Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
415 AM EDT Sun Mar 30 2025

Day 1
Valid 12Z Sun Mar 30 2025 - 12Z Mon Mar 31 2025

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

...Central Gulf Coast through the Ohio Valley...
A pair of shortwave troughs emerge from the Intermountain West
into the Central U.S. today, eventually partially phasing with the
polar jet (and 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 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 rates of up to 2-3"/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. 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.


...Florida Peninsula/Southeasternmost Georgia...
A frontal boundary at 850 hPa slowly lifts through the state,
bringing precipitable water values to 1.5-1.75". MU CAPE maximizes
above 2000+ J/kg in South Florida. There appears to be enough
effective bulk shear for thunderstorm organization as well. The
model guidance --- mostly mesoscale -- indicates the potential for
local amounts in the 5" range. The Marginal risk was maintained.

Churchill/Roth


Day 2
Valid 12Z Mon Mar 31 2025 - 12Z Tue Apr 01 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHEAST & NORTHEAST...

...Southeast...
The aforementioned pair of shortwave troughs are progged to fully
separate by Day 2, as the northern stream shortwave continues to
phase with the parent longwave trough into the Mid-Atlantic and
Northeast, while the southern shortwave likely hangs back into the
Mid-South and Lower MS Valley. The best moisture and instability
lies over portions of the Southeast (where forcing from the
secondary shortwave should still be sufficient for organized
convective activity). The Marginal risk was generally maintained
for this portion of the Southeast (including much of AL/GA with
surrounding portions of East TN, southern MS and the FL Panhandle).
Localized totals of up to 2-3" are generally expected (per GEFS and
ECENS exceedance probabilities), though the inclusion of the full
CAM suite later today will probably push higher-end totals to 3-4"
(still mostly supportive of only a Marginal risk, given associated
3-6 hour FFGs of 2.5-4.0". 


...Northeast...
Thunderstorms with heavy rainfall are expected during the afternoon
and evening hours ahead of a northern stream shortwave as MU CAPE
peaks around 1000 J/kg and precipitable water values peak in the
1.25-1.5" range. Sufficient inflow and effective bulk shear is
available for organized convection for much of the period, with
activity expected to be in decline past Tuesday 06z. Hourly rain
totals up to 2" are possible where convection trains, which could
challenge the flash flood guidance across urban portions of the
Mid-Atlantic States. The inherited Marginal risk was maintained.

Churchill/Roth


Day 3
Valid 12Z Tue Apr 01 2025 - 12Z Wed Apr 02 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTHEAST NEBRASKA, SOUTHWEST IOWA, NORTHEAST KANSAS, AND
NORTHWEST MISSOURI...

A blocking pattern begins to emerge by Day 3, as the aforementioned
shortwaves lift northeastward into Canada with large-scale ridging
quickly builds across the eastern CONUS from the North Atlantic.
Meanwhile, large-scale troughing becomes amplified out West, with a
potent upper-low expected to develop over the Northern Rockies and
Plains (translating across the Rockies from the Pacific Northwest).
While global model suites are in overall good agreement with the
evolution of the synoptic scale, the GFS/GEFS is oddly an outlier
lacking substantial convection with the associated surface low
(which is depicted as a sub 990 mb low from the 00z GFS, ECMWF, and
CMC). Most deterministic solutions depict localized totals of 2-3"
(including the downscaled ECMWF, the UKMET, and the CMC-reg) with
remarkably good spatial agreement for this range, so the inherited
Marginal risk was maintained.

Churchill


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt