Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
846 PM EDT Sun Mar 30 2025
Day 1
Valid 01Z Mon Mar 31 2025 - 12Z Mon Mar 31 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE LOWER MISSISSIPPI, TENNESSEE, AND OHIO VALLEYS AND CENTRAL GULF
COAST...

...Central Gulf Coast through the Ohio Valley...

01Z Update...
With diminishing convection along the immediate Upper Gulf
coast...removed the Slight risk area introduced earlier in the day.
Was not inclined to remove the Marginal risk area which surrounded
the Slight risk given how hydrologically sensitive the area became
as a result of prolific rain-makers earlier today. The latest model
guidance kept the bulk of QPF well to the north and west during the
overnight hours. Even so...any convection still lingering near the
coast of Alabama westward into southeast Louisiana could result in
additional excessive rainfall concerns. Also trimmed the Marginal
risk area from the western Florida peninsula based on trends in
radar imagery. The remainder of the previously-issued outlook still
covered the higher model QPF and any associated potential for
excessive rainfall.

Bann

Previous Excessive Rainfall Discussion...

A pair of shortwave troughs emerge from the Intermountain West
into the Central U.S. today, eventually partially phasing with an
associated longwave trough by later tonight. This complex
interaction is expected to result in a broad area of organized
convective activity across much of the eastern half of the CONUS,
including portions of the MS, TN, and OH Valleys.

Despite the continued complexity of the synoptic and mesoscale
details, models remain in fairly good agreement in indicating
convection breaking out across the broad warm sector today,
initially mostly discrete (and supportive of splitting supercells
and multicell clusters) and evolving into a QLCS/LEWP ahead of the
associated cold front by this evening. Broad inflow from the Gulf
will result in precipitable water values of 1.25-1.75" (at or above
the 90th percentile, per CFSR climatology) and SB CAPE of 2000-4000
J/kg, as deep layer (0-6 km) shear increases to 40-60 kts. Hourly
rainfall of up to 3"/hr are possible in this environment,
initially highly localized in association with established
supercells, then more numerous to widespread with the passage of
the QLCS/LEWP. Localized totals of 2-4" are generally expected
within the Slight for the whole of the event (with the majority
falling in a 3-6 hour period in association with the QLCS passage),
with totals near the upper-end of the range (~4") being possible
where storms train and/or where two or more mesocyclones align.
While there were some changes per the new guidance, in general,
continuity was maintained.

Ongoing organized convection midday in the general vicinity of
Mobile hadn't been disrupted by diurnal convection in its
vicinity, which threatened mergers as disorganized convection
moves more northward while organized convection moves more
eastward. Cold cloud tops continued to show expansion with the
convective area midday, which led to the introduction of a midday Slight
Risk from southern MS across southern AL into the FL Panhandle.
Hourly rain totals to 3" with additional local amounts to 6" remain
possible for as long as this convective mass can persist.

...Florida Peninsula/Southeasternmost Georgia...
A frontal boundary at 850 hPa slowly lifts through the state,
bringing precipitable water values to 1.5-1.75". ML CAPE should
maximize in the 2000-3000 J/kg, particularly in the southern
Florida peninsula. There appears to be enough effective bulk shear
for some degree of thunderstorm organization as well. The model
guidance --- mostly mesoscale -- indicates the potential for local
amounts in the 5-6" range. The Marginal risk was maintained with
minimal change.

Roth/Churchill


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