Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
838 PM EDT Fri Sep 11 2026
Day 2
Valid 12Z Sat Sep 12 2026 - 12Z Sun Sep 13 2026

...THERE ARE SLIGHT RISKS IN PORTIONS OF THE OHIO VALLEY AND MID-
ATLANTIC...

21Z update... The QPF footprint saw a notable increase in areal
coverage and amounts for portions of the central Appalachian
spine/surrounding areas and for a few locations in the northern
Mid-Atlantic. This increase over already sensitive soil/steep
terrain/vulnerable areas necessitated the expansion of the Slight
Risk further north into Pennsylvania and east across West Virginia
and western Maryland in addition to a southward expansion of the
Slight over central/eastern Virginia. With the trough lifting out
portions of the Gulf Coast will see a reduction in QPF, thus a
small section of the Gulf Coast was trimmed from the Marginal Risk
area. A minor southward nudge was made across north-central
oklahoma to reflect the latest QPF guidance and WPC forecast.

Campbell

Previous discussion...

...Ohio Valley & Mid-Atlantic...

The broad 500mb troughing setup in the Mid-South will induce
healthy upper-level ascent over the Ohio Valley and Mid-Atlantic
on Saturday. A warm front is forecast to lift north through the
Mid-Atlantic on Saturday that will act as a trigger for storms,
while daytime heating and height falls support widespread showers
and thunderstorms from the Gulf Coast to the northern Mid-Atlantic.
PWs will rise above 2.0" as far north as the I-95 corridor from
Richmond and D.C. to Baltimore and Philly by Saturday afternoon
with increasing instability as the warm sector drifts north. Warm
cloud layers are likely to be at least 12,000ft, favoring highly
efficient warm rain processes within developing thunderstorms.
Where the heaviest thunderstorm activity unfolds remains unclear.
However, the preference was to favor a Slight Risk along the I-95
corridor largely due to the greater concentration of metropolitan
communities.

West of the Appalachians, soils throughout the OH Valley are
growing increasingly more sensitive with 1-hr FFGs hovering as low
as 1" over southeast OH. It will not take much from this favorable
atmospheric and hydrologic environment to support a concern for
scattered instance of flash flooding. For these reasons, a Slight
Risk was issued for portions of the Upper OH Valley for Saturday.

...Central Plains...

An approaching 500mb disturbance and a strengthening southerly
low-level jet will interact with a frontal boundary that results in
scattered thunderstorms from the central High Plains to the Mid-MS
Valley. PWs ranging between 1.5-1.75" are generally around the 90th
climatological percentile and ample instability will be supportive
of strong thunderstorms producing >2"/hr rainfall rates.
Guidance has trended lower in QPF in recent forecast cycles,
although the reduction in QPF is largely because model spread in
where the axis of heavier storms occur remains unclear. A Marginal
Risk remains in place given the favorable atmospheric setup with
localized flash flooding possible.

...Florida...

Much of the Sunshine State will remain mired in an environment
containing copious amounts of moisture that will continue to
support scattered thunderstorms containing torrential downpours.
PWs between 2.0-2.25" will be common and storms will be capable of
causing localized flash flooding, especially in poor drainage areas
and in the more urbanized sections of Florida.

...Rockies & Southwest...

A strong 500mb closed-low with heights below the 10th
climatological percentile will work with an increasingly anomalous
PW field to produce scattered thunderstorms with efficient rainfall
over the Northern Rockies. Farther south, the pool of anomalous PWs
that supported daily rounds of excessive rainfall in the Southwest
through the week will stick around and cause additional
thunderstorms that produce locally excessive rainfall rates.
Instability will be lacking over the northern Rockies, but soils
are more saturated following recent rainfall and may provide a
slightly more favorable environment for localized flash flooding.

Mullinax


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