Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
838 PM EDT Wed Aug 26 2026
Day 1
Valid 01Z Thu Aug 27 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, pushing a cold front into Arkansas shortly, which
remains in place overnight. A shortwave in the mid levels dives
down from the northwest, riding the eastern periphery of a warm
core ridge in the Southwest. Storms in Arkansas are expected to
ride down a boundary laid out by previous convection across MS,
with a convective cluster congealing and dropping south to south-
southeast with time through LA. With precipitable water values
approaching 2.25", hourly rain amounts to 3" are possible where
cells merge or train. Meanwhile, showers and thunderstorms across
southern IN will sag southward into KY/TN with time. Radar
reflectivity trends and the 18z HREF/12z REFS guidance led to a
southward shift in the Slight Risk area in the Lower MS Valley,
while northern portions of the Marginal Risk area closer to the
Great Lakes were removed.
...Southeast...
Lingering convection across portions of FL and the Southeast will
fade with time, eventually replaced with coastal convection close
to the Carolina shore and southwest FL coast. Used radar
reflectivity trends and the 18z HREF/12z REFS guidance to pare
back the extent of the Marginal Risk area.
...Southern & Central Plains...
Showers and thunderstorms near the Front Range of the Southern and
Central Rockies should slowly migrate eastward with time through
the High Plains. Wherever cells merge or manage to train, hourly
rain amounts to 2" are possible. The area near the KS/CO border
shows the greatest potential for heavy rainfall this evening and
overnight, but don't think cell coverage will be sufficient enough
for a Slight Risk. Localized Slight Risk impacts cannot be ruled
out.
Roth
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
838 PM EDT Wed Aug 26 2026
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
CENTRAL GULF COAST AND THE NORTHEAST/MID-ATLANTIC...
...2030z Update...
The slight risk in the Northeast/Mid-Atlantic was expanded south
and eastward into the Baltimore to NYC I-95 corridor due, in most
part, to antecedent conditions (soil moisture/FFGs). Urban areas
like Baltimore, Philadelphia and New York City will be susceptible
to flash flooding. The maximum axis of QPF is likely to develop
ahead of the approaching cold front and along an elevated PW and
MUCAPE axis. 1-2 inches of rain are expected within the slight risk
area with 2-4 inch maxima possible in the strongest storms.
Rainfall rates of 1-2" are also possible in some areas.
The Southeast and Gulf Coast slight risk area was shaved off and
confined to the immediate Central Gulf Coast due to a notable
decrease in QPF over portions of Georgia and north/central Alabama.
Kebede
...Previous Discussion...
...Northeast/Mid-Atlantic...
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 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
838 PM EDT Wed Aug 26 2026
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...
...2030z Update...
A marginal risk was introduced over portions of the Sierra Nevada
up into the Great Basin for this Friday. Anomalous mid-level moisture,
on the order of 4-5 standard deviations above average, will stream
northward from Tropical Storm Iselle and get swept into a deep
mid-level trough propagating through the Northwestern U.S. on
Friday. MUCAPE values between 250-500 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.
Kebede
...Previous Discussion...
...Southeast/Mid-Atlantic...
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
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
838 PM EDT Wed Aug 26 2026
Day 4 and Day 5
Valid 12Z Sat Aug 29 2026 - 12Z Mon Aug 31 2026
...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PARTS OF
THE WEST, SOUTHEAST, PLAINS, UPPER MIDWEST, & GREAT LAKES...
...Southwest/Great Basin/Intermountain West...
A warm core high weakens 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
this weekend as above average moisture for the monsoon fuels
diurnally driven widely scattered to scattered convection. 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...
Showers and thunderstorms capable of producing heavy to excessive
rainfall each day should be focused in the vicinity of a quasi-
stationary front near the Gulf coast region and behind a
retrograding upper level disturbance moving through the central
Gulf, luring the dying front west as an easterly wave Sunday and
Sunday night. 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.
...Upper Midwest/Northern Plains/Great Lakes...
There is a risk of excessive rainfall persisting through the
weekend across parts of the Upper Midwest/Northern Plains/Great
Lakes. Precipitable water values of 1.5 to 1.75 inches should
already be in place...having been drawn into the region prior to
this weekend. That should be enough to result in hourly rain
amounts to 2" are possible wherever cells can merge or train.
Roth
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
838 PM EDT Wed Aug 26 2026
Day 4 and Day 5
Valid 12Z Sat Aug 29 2026 - 12Z Mon Aug 31 2026
...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PARTS OF
THE WEST, SOUTHEAST, PLAINS, UPPER MIDWEST, & GREAT LAKES...
...Southwest/Great Basin/Intermountain West...
A warm core high weakens 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
this weekend as above average moisture for the monsoon fuels
diurnally driven widely scattered to scattered convection. 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...
Showers and thunderstorms capable of producing heavy to excessive
rainfall each day should be focused in the vicinity of a quasi-
stationary front near the Gulf coast region and behind a
retrograding upper level disturbance moving through the central
Gulf, luring the dying front west as an easterly wave Sunday and
Sunday night. 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.
...Upper Midwest/Northern Plains/Great Lakes...
There is a risk of excessive rainfall persisting through the
weekend across parts of the Upper Midwest/Northern Plains/Great
Lakes. Precipitable water values of 1.5 to 1.75 inches should
already be in place...having been drawn into the region prior to
this weekend. That should be enough to result in hourly rain
amounts to 2" are possible wherever cells can merge or train.
Roth
