Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1145 AM EDT Wed May 21 2025

Day 1
Valid 12Z Wed May 21 2025 - 12Z Thu May 22 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
CENTRAL APPALACHIANS AND EASTERN OHIO VALLEY...

...Ohio Valley through the Mid-Atlantic States...
A series of shortwaves will interact and consolidate into a large
upper low centered over the Great Lakes by the end of D1 /12Z
Thursday/. During this evolution, impulses/shortwaves rotating
around the amplifying low will spin northeast from the Ohio Valley
into the Mid-Atlantic and Northeast, providing periods of enhanced
ascent. This lift will encounter favorable thermodynamics
characterized by PWs as high as 1.25 inches, overlapped by a plume
of modest MUCAPE reaching above 500 J/kg, especially across western
PA. Low-level flow at 850mb will vary in direction, but there is
likely to be an area of enhanced convergence across the Central
Appalachians as far north as northern PA where westerly flow south
of the primary gyre meets locally backed SE flow from the Atlantic
spreading inland from the NJ/DE coasts. This will result in
enhanced convergence and moisture confluence, combining with at
least subtle upslope flow to enhance lift across the region. Where
temporally this combines with the greatest instability, which will
likely peak within a narrow warm sector downstream of a surface low
moving near Lake Erie, showers and thunderstorms with heavy rain
are expected.

Although activity is ongoing this morning, with clouds and some
weak subsidence behind the primary surge of moisture will delay the
convection this aftn at least a few hours, but the high res CAMs
support a resurgence of convection with rain rates of 1-2"/hr at
times this aftn. This activity is progged to congeal into a cluster
or MCS moving across OH, PA, and WV this evening as convection
becomes organized through 0-6km bulk shear of 30-40 kts, and lapse
rates steepen beneath the developing upper low. Although this
cluster, or clusters, will move rapidly on mean cloud-layer winds
of around 25 kts, some short duration training is possible along
the line to enhance the temporal length of these rain rates.
Additionally, showers and thunderstorms blossoming behind the
primary line and beneath the steepening lapse rates may produce
repeated rounds of heavy rainfall across the same areas into
tonight. FFG across this region is only 1-2"/3 hrs, which has a
10-20% chance of exceedance according to the REFS and HREF,
supporting the inherited SLGT risk which was modified only
cosmetically for recent guidance.

Farther east towards the coasts of DE/NJ and the eastern shore of
MD, the 24-hr ensemble probabilities, including rain occurring
this morning, feature a low end risk (10%) for 3" of rainfall in a
few rounds through tonight. However, instability is minimal which
will generally limit the potential for any heavy rainfall rates.
For this reason, the recently trimmed MRGL risk for these areas
remains left out despite some areas likely picking up 2-3" of
rainfall today.

...South Texas and Gulf Coast...
The tail end of a cold front draped along the Gulf Coast will serve
as the impetus for convection today, some of which has already
developed across Louisiana. Plentiful ascent along this front
through convergence combined with impressive PWs and high
instability will support rainfall rates in excess of 2"/hr at
times, which has already been occurring within an FFW over SW LA.
Storm motions are progged to remain 10-15 kts along the front to
the east, but will be aligned to the Corfidi vectors, and with
upstream development likely, this will result in at least short-
term training for parts of the region. Despite dry antecedent
conditions and high FFG, training of these rates could support
isolated flash flood instances today. After coordination with the
metwatch forecasters and the Gulf Coast WFOs, a MRGL risk has been
expanded from the Lower Texas Coast through eastern Louisiana.

Farther SW, the setup is favorable for convection to blossom
rapidly across the Sierra Madre of Coahuila, Mexico as a modest
shortwave lifts eastward and interacts with extreme thermodynamics
this afternoon and evening. PWs as high as 2+ inches and MUCAPE
above 3000 J/kg will drift westward as modest 850mb inflow from
the Gulf pushes onshore the lower Texas Coast and across the Rio
Grande Valley. This will effectively upslope into the Sierra Madre,
providing additional impetus for convective development in the
terrain. With 0-6km bulk shear progged at 30-45 kts, this will
quickly organize into clusters or an MCS which will then follow the
moisture eastward into South Texas. Mean 0-6km winds of just 5-10
kts combined with aligned Corfidi vectors of 15-20 kts suggests
some training is likely as this MCS pivots east, and with rain
rates likely exceeding 2"/hr, this will result in areas of QPF of
more than 3", with both the HREF and REFS probabilities indicating
a low-end potential (10-15%) for 5+ inches along the Rio Grande
Valley. This area has been dry recently (7-day rainfall less than
5% of normal according to AHPS) which will limit the flash flood
risk, but these intense rain rates could still locally result in
instances of flash flooding.

Weiss

Day 2
Valid 12Z Thu May 22 2025 - 12Z Fri May 23 2025

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

A couple areas will be monitored on Thursday, including far north
Texas along the Red River, as well as for the urban corridor of
southeast Florida. Convection in either of these areas may cause
isolated and localized flash flooding, but there is enough
uncertainty with especially the coverage, and in Florida the
potential movement (or lack thereof) of the convection to forego a
Marginal for now, but one may be needed with future updates.

Wegman

Day 3
Valid 12Z Fri May 23 2025 - 12Z Sat May 24 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE PLAINS AND OZARKS...

Return/southerly flow off the Gulf will continue to increase the
available atmospheric moisture across the southern Plains as much
of Texas and Oklahoma is parked under a broad southerly flow regime
through the day Friday. Showers and thunderstorms are expected to
develop at the nose of this jet as early as Friday afternoon, but
the strongest storms and those most likely to cause flash flooding
are expected to hold off until after midnight Friday night across
the Marginal Risk area. This is largely due to the nocturnal
strengthening of the low level jet. This will advect air with PWATs
over 1.5 inches into the southern and central Plains.

While there will be a leeside low developing over southwest Kansas
Friday night that may help to focus the forcing against a large
Canadian area of high pressure over much of the eastern half of the
country, the convection will have to contend with a large upper
level ridge over the area, which will send any help from the upper
levels well north of the Marginal Risk area, and limit instability
a bit with warm air aloft. Further, as with most forecasts of
convection, there are likely to be substantive changes as to where
the strongest storms set up. Thus, while the rainfall forecast has
come up quite a bit, the confidence is not quite there yet for a
Slight, though trends would absolutely favor the issuance of an
upgrade somewhere in the general area over the next couple days.

South Florida will also need to be monitored as the classic
"stalled front" will remain over the area, acting as a focus for
additional afternoon showers and thunderstorms, that if tied to a
surface feature like the sea breeze, could result in localized
flash flooding. Here too a Marginal Risk might be needed with
future updates.

Wegman


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