FOUS30 KWBC 120739
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
339 AM EDT Sat Sep 12 2026

Day 1  Valid 12Z Sat Sep 12 2026 - 12Z Sun Sep 13 2026

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

...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 2  Valid 12Z Sun Sep 13 2026 - 12Z Mon Sep 14 2026

...THERE ARE SLIGHT RISKS FOR EXCESSIVE RAINFALL OVER PORTIONS OF
THE NORTHEAST & MIDWEST...

...Northeast...

The region will reside beneath the right-entrance region of a
strengthening 250mb jet streak, creating strong 200-400mb layer
Q-vector divergence over the Northeast. This is allowing for
unusually strong upper-level frontogenesis atop southern New
England, in addition to the surface warm front that is forecast to
span a strengthening surface low over Long Island early Sunday
morning. This is a continuation of the bands of strong
thunderstorms containing highly efficient rainfall rates from
Saturday night. PWs up to 2.25" are expected and low-level theta-e
advection into the warm front will direct MUCAPE between 500-1,000
J/kg into southern New England.Area averaged soundings across
multiple CAMs members paint a concerning picture; highly saturated
profiles within a sheared environment that could support enhanced
mesoscale rainfall rates >2"/hr within an urbanized corridor. The
rainfall threat moves into RI and southeast MA late morning and
into the afternoon before exiting off the coast late afternoon.

This setup led to the issuance of a Marginal Risk that extends
from northern NJ to as far north and east as southern NH/ME, and a
Slight Risk from the NYC metro on east to the Boston metro and MA
Capes. There is greatest concern for flash flooding impacts over
Long Island and the coastal portions of CT and RI where 00Z HREF
probabilities should moderate chances (40-60%) for 3-hr QPF >3"
across the mid-late morning hours on Sunday.

...Midwest & Central Plains...

A frontal passage and a fast moving 500mb shortwave trough heading
for the Great Lakes will trigger rounds of showers and
thunderstorms from South Dakota and the Nebraska Sand Hills to as
far south as northern KS/MO and as far east as the MS River. As the
first shortwave trough exits east, a strong 500mb closed low will
head through the Northern Rockies, prompting the development of low
pressure in the northern High Plains. This area of low pressure
will work in tandem with high pressure over the Great Lakes Sunday
night to generate a 30-40kt LLJ over the Central Plains. The LLJ
will advect rich moisture and warmer theta-e air into the Central
Plains with the frontal boundary that passed through early Sunday
morning lifting slowly as a warm front Sunday night and into early
Monday morning.

Latest ECMWF guidance shows a pool of anomalous PWs (above the
97.5 climatological percentile) sitting over the Central Plains on
Sunday that lifts north into the Midwest by early Monday morning.
With the LLJ intersecting the frontal boundary, and elevated
instability available, additional strong thunderstorms are likely
to form late Sunday night and into early Monday. While exact
placement of the heaviest rainfall still differs, most guidance is
in agreement that the Midwest will be the focus for potential flash
flooding on Sunday and into Sunday night. The latest WPC QPF has
shifted the heaviest rainfall into the MO River Valley with 1-2"
rainfall amounts (locally higher possible) in northeast KS,
northwest MO, eastern NE, and southwest IA.

...Southern Rockies & Four Corners...

The troughing pattern in the Northwest and the mid-upper level
ridge axis over the Southern Plains will direct additional
monsoonal moisture into the Southwest and Southern Rockies. ECMWF
percentiles suggest PWs will rise to above the 97.5 climatological
percentile Sunday evening as IVT values >300 kg/m/s direct moisture
into the regions. Some locations continue to sport saturated soils,
thus making these areas more prone to potential flash flooding.
While confidence in higher rainfall rates is lower at the moment,
the rgion's sensitive soils and potential for excessive rainfall
rates >1.0"/hr continues to support the Marginal Risk in place.

Mullinax
 

Day 3  Valid 12Z Mon Sep 14 2026 - 12Z Tue Sep 15 2026

...THERE ARE SLIGHT RISKS FOR EXCESSIVE RAINFALL OVER PORTIONS OF
THE MIDWEST & SOUTHERN ARIZONA...

...Midwest...

A strong closed 500mb low, sporing 500mb heights below the 10th
climatological percentile per ECMWF, will work in tandem with a
strong 500mb ridge over the Mid-South to produce an exceptional IVT
throughout much of the North Central U.S.. NAEFS shows the IVT
topping 750 kg/m/s over the MO River Valley and into MN Monday
morning, then strengthening to >1,000 kg/ms/s over the UppeR MS
Valley by Monday afternoon. Strong theta-e advection and anomalous
PWs that surpass the 99th climatological percentile will set the
stage for an active day, consisting of widespread showers and
thunderstorms that contain highly efficient warm rain processes. The
area of greatest concern is along the warm front, closest to the
surface low track, as veering low-level wind profiles ahead of the
approaching cold front support clusters of organized thunderstorms.
Latest WPC QPF shows widespread 1-2" of rainfall from eastern NE
to as far north as the Upper Great Lakes with localized totals
surpassing 3" possible. The inherited Slight and Marginal Risks
remain in good shape, but extended the Slight Risk into southern NE
to account for recent south and west trends in QPF.

...Southwest...

Broad upper-level troughing over the West and upper-level ridging
in the Southeast will continue to support a healthy monsoonal flow
into AZ/NM and as far north as the Central Rockies. NAEFS shows IVT
values still topping the 90th climatological percentile for mid-
September. In addition, the Four Corners region is ideally placed
beneath the right-entrance region of a 250mb jet streak that favors
healthy upper-level ascent. The region has remained fairly active
in recent days and will continue to see daily rounds of scattered
showers and storms over the weekend, helping to further saturate
soils ahead of Monday. QPF is heaviest in southeast AZ at the
moment, and given soils are more likely to moisten rather than dry
out in the coming days, a Slight Risk was issued in ands around the
Tucson area for Monday. A broad Marginal Risk accounts for the
potential of localized flash flooding from central AZ on east
through much of NM and northward into the CO Rockies.

Mullinax
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