Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
855 PM EDT Sun May 11 2025
Day 2
Valid 12Z Mon May 12 2025 - 12Z Tue May 13 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHEAST AND SOUTHERN MID-ATLANTIC...

20z Update: Continue to be very close to needing a MDT risk
upgrade across the upslope areas of the southern Appalachians, but
we will hold at a higher end Slight risk for now. Orographically
enhanced rainfall and embedded convective elements periodically
pushing rainfall rates over 1"/hr will likely drive at least a
minor flash flood risk across this portion of the western
Carolinas and southwest VA. However confidence on seeing more
significant higher end impacts remains lower. With easterly low
level flow in place over this corridor the better instability may
tend to stay farther south over GA/SC and over the eastern
Carolinas. Probabilities of exceeding 1"/hr in the HREF peak
around 40-50% in the 12z HREF and REFS, likely indicative of some
exceedance, but probably not widespread. It will likely take these
higher rates of 1-2"/hr or greater to drive more significant
impacts.

The currently forecast 3-7" of rainfall, combined with
embedded rates of 0.5"-1"/hr should be enough to drive at least
scattered instances of minor flash flooding, with localized more
significant impacts still possible. For now the expectation of
scattered minor flash flood and localized higher end impacts falls
more into the high end Slight risk category. If confidence in
greater coverage of higher rainfall rates increases, that would in
turn increase the potential for higher end impacts and justify a
MDT risk upgrade. Will thus continue to closely monitor.

The southeastern FL urban corridor is another area of concern for
flash flooding Monday. Neighborhood probability of exceeding 8"
are over 50% in both the 12z HREF and REFS and some deterministic
members are over 10". Overall seems like a favorable synoptic
setup for training convection and thus higher end urban impacts
are possible. Not confident enough yet to go with a MDT risk
upgrade, however do consider this a higher end Slight risk and the
potential is certainly there for significant urban impacts. We
will continue to monitor observational and model trends.

Chenard

...Previous Discussion...

The nearly stationary upper low that impacted the Southeast over
the weekend will finally gain some momentum and shift north towards
the Tennessee Valley on Monday, pulling the surface low pressure
system along with it. The trailing cold front will gradually push
towards the Southeast Atlantic Coast while the warm front lifts
into the southern Mid-Atlantic, and another round of widespread
showers and storms are forecast Monday into Monday night in the
warm sector of the system. Deep moisture will surge north ahead of
the cold front, extending from Florida through the Carolinas to
southern and central Virginia. PWATs are anticipated to reach
1.5-2+ inches in these areas with even higher values possible over
the Florida Peninsula. Strong south-southeasterly mean flow will
remain in place, which will likely result in two convergent zones,
one along the Southeast Coast and one along upslope regions of the
southern Appalachians where high QPF totals will be possible.
Strong diurnal convection over the Florida Peninsula will also
contribute to high QPF values over portions of Central and South
Florida.

The largest flooding concerns will come with repeat/training
activity, which could result in high QPF totals over a relatively
short period of time, potentially reaching or exceeding the 1 and 3
hour FFGs in these regions. To account for this flash flood risk,
there is a Slight Risk area stretching from Florida up the
Southeast Coast to North Carolina and southern Virginia and also
for the upslope regions of the Appalachians from Georgia to
Virginia. A Marginal Risk area extends even further into the Mid-
Atlantic where moderate rain rates may contribute to localized
flash flooding and into the Tennessee and Mid-Mississippi Valleys
where enough moisture will likely wrap around the surface low to
create locally heavy rainfall in showers and storms.

Upslope regions of the southern Appalachians are particularly
sensitive to heavy rain, especially in areas that were impacted by
Hurricane Helene. Impacts from heavy rain on Monday will likely
fall into the high-end Slight Risk category, and ERO upgrades may
be needed in the future if high QPF trends hold,. However,
instability may be a limiting factor as the better instability is
expected to reside along the coast. After coordinating with local
forecast offices, it was decided to hold off on any upgrades at
this time. By this afternoon, the CAMs will go out far enough to
cover the entire Day 2 period and will likely give us a better idea
of the magnitude of potential flooding impacts.

Dolan

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