FOUS30 KWBC 260850
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
450 AM EDT Wed Aug 26 2026

Day 1  Valid 12Z Wed Aug 26 2026 - 12Z Thu Aug 27 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE
LOWER MISSISSIPPI VALLEY...

...Lower Mississippi Valley through the Upper Great Lakes...

An upper-level trough over the Upper Great Lakes will continue
digging throughout the period, pushing a cold front into Arkansas
in the evening, and remaining in place in the overnight hours.
Meanwhile, a shortwave in the mid levels will dive down from the
northwest, riding the eastern periphery of a ridge in the
Southwest. As this shortwave approaches, localized enhancement of
upper-level divergence at the 250mb level is expected,
particularly over Arkansas. As such, storms are expected to fire
over Missouri and Arkansas, and move into Mississippi and Louisiana
by the evening and overnight. Efficient rainfall is expected, with
PWATs approaching 2.25". There stands medium-to-high chances of
rain rates exceeding 1"/hr per the HREF. Storms will also have the
potential to backbuild given weak (5kts) and near- orthogonal
Corfidi vectors relative to the mean 850-300mb winds. All that
said, the latest run of CAMs guidance raised more uncertainty as to
the placement to the expected evening-to-overnight rainfall than
desired. There is a surprising amount of discrepancy between the
00Z runs of the HREF & RRFS particularly, with the RRFS favoring
heavier rain over northern Louisiana compared to the HREF's
Mississippi and Arkansas solution. The insistent heavy rainfall
suggested by the RRFS will struggle to exceed FFGs in Louisiana
anyway, and as such the MRGL remains in place there. Otherwise,
minor expansions were made for the inherited SLGT risk area into
northeast Arkansas and also into Alabama.

Elsewhere, the aforementioned cold front will bring potentially
heavy rainfall across portions of Illinois, Indiana, and Ohio.
Rainfall rates associated could reach 0.5-1.0"/hr, spanning from
Michigan through Indiana. Storms will be quite progressive and
short lasting, and as such, the inherited Marginal Risk was largely
kept intact besides cosmetic adjustments to account for the last
run of models.

...Southwest and Rockies...

Monsoonal moisture will again bring diurnal thunderstorms over the
southwest, although a southwest upper-level ridge will continue to
move moisture away from the Arizona and New Mexico area. As such,
the inherited MRGL Risk area was merely adjusted to match the
latest suite of model guidance, but remains largely unchanged.

Blanco-Alcala
 

Day 2  Valid 12Z Thu Aug 27 2026 - 12Z Fri Aug 28 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
LOWER MISSISSIPPI VALLEY AND THE NORTHEAST...

...Northeast...

A Slight Risk upgrade was introduced over the Northeast with this
update. An upper-level trough will continue to slowly inch its way
eastward, enhancing upper divergence aloft, especially upwind over
southern NJ and southeast Pennsylvania. A healthy influx of
moisture is forecast to flow onshore from the Gulf stream to the
south, with around ~20kts 850mb southerly flow expected. PWATs
around 1.75-2" will support heavy rainfall which will be
continuously fueled by the aforementioned low-level flow. Latest
global model runs are in agreement as to the general area of the
QPF core in the northeast. The 00Z run of the RRFS appears to also
converge toward a northern Mid-Atlantic to Northeast solution,
albeit much more bullishly compared to the global solutions. Given
this, the Slight area was introduced, and the availability of more
CAMs guidance should result in refinement/fine tuning of the risk area.

...Southeast and Gulf Coast...

An upper-level shortwave over the eastern portion of the country
will continue pushing southward, with a surface cold front moving
southeastward. Upon arrival to the Gulf Coast, it will stall over
Louisiana and southern Mississippi for the next few days. While
approaching and eventually residing near the coast, the boundary
will contend with ample moisture moving northward from the Gulf,
yielding multiple rounds of storms with the potential for heavy
rainfall. Flood prone urban areas such as New Orleans, Baton Rouge,
Mobile, and some of the more interior areas of Mississippi dealing
with the latest of multiple days of heavy rains will all support
widely scattered instances of flash flooding. Rainfall and
thunderstorms will bring heavy rainfall to Alabama and Georgia as
well, and the previously mentioned trough encounters a similarly
moist airmass being pushed northeastward in the lower levels. This
will support several rounds of storms in the form of multiple lines
across Georgia and parts of South Carolina. The storms moving
northeastward could allow for some training as the cold front moves
through, and could bring widely scattered instances of flash flooding.

...Southwest...

As similarly stated in previous outlooks, a very gradual eastward
progression of the upper ridge across the Southwest should allow
for a shifting of the heaviest precipitation to the east out of
Utah and more into the central Plains. The ridge should largely
preclude much organization from the storms, so the Marginal
continues to focus on the areas where either terrain or prior
rainfall could locally increases the risk of flash flooding.

Blanco-Alcala/Wegman
 

Day 3  Valid 12Z Fri Aug 28 2026 - 12Z Sat Aug 29 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE COASTAL CAROLINAS...

A Slight Risk was introduced in this update on the coast of the
Carolinas, as a stationary boundary remains in place fueling
repeated thunderstorms. The upper-level trough mentioned over the
previous days' discussion continues to linger in place while
continuing to dig. A mid-level shortwave will move to the base of
the trough and will focus convection to the vicinity of a
stationary front. This front will continue to remain in place
throughout the day, while a secondary cold front moves in behind
from the Great Lakes. Low level flow will begin to take the shape
of an inverted trough, with southerly flow moving onshore from just
off the coast in the Atlantic. PWATs at this time are expected to
approach 2.25" per the ECMWF and EC-AIFS. As it moves onshore, an
axis of moisture convergence is expected to develop and aid in
lifting moist air for the development of thunderstorms. High PWATs
will bring the potential of hourly rain rates of up to 3",
particularly where cells merge or train. As such, scattered
instances of flash flooding will be possible along the coast. Heavy
rainfall is also expected in the surrounding areas extending from
the Gulf Coast to southern Virginia, in which isolated instances of
flash flooding are possible.

...Northern Plains...

A plume of upper-level energy by way of a shortwave trough will
quickly move through the eastern Dakotas and western Minnesota and
Iowa. This will bring a progressive complex of thunderstorms into
the area, with potential for heavy rainfall. Healthy moisture and
solid instability will be present, with PWATs exceeding 1.5", and
MUCAPE exceeding 2000 J/kg. However, given the progressive nature
of thunderstorms, the Marginal risk area was maintained.

...Southwest...

Monsoonal moisture continues the fuel diurnally drive convection
along the southwest. A plume of moisture will drive PWATs around
1.5-1.75" northward along much of the Southwest. This will lead to
renewed chances of isolated flash flooding, as has been similar for
much of the monsoon season.

Blanco-Alcala
$$
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