Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
310 PM EDT Mon May 19 2025
Day 2
Valid 12Z Tue May 20 2025 - 12Z Wed May 21 2025

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

20Z Update: 12z guidance remained pretty consistent within its run
to run variance on the expected evolution across the Central and
eventually Eastern U.S. CAMs output was strikingly similar in many
regards with only deviations being the relative maxima across the
Ohio Valley. In any case, the setup remains coincident with a
SLGT risk prospect as the greatest forcing will occur the period
prior with a step down in ascent magnitude moving into Tuesday.
The corridor of interest remains over the Ohio River Basin down
through KY and TN where instability will be greatest and the
frontal positioning overhead would allow for steady motion of
convection over the area. The good news is the instability field
will be meager compared to what we saw during the D1 so the threat
of widespread flash flood prospects are less likely. That said,
antecedent wet soils across the region will promote a slightly
better flash flood opportunity due to higher runoff capabilities as
moisture absorption will be harder to occur given the soil
moisture percentiles between 60-80% as indicated via NASA SPoRT.
Areal average of 1-2" with locally as high as 4" are forecast
across the Central Ohio Valley encompassing Southern IL/IN/OH down
through the northern half of KY. This is coincident with a modest
neighborhood probability (30-50%) for >3" with a sharp decline
towards >5". Expected rates between 1-2"/hr max will curb the
higher end scenarios, but the combination of saturated grounds and
locally heavy rainfall will still induce scattered bouts of flash
flooding. The previous SLGT was generally maintained given the
above variables.

Kleebauer

..Previous Discussion..

The deep trough across the middle of the country and its 2 separate
shortwaves forcing the heavy rainfall from the Day 1/Monday period
will phase the shortwaves by the start of the Day 2/Tuesday period,
and also quickly weaken as the flow quickly becomes zonal. The
result will be a widespread shearing front and surface low which
will propagate east-southeastward from southern Iowa to northern
West Virginia through the period. The low tracking somewhat
parallel to its leading front will result in an elongated area of
rain stretching from the northern Plains to the Mid-Atlantic. The
greatest forcing will be near the surface low and the phased and
weakening shortwave energy in the upper levels rapidly flattening
to zonal flow. The further north and west along the line you go,
the more "used up", the moisture and instability will be, favoring
more stratiform rain, especially across Minnesota and Wisconsin.
Due to the lack of instability which would be critical for heavy
rain rates in this area, the Slight Risk was trimmed out of those
states with this update. Elsewhere the Slight remains largely
unchanged, especially for portions of Kentucky and West Virginia,
which have been hard hit with recent storms, thus have sensitive
soils to additional rainfall. The long duration of the rainfall may
play as big a role as individual storms' rainfall rates in many
areas.

The surrounding Marginal Risk area was expanded in the Mid-
Atlantic, particularly across Virginia, where recent heavy rains
may have made soils in the most sensitive areas prone to isolated
instances of flash flooding, even as far east as the coast.
Elsewhere, only minor changes were made.

Wegman

Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt