Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
411 PM EDT Sat Sep 12 2026
Day 1
Valid 1948Z Sat Sep 12 2026 - 12Z Sun Sep 13 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE OHIO VALLEY, MID-ATLANTIC AND SOUTHERN ARIZONA...

20z Update: A Slight risk was added to southern AZ where convection
is developing within an environment of overlapping high CAPE and
PWs. Observational trends and HREF/REFS FFG exceedance
probabilities support a scattered flash flood risk over this area.

Chenard

1630Z Excessive Rainfall Discussion...

Maintained the Slight Risk area in the Ohio Valley and adjacent
portions of the Mid-Atlantic region given the ingredients in place
to focus and support convection capable of producing downpours. 12Z
upper air analysis showed precipitable water values around 1.75
inches in place with the 12Z NAM and HREF both supporting a surge
of precipitable water values of 2 inches or more reaching the
southeast Pennsylvania by early this evening and reach northern New
Jersey and the Tri-State area in the predawn hours Sunday. The
previous forecast had a good handle on this so the changes tended
to be pretty minor. The heaviest rainfall still looks to be focused
farther west through central PA and into the Upper OH Valley, a
convergence of elevated low-level theta-e within the strengthening
area of upper level divergence associated with the approach of a
surface cold front will support convection capable of being
efficient rainfall producers. The 12Z HREF/REFS probabilities for
2 inch amounts in 3 hours remained in the 50 to 60 percent range
from late this afternoon into the evening. Given the lowered Flash
Flood Guidance from recent rainfall in the area going back a few
days...still think this is a "higher-end" Slight Risk for the Upper
OH Valley, with potential for these rates.

Elsewhere...the Marginal Risk areas looked to be in good shape
given the anomalous moisture in the Central Plains as a wave lifts
northeastward later today and overnight while the monsoon-fueled
convection ramps up this afternoon with daytime heating from the
Southwest US into portions of the Rockies.

Bann

0830Z Excessive Rainfall Discussion...

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

The broad 500mb trough in the Mid-South will induce healthy upper-
level ascent over the Ohio Valley and Mid-Atlantic through Saturday
night. A warm front lifting north through the Mid-Atlantic 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 between 2.0-2.25" will stream
north along the I-95 corridor from Richmond and D.C. to Baltimore
and Philly by Saturday evening, then into the NYC/Tri-State areas
in the pre-dawn hours Sunday. With instability increasing as the
warm sector drifts north and warm cloud layers likely to be at
least 12,000ft, highly efficient warm rain processes within
developing thunderstorms are likely. The 00Z HREF shows low-to-
moderate chances (20-40%) for 3-hr QPF >2" from the Richmond, VA
area late Saturday afternoon, then from northeast MD into the
Delaware Valley and southern NJ Saturday night. The Slight Risk was
extended north through eastern PA, northern NJ, and into the NYC
metro and lower Long Island. It is important to note for the
northern NJ suburbs, NYC, and Long Island that the Slight Risk is
primarily for late Saturday night with the excessive rainfall
threat lingering into Sunday (Day 2).

Farther west through central PA and into the Upper OH Valley, a
convergence of elevated low-level theta-e within the strengthening
area of upper level divergence in the 200-400mb layer will generate
bands of thunderstorms that track parallel to the mean 850-300mb
wind flow. Soundings are highly saturated from just above the
surface to as high as 300mb (>90% RH). Similar to areas farther
east, warm cloud layers are at least 12,000ft deep and FFGs are
less than 2" in northern WV and western PA. The 00Z HREF shows
worrying signals with moderate-to-high chances (50-80%) for 3-hr
and 6-hr FFG exceedance within the Upper OH Valley, and moderate
chances (40-60%) for 2" of rainfall within 3-hours Saturday
afternoon. This is a "higher-end" Slight Risk for the Upper OH
Valley, with potential for these rates to spills over into north-
central PA early Saturday evening.

...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.
The latest 00Z HREF guidance shows 24-hr QPF >3" probabilities
remain on the low end (10-30%), and are isolated across eastern KS
and western MO. For those reasons, decided to hold on with a
Marginal Risk this forecast cycle.

...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 Central Rockies, but soils
are fairly saturated in parts of AZ and NM following recent
rainfall and provide a slightly more favorable environment for
localized flash flooding.

Mullinax


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt