FOUS30 KWBC 150804
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
404 AM EDT Thu May 15 2025

Day 1
Valid 12Z Thu May 15 2025 - 12Z Fri May 16 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
NORTHERN PLAINS AND MID-ATLANTIC...

...Northern Plains...

A well-developed low will track north into the eastern Dakotas into
this afternoon, with a slow and chaotic track through tonight. This
track is supported by a nearly stationary upper level low, with a
potent shortwave trough rounding its southern periphery, thus
increasing the divergence aloft. While the low itself will be able
to tap into some of the Gulf moisture across much of the eastern 
half of the country, this is not expected to result in significant 
flash flooding. For North Dakota, any associated instability is 
likely to stay east of the low center. Meanwhile, the cold conveyor
belt feeding the comma-head region of the surface low will be 
unable to also advect instability into the region. Thus, while the 
24 hour rain forecast remains consistent with continuity with a 
forecast for 2-3 inches of rain areally across western North Dakota
through 12Z Friday, the lack of instability will keep that 
rainfall as stratiform, and therefore unlikely to cause flash flooding.

...Mid-Atlantic...

Multiple rounds of rainfall are likely to impact portions of the
Mid-Atlantic through this Day 1 period. This will be on the heels
of several days of more widespread heavy rain that has occurred
over the past few days. While today should be a wind down of the
coverage of heavy rain, it's still possible that any storms that
form today into tonight will merge together, train, or remain so
slow moving as to cause localized flash flooding. As an occluded
front associated with the low over the Northern Plains approaches,
it will turn the predominant storm track towards the east and
northeast. This will be a significant change in the storm motions
from previous days, and one that should ultimately keep any flash
flooding producing storms as few and far between. Since the soils
are very moist and prone to flash flooding, the lessening coverage
of storms over the area would still favor the continuation of the
Marginal Risk for today.
 
Wegman


Day 2
Valid 12Z Fri May 16 2025 - 12Z Sat May 17 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE 
OHIO VALLEY...

A stalled and shearing apart occluded front over the Ohio Valley
early Friday morning will dissipate through the day as its
associated rainfall ends. However, a reinforcing strong shortwave
will round the southern periphery of the vertically stacked low,
which will be pushing east from the Northern Plains and into the
upper Great Lakes. This shortwave and associated push of cooler air
in the form of a cold front will tap into the moisture and
instability largely in place ahead/southeast of the cold front over
the Ohio Valley, resulting in multiple areas of storms forming
Friday afternoon from southern Illinois through West Virginia.
While some training is possible as the storms are forming, they're
likely to develop into a single line, where any training would be
from pre-line convection that will likely be moving into and being
absorbed by the line. The line of storms will continue south into
the Mid-South through Friday night, albeit with lessening flash
flooding potential as the line becomes the only area of storms
around. As with areas further east, soil moisture levels all up and
down the Ohio Valley are at or above normal for this time of year,
and the presence of moisture to the order of 1.5 to 1.75 inches of
PWAT will support strong thunderstorms capable of 2+ inch/hour
rainfall rates. Given the sensitivities of the rivers and streams
in the area, the combination of heavy rain and sensitive soils
supported the continuance of the Slight Risk area, with some
expansions included to account for more the suite of guidance.

Wegman


Day 3
Valid 12Z Sat May 17 2025 - 12Z Sun May 18 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM CENTRAL
TEXAS THROUGH NORTHERN ALABAMA AS WELL AS FOR PORTIONS OF THE
NORTHEAST...

...Texas to Alabama...

Another round of afternoon and evening thunderstorms is expected
from northeast Texas into northern Alabama Saturday and Saturday
night. Continued influx of Gulf moisture and instability on
southerly flow will clash with the cooler, dry air mass behind a
cold frontal passage over much of the Midwest on Friday. The storms
will form along this frontal interface. With that said, the forcing
in the upper levels should be greatly reduced over this region as
compared with previous days, as the upper forcing on Saturday will
be associated with a fast moving and weaker shortwave. This should
limit the coverage of storms. Once again however, recent heavy
rains in this area continue to keep the soils wet, and therefore
more prone to flash flooding. Should the forcing increase or amount
of moisture off the Gulf increase compared to current forecasts,
then it's possible a targeted Slight may eventually be needed,
with the DFW Metroplex area currently appearing to be at greatest
risk for an upgrade.

...Northeast...

Periods of heavy rain will move across the Northeast Saturday
afternoon ahead of a potent upper level low moving across the Great
Lakes. The storms will occur ahead of the upper low. There is some
uncertainty on storm coverage and intensity with moisture amounts
being limited in this area, but it is made up for by wetter than
average soils in this area of New England and New York. The
inherited Marginal Risk was trimmed out of most of Maine, as soils
there are drier than normal, which should allow any rainfall (which
should be less than an inch) to be absorbed by the dry soils.

The greatest flash flooding risk is likely in the Green and White
Mountains where terrain will worsen any potential flash flooding.

Wegman

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