Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
900 PM EDT Sat Sep 12 2026
Day 2
Valid 12Z Sun Sep 13 2026 - 12Z Mon Sep 14 2026

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

2030Z Excessive Rainfall Discussion Update...

Northeast...

Rainfall should be on-going as the Day 2 period begins at 13/12Z
over portions of northern New Jersey eastward into southern New
England as mid-level height falls induces surface low
pressure...albeit weak...in proximity to the Southern New England
coastline. This is a bit delayed from the previous suite of model
guidance. As a result...there may be a bit more time for moderate
to heavy rainfall and the Slight Risk was expanded a bit northward
and westward...including more of Long Island and adjacent coastal
areas in addition to the previously mentioned portions of
Connecticut and Rhode Island into Massachusetts. The 12Z HREF
probabilities depicted 45-60%) for 3-hr QPF >3" across the area
during the mid- to late morning hours on Sunday.

Midwest...
The 12Z suite of model guidance tended to show somewhat better
clustering in the QPF guidance as flow over the Upper Midwest backs
in response to height falls coming out of the Rockies, As a
result...the Slight Risk and surrounding Marginal Rusk areas were
adjusted. There may be room for additional adjustment but kept the
changes modest at this point in case the model opinion swings back.

...Southern Rockies & Four Corners...
Monsoonal moisture remains in place as flow aloft strengthens in
response to the mid-level system approaching the international
border late in the period. Current outlook still appears to capture
the area with the greatest chances of heaviest rain based on model
spaghetti plots.

Bann

0830Z Excessive Rainfall Discussion

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