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

Day 1
Valid 01Z Mon May 12 2025 - 12Z Mon May 12 2025

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

Isolated to scattered flash flooding will remain a concern
overnight across portions of southeast GA and southern SC. Some of
this area has already seen training convection and some flash
flooding earlier today, and additional convection is streaming
northward back into the region. The latest SPC mesoanalysis depicts
an area of instability offshore, and with easterly low level flow,
this will likely advect inland and help sustain convection across
these areas. Expect periodic backbuilding/training to persist, and
some additional flash flooding is likely. The REFS has been
favoring this corridor for heavy rainfall with both its 12z and
18z run, but the HREF and recent HRRR runs have not been as
aggressive. However the 23z HRRR run did make a pretty significant
jump towards a wetter scenario...and based on observational
trends...it does appear like something closer to the wetter
guidance may indeed pan out. Opted against a MDT risk given some
lingering uncertainty on the location and coverage of higher
amounts, along with much of the coastal plain having high FFG...but
do now consider this a higher end Slight risk and localized higher
end impacts are possible.

We also maintained a Slight risk across portions of central AL
into northern MS. Convection to the east of the deep layered low
will continue to pose an isolated to scattered flash flood risk.
Some recent indications that an uptick in activity may be starting
to occur across portions of central AL. Guidance indicates some
maintenance of this convection through the evening hours, so do
anticipate some increase in the flash flood potential.

A Marginal risk covers much of FL. Convective evolution is less
certain here...but do expect to see scattered to numerous showers
and thunderstorms. The combination of low confidence in model
solutions over FL and the high FFG keeps the risk at Marginal. It
is possible that none of the convection organizes enough or trains
long enough to cause flash flooding overnight...but can not rule
out some training segments hanging up and resulting in isolated
flash flood concerns.

Chenard


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 3
Valid 12Z Tue May 13 2025 - 12Z Wed May 14 2025

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

20z Update: The Slight and Marginal risk areas were expanded a bit
to the west with this update. The Marginal risk covers more of the
OH valley where slow moving convection ahead of the deep layered
low could result in localized flash flooding. The Slight risk is
expanded into more of the central Appalachians. Overall still
looks like a solid Slight risk from northeast NC into the portions
of eastern WV. Convection near a stalled boundary should result in
an isolated to scattered flash flood risk. Will need to closely
monitor, as the slow moving closed low and a stationary boundary
does suggests some higher end impact potential Tuesday within the
Slight risk area.

Chenard

...Previous Discussion...

The slow-moving upper low and occluded surface low will continue
to move north, triggering showers and storms across the Mid-
Atlantic on Tuesday. A deep stream of moisture ahead of the system
will push PWAT values above 1.5 inches, and there will be enough
instability (MUCAPE > 1500 J/kg) across portions of North Carolina
and Virginia to support high rain rates in showers and storms. It
seems likely that rain rates could reach or exceed the 1-2 inch per
hour FFGs in Central Virginia (especially in upslope and urban
areas), and possibly the 2.5-3 inch per hour FFGs in southern
Virginia and northeastern North Carolina as well. Models have been
picking up on a faster trend with this system, but there is some
uncertainty in how fast or slow it will move. This will affect the
location of the heaviest rainfall in the warm sector and
along/ahead of the cold front. The general consensus is for the
highest totals to fall over Virginia and northern North Carolina,
but some models are advancing heavy rain even further north.

To account for the faster trend in guidance, the inherited Slight
and Marginal Risk areas from the previous Day 4 ERO have been
shifted north, with the Marginal extending up into portions of
Pennsylvania and New Jersey. The southern edge of the Slight Risk
area has been trimmed up to match QPF trends, and it was extended
east to coastal Virginia where urban areas tend to be more
sensitive to heavy rain and flooding. It's possible that the
location of the Slight Risk will shift in future updates as the
speed of the system becomes more certain.

Dolan


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