FOUS30 KWBC 280806
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
406 AM EDT Fri Aug 28 2026

Day 1
Valid 12Z Fri Aug 28 2026 - 12Z Sat Aug 29 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE COASTAL
CAROLINAS INTO SOUTHEAST VIRGINIA AND COASTAL GEORGIA...

...Southeast/Mid-Atlantic...

The HREF and REFS guidance continue to depict much of the Carolina
Coastline and surrounding areas to reach/exceed 2 inches during
this period. Local enhancement can be expected during the afternoon
and evening hours. A sharpening upper-level jet streak on the lee
side of the upper trough will enhance deep-layer ascent as well and
low-level FGEN, increasing the convective coverage and intensity
during the peak heating hours Friday afternoon and evening. This
forcing combined with PWs exceeding 2", 1000-2000 J/kg MUCAPE, and
a stalled front will likely lead to multiple rounds of heavy
rainfall with the strongest cells capable of hourly rain rates of
up to 3".

The latest guidance concentrates the highest rainfall QPF along the
coastline from eastern Georgia to southeast Virginia. The inherited
Slight Risk was expanded southward to cover all of South Carolina
coastline and into eastern Georgia. Of note, a signal persists for
a weak circulation to develop offshore of the Florida Panhandle
over the next 24 hours, which focuses convection near the
coastline. While FFGs in the are are high, a categorical upgrade
could be needed if the heavy rainfall advertised from this feature
trends further inland.

...Northern Plains...

A progressive thunderstorm complex is expected to move through the
northern states as a shortwave trough and associated surface warm
front rapidly advances through/somewhat flattens the upper ridge.
Although PWs exceeding 1.5" and MUCAPE exceeding 2000 J/kg will be
favorable for for producing heavy rainfall, the threat for
localized flash flooding will be limited given the progressive
nature of individual storms. A Marginal Risk was maintained.

...Southwest and Portions of the Great Basin...

Monsoonal moisture will continue to 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.

Over the Great Basin, anomalous mid-level moisture, on the order
of 4-5 standard deviations above average, will stream northward
from Post-T.C. Iselle and get swept into a deep mid-level trough
propagating through the Pacific NW. MUCAPE values near 500 J/Kg
working their way north through the southern Great Basin/Sierra
should be enough to produce heavy rainfall and at least isolated
flash flooding out of the available moisture. The Marginal Risk
area remains in effect across the region.

Campbell/Asherman
 

Day 2
Valid 12Z Sat Aug 29 2026 - 12Z Sun Aug 30 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST AND GREAT BASIN INTO THE ROCKIES AND FRONT RANGE,
NORTHERN PLAINS INTO THE UPPER MIDWEST, AND THE SOUTHEAST TO THE
CAROLINAS...

...Southwest and Great Basin into the Rockies and Front Range...

The upper high weakens/flattens across the Southern Plains as
troughing becomes increasingly established over portions of the
West and Northern Plains. Moisture gets funneled in between the two
features, which may have a connection to a future northeast
Pacific tropical cyclone. A Marginal Risk of excessive rainfall
remains for Sat-Sat night as above average moisture for the
monsoon fuels diurnally driven widely scattered to scattered
convection. The pattern would support isolated flash
flooding, with favored terrain, urbanized areas, box/slot canyons,
arroyos/dry washes, and burn scars being the most prone to
flooding.

..Southeast...

A quasi-stationary frontal boundary will be draped across the Gulf
Coast region during this period and will continue to provide a
focus for convection capable of producing heavy to excessive
rainfall along with aforementioned disturbance in the central
Gulf. Increasing ridging poking in from the western subtropical
North Atlantic should lead to a decreasing heavy rain threat across
the Carolinas with time. With precipitable water values of 2"+
expected, hourly rain amounts to 3" are possible where cells merge
or train, particularly near the Gulf Coast.

...Northern Plains into the Upper Midwest...

Shortwave energy will be traversing the edge of the upper ridge along
with the persistent moisture anomaly in place (PWATs ~2 standard
deviations above normal. Given the degree of instability and
moisture in play, some of the stronger storms will be capable of
intense short-term rainfall rates and localized flash flooding. A
Marginal Risk spans from the Dakotas to Wisconsin and portions of
Upper Michigan.

...Southeast and Gulf Coast...

The tail-end of the longwave upper trough, with distinct shortwave
energy traversing the Gulf Coast, along with the attendant
(eventually quasi-stationary) surface front, will provide the
dynamical focus for more organized (widespread) areas of convection
across this region. Multiple rounds of potentially heavy rainfall
for this period will 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. A Marginal Risk area remains in effect
for this period.

...Southwest...

The upper ridge will continue its gradual eastward shift across the
Southwest which in turn will also shift the footprint of the
heaviest precipitation. For this period, it is expected to set up
mainly east of Utah into the Central High Plains. Given the ridge,
storms will generally lack organization therefore the Marginal Risk
continues to focus on the areas where either terrain or prior
rainfall could locally increases the risk of flash flooding.

Campbell
 

Day 3
Valid 12Z Sun Aug 30 2026 - 12Z Mon Aug 31 2026

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PARTS OF
THE WEST,NORTHERN TIER OF THE COUNTRY AND THE GULF COAST...

...Southwest/Great Basin/Intermountain West...

A Marginal Risk of excessive rainfall remains for above average
moisture fuels diurnally driven widely scattered to scattered
convection across the Southwest, portions of the Rockies, and parts
of the Great Basin. The degree of moisture expected could lead to
hourly rain amounts to 2" wherever cells sit, merge, or train. This
could lead to isolated flash flooding...with favored terrain,
urbanized areas, box/slot canyons, arroyos/dry washes, and burn
scars being the most prone to flooding.

..Southeast...

Convection near the stalled/dying frontal boundary and retrograding
upper level disturbance moving through the central Gulf will
persist through this period to continue to the threat for isolated
instances of flash flooding. The combination of what becomes an
easterly wave and an increasing amount of ridging building westward
from the western subtropical North Atlantic should lead to the
heavy rain threat being constrained to portions of the northeast
and north-central Gulf coast. With precipitable water values of 2"+
expected, hourly rain amounts to 3" are possible where cells merge
or train.

...Upper Midwest/Northern Plains/Great Lakes...

The upper ridge will continue to shift eastward during this period
while the lower levels see and uptick in Gulf moisture spreading
across the Plains and Midwest. Precipitable water values are
expected to be in the 1.5-2" range. Where cells train or merge,
hourly rain amounts to 2.5". At this time Marginal Risk areas
remain in effect, however with time, there may be a need for
a Slight Risk somewhere between the Great Lakes and Northeast as
we get closer in time.

Campbell/Roth
 
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