FOUS30 KWBC 130746
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
346 AM EDT Sun Sep 13 2026

Day 1
Valid 12Z Sun Sep 13 2026 - 12Z Mon Sep 14 2026

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

...Northeast...

The Northeast resides 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
spawn 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.

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. The 00Z HREF also
shows moderate chances (40-60%) for 6-hr QPF surpassing 6-hr FFGs.
Both the 00Z HREF and REFS show high-chance probabilities (>70%)
for rainfall totals over 3" through early afternoon along the
southern New England coast. The Slight Risk continues to remain in
good standing, with the flash flood threat likely ending throughout
southern New England by late afternoon.

...Central Plains...

A frontal boundary draped over the Central Plains and a prolonged
SWrly LLJ in the Southern High Plains will act as a source of lift
and moisture transport throughout much of the day. By evening, an
area of low pressure will work in tandem with high pressure over
the Great Lakes Sunday night to strengthen the LLJ to as much as 40kt
LLJ over the Central Plains. A plume of high theta-e air will
continue to be oriented along the frontal boundary, with
additional storms likely to develop Sunday night. 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
now showing eastern KS and western MO becoming favored for the
locally heavier rainfall totals. 00Z HREF guidance shows low-to-
moderate chances (20-40%) for >3" of rainfall over the Kansas City
metro area. By Sunday night, the emergence of a strong closed 500mb
low over the northern Rockies will direct a rich plume of >99th
climatological percentile PWs into the Northern Plains. Given the
progressive nature of these storms and modest elevated instability,
the flash flood potential is not too concerning at the moment, but
the highly saturated profiles do support at least a Marginal Risk
through Sunday night.

...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. The footprint of saturated soils has only grown
following the active Saturday in AZ, thus making these areas more
prone to potential flash flooding. While confidence in higher
rainfall rates is lower at the moment, the region's sensitive
soils and potential for excessive rainfall rates >1.5"/hr
continues to support the Marginal Risk in place.

Mullinax
 

Day 2
Valid 12Z Mon Sep 14 2026 - 12Z Tue Sep 15 2026

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

...Midwest...

A strong closed 500mb low, sporting 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.. The ECMWF SATs show the
IVT topping 1000 kg/m/s over the MO River Valley and into MN
Monday morning, then strengthening to >1,250 kg/ms/s over the
Upper MS Valley by Monday evening. These values are approaching
levels that could challenge the highest observed IVTs for mid-
September in the CFSR database (1979-2009), thus showcasing the
remarkable moisture transport at play. Strong theta-e advection
and anomalous PWs exceeding 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. PWs will also surpass the 97,5 percentile for much of
eastern AZ and western NM. 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
additional scattered showers and storms on Sunday, helping to
further saturate soils ahead of Monday.

QPF has trended wetter over eastern AZ and western NM, and given
soils are more likely to moisten rather than dry out in the coming
days, the Slight Risk was expanded to include more of southeast AZ
and far western NM. 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
 

Day 3
Valid 12Z Tue Sep 15 2026 - 12Z Wed Sep 16 2026

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

...Central Plains & Midwest...

There is increasing concern over a multi-day excessive rainfall
across portions of the Central Plains and Midwest. For Tuesday,
ensemble means (ECMWF/GEFS/EC-AIFS ENS) all show PWs above the 99th
climatological percentile taking shape from the central High
Plains and stretching towards the MO River Valley. Rounds of storms
are likely to unfold during the day Tuesday as a persistent
southerly fetch of 850mb moisture is directed towards a stationary
front that is forecast to be oriented WSW-ENE from KS to central
IL. By Tuesday night the LLJ looks to strengthen, directing high
theta-e air and an increasingly unstable air-mass towards the
frontal boundary. Moisture for this setup not only comes from the
Gulf, but as far east as the western Atlantic, as the strong ridge
over the Southeast U.S. causes an extensive fetch of easterly flow
across these bodies of water and directs the moisture into the
Nation's Heartland.

This meteorological setup will not only feature moisture from
tropical origins, but a robust 250mb jet streak over the Great
Lakes that will place its divergent right-entrance region over the
Central Plains. Excellent upper-level ascent will allow for
widespread showers and storms that, given the stagnant SWrly low-
level flow into the frontal boundary, will provide an ideal setup
for training and back-building thunderstorms. There is a Slight
Risk that extend from central KS to as far east as northern IL. The
greatest concern is for northeast KS and northern MO, where WPC
QPF now shows 2-3" of rainfall, implying localized amounts over 4"
are possible. This area is deemed to be a "higher-end" Slight Risk.
There does remain some uncertainty in the rainfall axis, which is
why no higher threat levels were issued this forecast cycle.

...Eastern AZ & Western NM...

The same robust 250mb jet streak over the Great Lakes that will
place its divergent right-entrance region over the Central Plains
will actually stretch as far south and west as the Four Corners
states. PWs remain above the 97.5 climatological percentile on both
ECMWF and NAEFS SATs, and soils continue to grow more saturated in
wake of the active day on Monday. WPC QPF has increased over
eastern AZ, much of NM, and both southern and western CO. There is
some uncertainty in how much instability will be available
following what appears to be an active Monday for much of the
region. But given the anomalous PWs, excellent synoptic scale
forcing, and at least some minor levels of instability, a Slight
Risk was introduced to highlight the potential for scattered
instances of flash flooding in portions of eastern AZ and western
NM on Tuesday.

Mullinax
$$
--- MultiMail/DOS
 * Origin: capitolcityonline.net * KY, USA (618:250/10)