FOUS30 KWBC 230823
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
423 AM EDT Sun Aug 23 2026

Day 1
Valid 12Z Sun Aug 23 2026 - 12Z Mon Aug 24 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE WESTERN AND CENTRAL U.S. AND IN THE LEE OF LAKE ERIE...

...Northern Plains to the Ozarks...

Models continue to present a strong signal for convective
development, spurred by the interaction of mid-level energy
spilling off of the top of the western U.S. ridge along with a
ribbon of deepening moisture (1.50-1.75+ inches). However, they
continue to differ greatly on the details, especially with regards
to how far south storms will develop and propagate -- limiting
forecast confidence. The spread amongst the HREF members was more
notable, with some, including the HRW-ARW2, bringing heavy-
rainfall-producing storms farther south through the lower Missouri
Valley into the Ozarks than some others, such the HRW-ARW. The
REFS suite suggested greater confidence, with storm development and
heavy rainfall placement remaining more focused over the Dakotas
and showing high probabilities for accumulations over 2 inches
along an northwest-southeast axis extending from western North
Dakota to eastern South Dakota. The HREF showed a similar axis, but
with lower probabilities. Given the uncertainty, the Marginal Risk
drawn incorporates elements from both hi-res ensemble suites
including the southern extent drawn into the Ozarks.

...Lake Erie...

Shortwave energy digging south of a deep low closing off over
southern Ontario will support periods of lake-enhanced shower
activity later today, with the hi-res guidance continuing to
present a signal for localized heavy amounts. Maintained a small,
targeted Marginal Risk centered over northwestern Pennsylvania and
far western New York, where both the HREF and REFS show high
probabilities for amounts over 2 inches.

...Southwest and Great Basin to the Central High Plains...

The overall pattern is expected to remain relatively unchanged,
with the upper ridge remaining anchored over the Southwest and
southern Plains. Moisture and energy moving along the western and
northern periphery of the ridge will contribute to daily shower and
thunderstorm activity and the potential for isolated flash
flooding extending from the Southwest northward into parts of the
Great Basin and east across the central Rockies into the High Plains.

Pereira
 

Day 2
Valid 12Z Mon Aug 24 2026 - 12Z Tue Aug 25 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE WESTERN U.S., THE LOWER MISSISSIPPI VALLEY, AND ALONG PORTIONS
OF THE GULF AND SOUTHEAST COASTS...

...Lower Mississippi Valley to the Gulf and Southeast Coasts...

Models that show convection developing and propagating south into
the Ozark region late in the Day 1 period indicate the potential
for storms to continue, with heavy rainfall persisting as they
track south into parts of the lower Mississippi Valley during the
mid to late morning on Monday. Therefore, the Marginal Risk
extending from the Ozarks down into the lower Mississippi Valley
was maintained for now.

Farther south and east, the overnight guidance presented a growing
signal for heavy amounts and isolated concerns along a ribbon of
deepening moisture south of a slow-moving front extending from the
Louisiana to the Carolina coasts. PWs of 1.75-2 inches interacting
with weak energy moving along the base of an upper trough,
including the previously noted energy dropping though the lower
Mississippi Valley, may contribute to enhanced diurnal activity. A
Marginal Risk was drawn along the coastal areas where the REFS
shows high probabilities for accumulations exceeding 2 inches.

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

As drier air spreads into the region from the west, a decrease in
activity is expected across the Great Basin. Otherwise, with the
pattern remaining much of the same, another day of diurnal storms,
producing isolated flooding concerns, is expected across much of the
same area as Day 1.

Pereira
 

Day 3
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 Rockies and Central Plains...

As the upper high centered over the Southwest shifts slowly west,
expect decreasing diurnal storm activity across parts of the
region, especially over southern Arizona and New Mexico. A better
chance for showers and storms with isolated flash flooding concerns
will continue farther north from the Four Corners into parts of
the northern and central Rockies.

Activity is expected develop farther east out into the central
Plains and perhaps as far east as the lower Missouri Valley as
moisture pools ahead of a cold front pushing south through the
central part of the country. While there are still plenty of
differences in the details, there is a growing camp of solutions
showing organized activity, with potentially heavy amounts,
centering in the vicinity of western Kansas as energy moving off of
the top of western ridge interacts with this deepening moisture
and increasing instability.

...Central Gulf to the Southeast Coast...

With ample moisture remaining in place (PWs ~2 inches), another
day of slow-moving, efficient storms can be expected -- raising the
threat for at least isolated flash flooding concerns, especially
within urbanized areas along the coast from Louisiana to South Carolina.

Pereira
 

Day 4 and Day 5
Valid 12Z Wed Aug 26 2026 - 12Z Fri Aug 28 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF
THE SOUTHWEST AND INTERMOUNTAIN WEST AND TO PORTIONS OF THE SOUTHEAST
AND lOWER MISSISSIPPI VALLEYS EACH DAY EXPANDING INTO MID ATLANTIC ON DAY 5...

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

Maintained a Marginal Risk both days as monsoonal moisture
continuesto fuel diurnally driven widely scattered convection
covering portions of Arizona and New Mexico northward through the
Four Corners into parts of the central and northern Rockies. Given
the magnitude of the moisture anomalies...locally heavy rainfall
rates may lead to isolated instances of flash flooding.

...Southeast to Mid-Atlantic Region...

Maintained a Marginal Risk covering portions of the Lower Tennessee
Valley and the Southeast US on Day 4 and Day 5 as mid- level
energy continues to slide southeast into the region. As it does
so,...the energy interacts with a pool of deepening moisture
pooling along a weak boundary draped across the region each day. The
footprint for heavy rainfall may expand and the potential for
flash flooding may increase across the region and extend back into
parts of the lower Mississippi Valley.

By Day 5...The coverage of convection over the Mid Atlantic region
should increase as a cold cold front approaches from the
west/north and upper flow becomes more difluent and the low level
flow taps deeper moisture. This could lead to some locally heavy
rainfall rates developing over parts of the Mid-Atlantic...perhaps
as far north as Pennsylvania...on Day 5.

Bann
$$
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