Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
400 AM EDT Tue May 20 2025

Day 1
Valid 12Z Tue May 20 2025 - 12Z Wed May 21 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
OHIO AND TENNESSEE VALLEYS...

A deeply negatively tilted longwave trough stretching from Montana
to the mid-Mississippi Valley will continue flattening by becoming
even more negatively tilted through the period as the trough itself
lifts back into the broader more zonal jet stream. While this
trough will continue to support a surface low as it moves east from
Iowa to roughly the IN/OH/MI tripoint by 12Z Wed, the low will be
weakening with time. The low will be filling in part because the
zonal pattern will gradually decouple the low from the plume of
Gulf moisture as the LLJ that has been supporting the storms over
the middle of the country the past couple days gets shunted south
back towards the Gulf Coast. The result of this will be decreasing
amounts of atmospheric moisture available for the storms with time,
resulting in a subsequently decreasing flash flooding threat.
However, for this period, the southwesterly influx of Gulf moisture
will feed training storms across the Ohio and Tennessee Valleys,
including portions of eastern Kentucky and southern West Virginia 
which were hard hit with heavy rain and severe storms over the past
week or so. 

The inherited Slight Risk area was trimmed on the northwest side
due to a lack of instability over much of Illinois, but expanded a
few rows of counties across Tennessee towards the south and west. 
No significant changes were made to the surrounding Marginal Risk.

Wegman

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...

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 3
Valid 12Z Thu May 22 2025 - 12Z Fri May 23 2025

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

The inherited Marginal across eastern New England was downgraded
with this update in coordination with BOX/Taunton, MA and
OKX/Upton, NY forecast offices. The broad 1-2 inches of rainfall 
expected through the period will be from a wound up low that will 
develop off the coast. The low should shove any instability to its 
east out to sea, resulting in only stratiform rain across New 
England. While there may be a few urban concerns in Boston and 
Providence, the low rainfall rates should favor more ponding and 
very little, if any, flash flooding.

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