FOUS30 KWBC 240817
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
417 AM EDT Mon Aug 24 2026

Day 1  Valid 12Z Mon Aug 24 2026 - 12Z Tue Aug 25 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG THE GULF
COAST FROM SOUTHEASTERN LOUISIANA TO SOUTHERN ALABAMA...

...Ozarks-Lower Mississippi Valley and Central Gulf Coast to the
Coastal Carolinas...

Elevated storms are expected to develop later this morning and
propagate southeast from northern Arkansas into the lower
Mississippi Valley mid to late morning. Following an expected
decrease in activity, storms are forecast to re-intensify in the
afternoon, with the consensus of the CAMs showing surface based
storms organizing over portions of southeastern Louisiana, southern
Mississippi, and perhaps as far east as southern Alabama.
Shortwave energy dropping into the region will likely interact with
a pool of deepening moisture, with PWs increasing to over 2.25
inches according to recent RAP runs. This will support very heavy
rates with localized heavy accumulations likely. A Slight Risk was
introduced where both the HREF and REFS indicate localized totals
over 3 inches are likely. While the antecedent conditions are dry,
both the HREF and REFS show probabilities for amounts exceeding
3-hr FFGs nearing 30 percent across portions of the highlighted
region during the late afternoon-early evening hours -- with
urbanized and poor drainage areas likely to be the most susceptible
to flash flooding.

Further east through northern Florida and along the Southeast Coast
to the Carolinas, a ribbon of deep moisture (PWs 1.75-2+ inches)
centered along and south of a slow-moving boundary will support
enhanced diurnal activity, with developing storms likely to produce
localized heavy rainfall rates and amounts. A Marginal Risk was
maintained where both the HREF and REFS show high probabilities for
amounts over 2 inches.

...Southwest to the Central High Plains...

With not much change to the overall pattern, and the development of
monsoon-moisture-fueled storms expected to continue, maintained a
Marginal Risk covering much of the same area as previous day --
extending from portions of the Southwest northward into the Great
Basin and northern Rockies, and east into the central Rockies and High Plains.

Pereira
 

Day 2  Valid 12Z Tue Aug 25 2026 - 12Z Wed Aug 26 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST AND ROCKIES TO THE CENTRAL PLAINS AND ALONG THE GULF
AND SOUTHEAST COASTS...

...Southwest to the Central Plains...

Diurnal shower and thunderstorms activity will develop once again
across portions of the Southwest into the central and southern
Rockies. However, the footprint for these storms will likely be
less than earlier in the week as the upper high begins to shift
west -- capping or directing moisture away from parts of the
Southwest and Great Basin. Therefore, continued to leave southern
Arizona and New Mexico and much of the Great Basin out of the
Marginal Risk.

The models continue to signal a chance for organized storms and
potentially heavy amounts across portions of the central Plains.
Energy emanating from the West and sliding off of the top of the
western U.S. ridge is expected to interact with an area of
increasing moisture that will be deepening ahead of cold front
dropping through the central U.S. While models continue to differ
on the details, they have presented a stronger signal for organized
activity over portions of southern Nebraska into Kansas. But given
the uncertainty and what are expected to be fairly progressive
storms, maintained just a Marginal Risk for now.

...Gulf and Southeast Coasts...

Another day of potentially heavy diurnal showers and storms are
expected from portions of the Florida Panhandle to coastal South
Carolina. PWs are forecast to remain at or above 1.75 inches,
fueling efficient-rainfall-producing-storms. Farther west, the
consensus of the guidance shows much of the heavier activity
developing and remaining offshore with a weak wave. There is some
potential for heavy amounts developing onshore, but at this point
they look to be mostly confined to the immediate coast of
southeastern Louisiana, where flash flooding is not a typical concern.

Pereira
 

Day 3  Valid 12Z Wed Aug 26 2026 - 12Z Thu Aug 27 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST, ROCKIES, AND CENTRAL SOUTHERN HIGH PLAINS AND FROM
PARTS OF THE LOWER MISSISSIPPI TO THE TENNESSEE VALLEY...

...Lower Mississippi Valley to the Tennessee Valley...

A series of shortwaves sliding off of the western U.S. ridge and
then along the base of a trough amplifying over the eastern U.S.
are expected to generate showers and storms across portions of the
region. While the models differ greatly on the details, there is
some general indication that one of the more defined waves moving
across the region will help develop organized storms and some
greater potential for heavy accumulations over parts of the lower
Mississippi Valley. Backing flow and deepening moisture may also
support heavy rainfall developing farther east into parts of the
Tennessee Valley, but model spread is significant there as well --
limiting forecast confidence.

...Southwest and Rockies to the Central and Southern High Plains...

Overall, not much change from the previous day with diurnal
activity likely to be very limited across southern Arizona and New
Mexico and the Great Basin. Farther east, storms that develop
along the eastern front of the Rockies are expected to propagate
more south than the previous day and remain mostly limited to the
High Plains. With the ridge shifting west, expect to see some
return of deeper moisture and convective activity over a greater
portion of the southern Rockies and High Plains this period.

Pereira
 

Day 4 and Day 5
Valid 12Z Thu Aug 27 2026 - 12Z Sat Aug 29 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF
THE SOUTHWEST AND INTERMOUNTAIN WEST AS WELL AS PARTS OF THE
SOUTHEAST AND MID ATLANTIC...

...Southwest and Intermountain West to Central and Southern Plains...

Maintained a Marginal Risk both days across portions of the
Southwestern US as monsoonal moisture fuels diurnally driven
widely scattered convection covering portions of Arizona and New
Mexico. Given the plume of precipitable water values nosing back
into to the Arizona desert on the western side of a broad mid- and
upper level high...local downpours leading to instances of flash
flooding are possible.

Farther north...deep moisture and weak shortwave energy riding
around the ridge axis associated with the upper high will combine
with southerly flow of deep moisture in low-levels to the lee of
the Front Range to support more showers and thunderstorms capable
of producing heavy rainfall rates and flash flooding. Locally heavy
rainfall rates may lead to isolated instances of flash flooding here as well.

...Southeast to Mid-Atlantic Region/Southern New England Coast...

Showers and thunderstorms capable of producing heavy to excessive
rainfall each day should be focused along a quasi-stationary front
extending over the Southeast US/Gulf coast region into parts of the
Mid Atlantic. Height falls associated with weak shortwave energy
will embedded within a broader trough will aid and focus the
convection in vicinity of the front.

In addition...the approach of a front in the northern stream trough
will tap into some deeper moisture being drawn northward ahead of
the above-mentioned front to bring an isolated threat of flooding
on Thursday before the northern stream front sweeps off shore later
in the Day 4 period.

...Upper Midwest/Northern Plains...
There is a risk of excessive rainfall developing across parts of
the Upper Midwest/Northern Plains on Friday as low level flow of
Gulf moisture is drawn into the region on 30 to 45 kt low level
jet and rides over a surface warm front over the region.

Bann
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