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 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt