Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
353 PM EDT Sat May 10 2025
Day 1
Valid 16Z Sat May 10 2025 - 12Z Sun May 11 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE FLORIDA PANHANDLE AND SOUTHEASTERN ALABAMA...

...16Z Update...

...Southeast...

No big changes were needed with this morning's update. The Slight
and Marginal Risk areas are largely unchanged, short of a small
carve out in the Marginal around the Big Bend region for higher
FFGs there.

The Slight remains in place for potential flash flooding
from ongoing convection, but perhaps a bit more concern for
potential additional convection in the area tonight. The jet stream
has brought a plume of dry air into the area, which is greatly
impacting convective coverage, even if locally increasing the
potential instability. This dry air will for sure prevent much
light rainfall outside of any convection, though that convection
may have enough moisture from the lower levels to support a flash
flooding threat. A large contributor to the flash flooding threat
remains recent heavy rains, especially in the Florida Panhandle
portion of the Slight, which have locally saturated the soils but
it will still take a lot of heavy rain from any one or line of
training storms to support the flash flooding threat there tonight
since the soils are sandy and swampy in the area.

The upper level low forcing the convection across the Southeast is
still getting its act together, so the better forcing and more
widespread convection is more likely into the Day 2/Sunday period.
The broad Marginal remains in place for localized training
convection, with little certainty on where exactly any associated
flash flooding will occur.

...New England...

In coordination with GYX/Gray, ME forecast office, the Marginal
Risk was removed with this update. All of the rain in the area is
of a stratiform nature, and is struggling to even reach 1/2 inch
per hour rates based on radar estimates. While any additional
rainfall especially into New Hampshire will contribute to any
ongoing riverine flooding, the flash flooding threat from this
system is very low. The rain will continue pivoting east with the
back edge over central New Hampshire now crossing Maine and into
New Brunswick by this evening. For much of Maine, soils are much
drier due to missing recent heavy rainfall, so the flooding threat
there is also very low.

Wegman

...Previous Discussion...

...Southeast...

A stationary upper low will remain parked over the Central Gulf
Coast through the weekend. A potent shortwave trough will pivot
around the upper low today, sparking convection across the
Southeast. At the surface, a low pressure system slowly tracking
north into the Central Gulf Coast states will become occluded, with
nearly stationary frontal boundaries extending south into the Gulf
and east to the Atlantic Coast. Thunderstorms are expected to form
in the warm sector of the system today, with some overrunning
precipitation on the northern side of the system as well. Heavy
rain should initially focus on the Florida Panhandle this morning,
then spread further into the Southeast and Florida peninsula
throughout the day.

Showers and storms tracking north along the frontal boundary that
extends into the Gulf will result in areas of repeat convection in
the Florida Panhandle and southeastern Alabama that will likely
cause high rainfall totals over the span of a few hours. Storms
will be capable of producing 2-3 inch per hour rain rates with
ample moisture (PWAT>1.5 inches) and CAPE (>1500 J/kg) to tap into.
FFGs in the Florida Panhandle and far southeastern Alabama have
decreased to 3-5 inches in 3 hours, which will be in danger of
being exceeded with areas of repeat convection today. Additionally,
heavy rain fell last night/early this morning in the Florida
Panhandle, which has helped to prime the ground and saturate soils
ahead of more heavy rain today. To account for this heightened
flood potential, a targeted Slight Risk area has been introduced
for portions of the Florida Panhandle and far southeastern Alabama.

Elsewhere in the Southeast, persistent areas of moderate rainfall
are expected on the northern periphery, north of the west-east
oriented stationary boundary. This should result in some decent
rainfall accumulations today from central and northern Mississippi
to South Carolina. The higher flood potential will be along and
south of the frontal boundary where thunderstorm development is
favored. Hi-res CAMs have been showing the potential for some more
organized convection late this morning into this afternoon. Given a
favorable environment with ample moisture and instability combined
with 35-45 knots of effective bulk shear, strong thunderstorms
that develop will be capable of producing high rain rates of 2+
inches per hour and could approach the 2.5-3 inch per hour FFGs in
these areas. By this afternoon, showers and storms should also pick
up along the Florida peninsula as diurnal heating increases. To
cover these threats, a broad Marginal Risk area is in place from
central Mississippi to South Carolina and to portions of North and
Central Florida.


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