Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
828 PM EDT Tue May 20 2025
Day 2
Valid 12Z Wed May 21 2025 - 12Z Thu May 22 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR A PORTION OF
THE CENTRAL APPALACHIANS...

The Slight Risk remains largely unchanged with only minor
adjustments based on the most recent guidance. Most of the 12Z
guidance (aside from the RRFS) has sped up timing of precipitation
and also shifted to the north/northeast. This would lower the
probability of flash flooding across portions of northern West
Virginia while increasing it somewhat over western/central PA so
the Slight was adjusted accordingly.

---previous discussion---

A Slight Risk upgrade was introduced with this update for a
portion of southwestern Pennsylvania, northern West Virginia,
eastern Ohio, and far western Maryland. New shortwave energy will
begin to tug an upper level low north of Lake Superior southward
into the Midwest on Wednesday. Meanwhile, an active subtropical jet
and associated 110 kt jet streak will also round the base of that
upper level low. The cold air aloft associated with the upper low
is expected to locally increase instability across the Slight Risk
area Wednesday afternoon, while the multiple shortwaves moving
through result in a rather compact area with multiple rounds of
showers and thunderstorms moving through in rapid succession. This
portion of the central Appalachians is particularly sensitive to
flash flooding, as well as the urban concerns in and around the
Pittsburgh metro. While there is still limited guidance on the
nature of the storms, the RRFS solution suggests the above
convective evolution Wednesday and into Wednesday night.

Two big questions still remain...how far north will significant
instability get and how will this instability align with the
greatest forcing, likely to set up in and just north of the Slight
Risk area. A total elimination of instability shouldn't prevent
training convection, but it's likely to be mostly showers and
unlikely to result in more than isolated flash flooding. Further,
any northward shift in the track of the repeating shortwave
impulses could also reduce the flash flooding threat. Meanwhile a
more northward expansion of the instability or a slightly further
south track to the upper level energy could both enhance the flash
flooding threat. Thus, the consensus was a low end Slight with a
southward bias towards the greater instability.

Elsewhere, the Marginal Risk across the Northeast was trimmed
largely due to a certain lack of instability. While areas such as
NYC are likely to see periods of light rain for much of the period,
the stratiform nature should limit rainfall rates to a half inch
per hour at most, precluding any more than some ponding on roads.

The Marginal Risk in deep south Texas was trimmed on its eastern
side but expanded north. Convection is expected to develop along
the mountains of Mexico, but is unlikely to drift too far east off
those mountains, narrowing the flash flooding threat to the
immediate Rio Grande Valley.

Wegman

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