FOUS30 KWBC 112029
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
429 PM EDT Sun May 11 2025

Day 1
Valid 16Z Sun May 11 2025 - 12Z Mon May 12 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE SOUTHEAST...

16z Update: The best signal for isolated to scattered flash
flooding this afternoon/evening appears to be over portions of
southeast GA into northeast FL where backbuilding cells will result
in rainfall locally over 3 inches. In fact both 12z HREF and 06z
REFS neighborhood probabilities of exceeding 5" are over 50%, and
if/where this occurs would expect at least some minor flood
concerns to develop.

By this afternoon convection should develop over portions of
central AL into MS closer to the deep layer low center. This
activity will also likely exhibit some training/backbuilding
characteristics resulting in an isolated to scattered flash flood
risk. Some localized exceedance of 3" appear probable here, and
both the HREF and REFS indicate some FFG exceedance probs as well.
The Slight risk was expanded a bit to the west to account for this
activity over MS.

The northern edge of the Slight risk over the Carolinas was
tightened a bit as the main flash flood risk here will likely be
on day 2 (after 12z Monday). The Marginal risk was expanded
southward to include the rest of the FL peninsula as slow moving
convection may try to move into south FL overnight.

Chenard

...Previous Discussion...

A stationary upper low will remain positioned over the Central
Gulf Coast today while waves of upper level energy pivot around it
over the Southeast. The low is expected to become slightly
elongated north to south, resulting in strong southerly mean flow
that will pump moisture from the Gulf directly into the Southeast.
Additionally, a southern stream jet is forecast to strengthen over
the northern Gulf, creating favorable upper level divergence over
North Florida and Georgia to support widespread shower and
thunderstorm development. At the surface, an occluded low pressure
system will be slowly moving north across the region, with a cold
front extending south across the Florida Panhandle and a stationary
front extending east across Georgia and South Carolina. The best
conditions for heavy rain will be in the warm sector over North
Florida and southern Georgia, but heavy rainfall will also be
possible along and north of the stationary boundary up into
portions of the Carolinas.

With plentiful moisture (PWATs 1.5-2+ inches) and instability
(MUCAPE 1500-2000+ J/kg), storms in the warm sector will be capable
of producing 2-3+ inch per hour rain rates from Florida to South
Carolina, which could lead to some flash flooding concerns given
2-3 inch per hour FFGs. Flash flooding may also be a concern back
into portions of Alabama and northern Mississippi where there will
likely be enough moisture and instability in the vicinity of the
surface low to produce showers and storms with 2+ inch per hour
rain rates. Previous heavy rains have already saturated soils from
Mississippi to Georgia, which will make these areas more sensitive
to flooding with additional heavy rain today. To account for this
flood potential there is a Slight Risk of excessive rainfall in
place for much of the Southeast, including areas from North Florida
to southern North Carolina and much of Alabama and northern
Mississippi. The Slight Risk area is encompassed by a broader
Marginal Risk that includes much of the Tennessee Valley, eastern
Arkansas, upslope regions of the southern Appalachians, and Central Florida.

Dolan


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

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