FOUS30 KWBC 220831
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
431 AM EDT Sat Aug 22 2026

Day 1
Valid 12Z Sat Aug 22 2026 - 12Z Sun Aug 23 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE COASTAL MID ATLANTIC REGION...

....Eastern United States...
The large scale pattern will continue amplify over the eastern
United States with a deepening mid- latitude trough extending from
Canada all the way into the eastern CONUS. A moisture-rich airmass
in place early in the outlook period from the coastal Carolinas
into the Mid-Atlantic region should start to get shunted eastward
in response to the lowering mid-level heights of the amplifying
upper trough. The larger scale models have been trending drier in
this part of the country compared with the past few runs but the
22/00Z HREF/RREF and machine learning guidance continue to focus
the potential for rainfall amounts in excess of 2 inches in 6-hours
into this afternoon across parts of southeast Virginia and the
southern Delmarva. Based on short term terms in satellite and
radar...thinking is that rainfall rates could exceed an inch per
hour but that the risk of flash flooding will be mitigated somewhat
by the progressive nature of the storms afforded by the
strengthening mid-leve flow. Confidence is growing that the focus
for heaviest rainfall is south of areas hard-hit by flash flooding
on Thursday.

Elsewhere in the eastern US...maintained the Marginal Risk area
over portions of the Ohio Valley and eastern Great Lakes given the
proximity of a strengthening low pressure system at the surface and
aloft. This pattern may allow for some lake-enhanced precipitation
across portions of Ohio, Pennsylvania, and New York. Given recent
heavy rains in the Ohio Valley. So there was little reason to make
make more than minor changes to previously issued Marginal area
given the potential for lingering sensitivities to flooding from
additional precipitation on Saturday.

...West...

Persistent ridging across the Western U.S. will finally begin to
shift back over the Plains as a robust trough over the Pacific
Ocean makes headway into the Pacific Northwest and begins pushing
the pattern downstream and flattening the northern portion of the
ridge in addition to increasing the moisture transport into the
region by the end of the period. Pronounced low and mid level dry
air in the subcloud layer due to the upper ridge will initially
result in little rainfall based on forecast soundings for parts of
Washington and northern Idaho this afternoon and early
evening...although the influx of moisture with the robust trough
should overcome that by late in the period. But elsewhere...the
precipitable water anomalies suggest the monsoonal pattern will
persist over the Desert Southwest and Great Basin into the Central
Rockies. A broad Marginal risk over much of the Western U.S, east
of the Pacific coast states will suffice.

Bann
 

Day 2
Valid 12Z Sun Aug 23 2026 - 12Z Mon Aug 24 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
CENTRAL US AND THE SOUTHWEST...

...Midwest...

Energy approaching from the west begins to flatten the northern
portion of the ridge...allowing for showers and thunderstorms to
develop across the northern Plains which then builds southeastward
towards the Ozarks. Even though flow aloft is from the
northwest...the 850 mb flow maintains a plume of precipitable water
values in excess of 1.75 inches from the Gulf coast northward into
the eastern Dakota which roughly defines the ridge in the thetae
field to help fuel locally heavy rainfall rates. And given the
potential for repeat convection...maintained the Marginal risk
area even though individual cells should remain progressive.

...Western US...

The overall pattern is expected to remain relatively unchanged
with the upper ridge remained 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 to the northern Intermountain
West and Rockies.

Bann
 

Day 3
Valid 12Z Mon Aug 24 2026 - 12Z Tue Aug 25 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE WESTERN US AND THE LOWER MISSISSIPPI VALLEY...

...Western US...

An upper ridge will remain anchored over the Southwest-southern
Plains ON Tuesday...with moisture and shortwave energy on the
backside of a high contributing to diurnal showers and storms that
will raise the potential for isolated flash flooding from parts of
the Southwestern US into the Rockies. Only changes to the Marginal
Risk area were minor and largely based on the precipitable water field.

...Lower Mississippi Valley...
Maintained the Marginal Risk area which was introduced on Friday
as a corridor of increasing instability persists due to a
persistent confluent flow of Gulf moisture. At this point...model
guidance would tend to suggest more of an urban concern.

Bann
 

Day 4 and Day 5
Valid 12Z Tue Aug 25 2026 - 12Z Thu Aug 27 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF
THE SOUTHWEST AND INTERMOUNTAIN WEST TO THE CENTRAL AND SOUTHERN
PLAINS, AND OVER PORTIONS OF THE SOUTHEAST TO THE TENNESSEE AND
LOWER MISSISSIPPI VALLEYS...

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

Maintained a Marginal Risk both days covering portions of Arizona
and New Mexico northward through the Four Corners into parts of the
central and northern Rockies as monsoonal moisture is expected to
continue to bring scattered, diurnally-driven showers and
thunderstorms, with isolated flash flooding possible.

Showers and thunderstorms are likely to extend east of the Rockies
into the Plains midweek as mid-level energy emanating from the
West moves off of the top of the Southwest ridge and interacts
with a pool of deepening moisture associated with a cold front
dropping through the central U.S. With the guidance continuing to
present some signal for potentially heavy amounts, a Marginal Risk
was maintained over much of Kansas and Nebraska south of the
Sandhills on Day 4.

Additional rounds of storms are expected on Day 5 from the Rockies
into the Plains as the series of mid-level shortwaves continue to
move out of the West. As the front continues to press farther
south, storms developing along the high terrain are expected to
propagate southeast into southern High Plains into the axis of
deeper moisture and greater instability.

...Southeast to the Tennessee and Lower Mississippi Valleys...

Maintained a Marginal Risk covering portions of the Tennessee
Valley and Southeast on Day 4 as mid-level energy sliding southeast
into the region interacts with a pool of deepening moisture across
the region. As moisture continues to deepen ahead of a cold front
approaching from the north while northwesterly flow directs a
series of mid-level shortwaves into the region, 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 on Day 5.

Pereira
$$
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