleward expansion into the southeastern Great Lakes. This 
allowed for moisture rich air exuding PWAT anomalies between +2 and
+3 within the corridor extending through much of WV up through 
Eastern OH and Western PA to the Lake Erie border. SBCAPE between 
1500-2500 J/kg will be a common occurrence during peak diurnal 
destabilization creating a corridor of enhanced thermodynamic 
prospects to coincide with the arrival of the better mid-level 
forcing downstream of the approaching trough. Regional shear will 
be improving as well, especially within the confines of a shallow 
warm front that will be lifting northward through the course of the
afternoon across the interior Mid Atlantic. Cell initiation will 
begin between 18-00z today with greater multi-cell cluster coverage
likely to occur towards the back end of the above time frame due 
to anchoring along the warm front as it approaches. Areas across 
Western PA have been hit relatively hard recently with convective 
rainfall adding to the areal decoupling of the FFG indices meaning 
the prospects for flash flooding have increased with the arrival of
this next disturbance.

00z HREF neighborhood probs for >2" are between 30-50% across a
large section of Western PA extending south into the northern High
Country of WV, rooted in the Appalachians Front, an area notorious
for flashy rivers and smaller towns located within the valleys.
There was enough consensus to warrant an upgrade considering the
antecedent conditions, atmospheric environmental favorability, as
well as the increased probs denoting a locally heavy rain prospect
over the region. A SLGT risk has been added to much of Western PA
down into portions of the Central Appalachians. 

Kleebauer


Day 2
Valid 12Z Fri May 02 2025 - 12Z Sat May 03 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS FROM THE
SOUTHERN PLAINS INTO THE LOWER MISSISSIPPI VALLEY...

Steady progression of a cold front through the Southern Plains will
lead to textbook convergent pattern with an addition of a strong 
upper diffluence signature within the bounds of the southern jet
proxy. High areal theta_E averages will denote the significant 
instability pattern positioned ahead of the approaching cold front 
leading to ample thermodynamic assistance for strong convective 
cores and expected heavy rainfall. Recent CAMs have finally come 
into view for the primary window of cell initiation with a
widespread convective pattern likely to extend from the Stockton
Plateau to points east within the I-10 corridor up through all of
Hill Country, especially over the I-35 section of south-central TX.
HREF blended mean QPF is over 2" now within a large area 
encompassing the terrain focused across the Central portion of the
state. This had led to elevated neighborhood probs of >2" for the 
12-00z time frame in the forecast running >70% over pretty much all
of the above region extending into portions of East TX. The East TX
corridor has become a newer development in terms of anticipated QPF
magnitude as the the CAMs and in some instances the global
deterministic are now catching on to the deep moisture advection
regime poking north out of the Western Gulf ahead of the front
arrival. Forecast soundings across the area between Houston up to
the latitude of Austin are now indicating MUCAPE or the order of
3500-5000 J/kg prior to any front arrival leading to significant
heavy cores to transpire due to cap breach from diurnal
destabilization initially, followed by more organized multi-cell
clusters as we move into the evening as the LLJ re-intensifies and
the cold front motions into the region. 

Soundings also note a period of mean flow and upshear components
that would lead to potential backbuilding within the confines of
the front. Very strong ascent between 850-300mb is also noted
within multiple CAMs outputs, a testament to the favorability for
the setup to become more organized and providing a vast areal
expanse of heavy convection once the setup matures. PWATs between
1.6-1.9" with locally >2" are being forecast along and ahead of the
front as it moves into Central TX, a forecast that traditionally is
favored for greater flash flood prospects, especially over the
favored I-35 corridor between Austin to San Antonio where high
runoff capabilities are found due to soil type and limestone
foundations. Local totals between 3-6" are likely over the entire
stretch of Hill Country through East TX with the eastern most
extent even shifting into Western LA as the convective pattern
evolves and grows upscale entering into the adjacent Lower
Mississippi Valley. Houston metro is now in play for convective
impacts, especially as thunderstorms to the north provide outflow
generation that could send cold pools south towards the Gulf coast
with additional convective development along the edge of any
approaching cold pool. 

The previous SLGT risk was relatively unchanged across Central TX.
Some expansion to the west was made into the Stockton Plateau where
initial cell formations will likely occur as the front arrives and
interacts with a formidable instability axis aligned over the
plateau itself. The SLGT was expanded southeast to cover the
Houston metro and points north where there is growing consensus on
convective impacts over and near the urban center. This is a high-
end SLGT over much of Central TX, especially Hill Country to the
I-35 corridor and points east. A risk upgrade is not out of the
question in subsequent updates given the environmental
favorability and flashy history of places expected to see the
greatest thunderstorm coverage/rates.  

Kleebauer

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