Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
828 PM EDT Tue May 20 2025
Day 1
Valid 01Z Wed May 21 2025 - 12Z Wed May 21 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
OHIO AND TENNESSEE VALLEYS...

...01Z Update...
Few changes to the forecast reasoning expressed in earlier
Excessive Rainfall Discussions. The western portion of the Marginal
and the Slight risk areas were trimmed out behind a cold front that
was ushering drier and more stable air into the region. Ahead of
the front...the airmass remained conducive for convection with
downpours and heavy rainfall amounts falling on increasingly
saturated soils.

Bann

...16Z Update...

There were some subtle adjustments to the Marginal Risk in the
Mid-Atlantic and Upper Midwest based on 12Z guidance trends, but
the most concerning area for flash flooding remains in the Ohio and
Tennessee Valleys. The setup features an ideal setup for both
synoptic and mesoscale forcing to go along with anomalous PWs
(above the 90th climatological percentile per NAEFS) and a strong
IVT for mid-late May (500-750 kg/m/s). Latest RAP guidance shows
the first round of storms continuing to progress east this
afternoon, while the next line of storms forms along and east of
the cold front tracking across the Middle Mississippi Valley where
surface-based heating is on going. As the atmosphere destabilizes,
additional storms will form and add to the repeated rounds of
storms throughout the Mid-South through this evening. RAP soundings
in portions of eastern KY show sfc-500mb RH values >90%, MLCAPE
>1,000 J/kg, and warm cloud layer depths of at least 10,000ft. The
potential for training storms, particularly from as far south as
northern MS/AL and and far north and east as western WV, also
aligns in areas where 10-40cm soil moisture is >70% in some cases.
These storms should be progressive movers, but given the potential
for some localized training and increasingly sensitive soils, the
Slight Risk still remains on track for parts of the Mid-South
through tonight.

Mullinax

---Previous Discussion---

A deeply negatively tilted longwave trough stretching from Montana
to the mid-Mississippi Valley will continue flattening by becoming
even more negatively tilted through the period as the trough itself
lifts back into the broader more zonal jet stream. While this
trough will continue to support a surface low as it moves east from
Iowa to roughly the IN/OH/MI tripoint by 12Z Wed, the low will be
weakening with time. The low will be filling in part because the
zonal pattern will gradually decouple the low from the plume of
Gulf moisture as the LLJ that has been supporting the storms over
the middle of the country the past couple days gets shunted south
back towards the Gulf Coast. The result of this will be decreasing
amounts of atmospheric moisture available for the storms with time,
resulting in a subsequently decreasing flash flooding threat.
However, for this period, the southwesterly influx of Gulf moisture
will feed training storms across the Ohio and Tennessee Valleys,
including portions of eastern Kentucky and southern West Virginia
which were hard hit with heavy rain and severe storms over the past
week or so.

The inherited Slight Risk area was trimmed on the northwest side
due to a lack of instability over much of Illinois, but expanded a
few rows of counties across Tennessee towards the south and west.
No significant changes were made to the surrounding Marginal Risk.

Wegman

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