Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
400 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...

A vigorous upper level shortwave tracking eastward to the south of
a broad upper level low over Ontario will bring a pair of lows into
the Great Lakes and Mid-Atlantic today. The leading front ahead of
these lows will be the focus for rainfall through the day and into
tonight. The northwestern-more of the surface lows will track 
north of the Slight Risk area towards Lake Erie, while the second 
low will form along the coast near the VA/NC border. This pattern 
will set up a west to southwesterly predominant flow into the 
Appalachians from the west, while also favoring broad easterly 
marine flow into the Mid-Atlantic. Where these two opposing flows 
meet, enhanced uplift over the terrain will wring out as 
occasionally heavy rainfall the decreasing available atmospheric 
moisture. This area of southwest Pennsylvania and northern West 
Virginia is particularly sensitive to heavy rainfall causing 
flooding. This afternoon expect limited instability to advect into 
the area, which should allow for more convective shower activity, 
which in the unidirectional flow will favor training of those 
showers into the mountains. This instability will have a hard time 
advecting too far north however, so the Slight Risk area was 
trimmed on the northern side, despite central PA's sensitivity to 
heavy rain as well, as the rain while persistent will be unlikely 
to fall heavily enough to cause more than isolated flash flooding. 
Meanwhile further south, more instability will be present but the 
forcing from the upper levels will diminish rapidly the further 
south you go across West Virginia, so the Slight is highlighting 
the small region where instability and forcing overlap.

Elsewhere, the Marginal Risk was trimmed westward away from the
coast of the Mid-Atlantic due to very limited, if not zero
instability. So here too the ability for the convection to produce
heavy rainfall will be very limited.

Finally, across Deep South Texas, other than a small trim off the
northern end of the Marginal, the risk area remains about the same.
Afternoon and evening convection is likely on the Mexico side of
the river, but that convection continues to be simulated in the
guidance to drift east into Deep South Texas. PWATs above 2 inches
in this area will be very supportive of that convection producing
very heavy rain, despite the bone dry soils in the area, so
isolated flash flooding is possible.

Wegman

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: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt