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

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
CENTRAL GULF COAST...

...Central Gulf Coast/Southeast... T
A pair of shortwave troughs are progged to fully separate, 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 from continuity. However,
due to recent convective events along the Central Gulf Coast,
upgraded to a Slight Risk for portions of southernmost MS, AL, and
the western FL Panhandle which should have increased sensitivity/
soil saturation. Localized totals of 2-4" are generally expected.


...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 showed minimal
change.

Roth/Churchill


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

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL PLAINS & MIDWEST...

A blocking pattern begins to emerge by Tuesday, as the
aforementioned shortwaves lift northeastward into Canada with
large-scale ridging quickly builds across the eastern United States
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 12z global model suites
are in overall good agreement with the evolution of the synoptic
scale, the GFS/NAM are drier than the rest of the guidance with the
associated surface low (which is depicted as a sub 990 mb low from
the GFS, ECMWF, and CMC). Within the GFS/NAM guidance, the idea of
light QPF is internally inconsistent as the forecast precipitable
water values (1-1.25") combined with the 1000-500 hPa thickness
values near 5610 meters suggest a nearly saturated atmosphere, and
there appears to be sufficient instability in the vicinity and
effective bulk shear for convective organization, so the wetter
guidance was more believable. However, nearly as soon as the
necessary ingredients are in place, they take off to the east due
to system progression. Most deterministic solutions follow that
idea and depict localized totals of 2"+ (including the downscaled
ECMWF, the UKMET, and the CMC- reg) with remarkably good spatial
agreement for this range. The ingredients above suggest hourly
rainfall up to 1.75" with local amounts in the 3-4" where/when
cells have the opportunity train or backbuild, so the inherited
Marginal risk remains, though the area showed some expansion.

Roth/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