Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
404 AM EDT Mon Apr 7 2025

Day 1
Valid 12Z Mon Apr 07 2025 - 12Z Tue Apr 08 2025

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
SOUTHERN ALABAMA AND THE WESTERN FLORIDA PANHANDLE...

A cold front moves along from the border of southernmost AL/FL
Panhandle eastward into the central FL Peninsula, picking up the
pace with time. In the wake of the excessive rainfall/flash
flooding episode near the FL Panhandle border with southernmost AL,
and due to convection filling in behind the surface front and ahead
of its 850 hPa position, pulled the Marginal Risk westward and
introduced a Slight Risk area to account for the recent convective
evolution. Precipitable water values near 1.75" will lie in the
area, while ML/MU CAPE of 500-1500 J/kg advects in from the Gulf.
Low- level inflow and effective bulk shear remain sufficient for
convective organization. The main hindrance from a flash flood
perspective is the progressive nature of the front as time marches
on, particularly after midday. Considered a Slight Risk for areas
farther east near the Apalachicola River in FL which could see an
outflow boundary reach the region and stall while convectively
active, waiting for the cold front to catch up before picking up
the pace. However, QPF amongst the global and mesoscale guidance
showed low chances for 3"+, so left the risk farther east as
Marginal, which was coordinated with the TAE/Tallahassee FL
forecast office.

Roth


Day 2
Valid 12Z Tue Apr 08 2025 - 12Z Wed Apr 09 2025

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS IN SOUTHERN
FLORIDA...

Deep southwest flow is expected ahead of an advancing cold front
across the FL Peninsula. Precipitable water values of 1.75-2" are
forecast to reside here, and ML CAPE should maximize in the
2000-3000 J/kg range per NAM guidance. Low-level inflow and
effective bulk shear are sufficient for convective organization.
Whether or not a flash flood risk fully materializes depends upon
whether or not an East Coast seabreeze front can develop and focus
convection, which could anchor convection in the large urban area
of South FL as it is unlikely to progress inland within this
synoptic pattern. In this sort of atmosphere, hourly rain totals
to 2.5" and overall totals to 5" are possible. The 00z HREF does
have some non- zero probabilities of 5"+ totals across sections of
the northern Keys, while the 3"+ probabilities are more prominent
along the Gold Coast. Considering the urban environment,
ingredients available, and mesoscale guidance signal, added a
Marginal Risk to South FL and the Upper Keys as a precaution as
isolated to widely scattered flash flooding appears possible.

Roth


Day 3
Valid 12Z Wed Apr 09 2025 - 12Z Thu Apr 10 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

...Southern Illinois...
Conceptually, there is a non-zero threat of heavy rainfall in and
near southern IL Wednesday evening as precipitable water values
rise near to above 1" within a region with 1000-500 hPa thickness
near 5550 meters, implying a completely saturated troposphere.  MU
CAPE is expected to rise above 250 J/kg per NAM guidance, which
tends to be underforecast at this time range. The area will still
be attempting to dry out from recent heavy rainfall/ saturated
soils. However, there is only about a six hour window for heavy
rain concerns to materialize near the parent cyclone's associated
frontal boundary before it moves through. There is the potential
for hourly rain totals to maximize near 1.5". The guidance is this
time range QPF-wise is underwhelming, so no risk area was
depicted. However, this may change as we move closer in time into
the high resolution guidance window.


Roth


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