Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1202 PM EDT Fri Aug 28 2026
Day 1
Valid 16Z 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 SOUTHERN GEORGIA...
...Southeast/Mid-Atlantic...
...16z Update....
Changes to the forecast update included removing northern New
Jersey and New York from the Marginal due to the latest model
guidance showing little signal for QPF exceeding 1-2 inches. Other
changes including trimming out the western portions of the
Marginal across the Louisiana and Mississippi Gulf Coast. Another
change was to bring the Slight Risk farther southwest to include
more of central-south Georgia with radar showing storms with
locally heavy rainfall moving towards the area from northern
Florida and higher HREF/RRFS probabilities of 3-5 inches. The rest
of the forecast remains on track.
Wilder
...Previous Discussion...
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...
...16z Update...
The latest forecast remains unchanged. A shortwave trough should
interact with instability and moisture to produce a progressive
complex of thunderstorms capable of dropping 1-2 inches of rain
over the Dakotas and Minnesota. The complex could then drop
southward into northern portions of Nebraska and Iowa. Flash flood
guidance exceedance remains marginal around 10-20%.
Wilder
...Previous Discussion...
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...
...16z Update...
The forecast remains on track. Some minor adjustments were made to
reflect the latest 12z model guidance. Instability looks a bit
higher over the Southwest compared to the Great Basin. However, as
noted below, the highly anomalous moisture sourced from the Pacific
will likely still lead to the threat of locally heavy rainfall
over the Great Basin as well.
Wilder
...Previous Discussion..
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 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1202 PM EDT Fri Aug 28 2026
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 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1202 PM EDT Fri Aug 28 2026
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
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1202 PM EDT Fri Aug 28 2026
Day 4 and Day 5
Valid 12Z Mon Aug 31 2026 - 12Z Wed Sep 02 2026
...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PARTS OF
THE WEST, MIDWEST TO NORTHEAST, AND EASTERN GULF COAST...
...Southwest/Four Corners/Easter Great Basin/Rockies...
For both Days 4 and 5, we are maintaining a Marginal Risk in areas
along the outer periphery of the flattened, broad upper ridge. The
guidance continues to show a fairly active northern stream, with
several jet streaks and attendant shortwave energy riding along
the perimeter of the upper ridge. On Tuesday-Tuesday night, this
includes the Four Corners and parts of the Southwest where diurnal
instability and continued favorable deep-layer moisture (PWAT
anomalies 1 to 2 standard deviations above normal per the 00Z GEFS)
will again favor scattered-numerous monsoonal activity with an
isolated risk for flash flooding.
On Day 5 (Wed-Wed night), a fairly vigorous longwave trough with a closed
500 mb low (H5 height anomalies around 2 standard deviations below
normal) will drop into the Pacific NW Coast. This dynamic system
will help draw the deep-layer moisture anomaly farther north across
the Western U.S., thereby allowing for an expansion of the Marginal
Risk area well northward (to the Canadian border). While the global
QPFs currently do not show much in the way of organized (focused)
areas of heavy rain, the combination of the deep moisture,
increasing upper difluence over a broad area, and diurnal
instability will maintain at least a localized flash flood risk.
...Upper Midwest/Lower Great Lakes/Northeast...
On Day 4, an upper level jet streak traversing far northern New
England will provide a focus for more organized convection within
the favorable right-entrance region of this jet streak, along the
outer perimeter of the upper ridge. The spread in the model QPFs
at this point is quite high, however the signal is there to support
at least localized areas of excessive rainfall across much of NY
and the Northeast outside of northern New England.
By Day 5, the Marginal Risk area broadens a bit (from the Upper
Midwest east through a bit more of the Northeast Region), again
with more upper jet streak energy and associated vort lobes riding
along the flatted outer rim of the upper ridge. While the activity
appears to be active across this corridor, it should also be fairly
transient. This along with the high spread in the guidance QPF
would support capping the ERO risk at Marginal for now.
...Eastern Gulf Coast...
Carried over the Marginal Risk area from the current Day 3 ERO to
the new Day 4 outlook (12Z Mon-12Z Tue), though given the guidance
trends, trimming much of eastern LA that is in the Day 3 outlook.
Shortwave energy moving westward underneath the closed 500 mb High
will be the foci for potentially more numerous (organized)
convective clusters during peak diurnal heating, especially when
interacting with the Atlantic and Gulf sea breezes. By Day 5 (12Z
Tue-12Z Wed), the mid/upper ridge presses closer to the Gulf Coast,
thus lowering the 0-6km bulk shear values and reducing the
convective coverage and intensity. The global QPFs are notably
lower on Day 5 compared to Day 4, and as such we opted to not
include a Marginal on Day 5.
Hurley
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1202 PM EDT Fri Aug 28 2026
Day 4 and Day 5
Valid 12Z Mon Aug 31 2026 - 12Z Wed Sep 02 2026
...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PARTS OF
THE WEST, MIDWEST TO NORTHEAST, AND EASTERN GULF COAST...
...Southwest/Four Corners/Easter Great Basin/Rockies...
For both Days 4 and 5, we are maintaining a Marginal Risk in areas
along the outer periphery of the flattened, broad upper ridge. The
guidance continues to show a fairly active northern stream, with
several jet streaks and attendant shortwave energy riding along
the perimeter of the upper ridge. On Tuesday-Tuesday night, this
includes the Four Corners and parts of the Southwest where diurnal
instability and continued favorable deep-layer moisture (PWAT
anomalies 1 to 2 standard deviations above normal per the 00Z GEFS)
will again favor scattered-numerous monsoonal activity with an
isolated risk for flash flooding.
On Day 5 (Wed-Wed night), a fairly vigorous longwave trough with a closed
500 mb low (H5 height anomalies around 2 standard deviations below
normal) will drop into the Pacific NW Coast. This dynamic system
will help draw the deep-layer moisture anomaly farther north across
the Western U.S., thereby allowing for an expansion of the Marginal
Risk area well northward (to the Canadian border). While the global
QPFs currently do not show much in the way of organized (focused)
areas of heavy rain, the combination of the deep moisture,
increasing upper difluence over a broad area, and diurnal
instability will maintain at least a localized flash flood risk.
...Upper Midwest/Lower Great Lakes/Northeast...
On Day 4, an upper level jet streak traversing far northern New
England will provide a focus for more organized convection within
the favorable right-entrance region of this jet streak, along the
outer perimeter of the upper ridge. The spread in the model QPFs
at this point is quite high, however the signal is there to support
at least localized areas of excessive rainfall across much of NY
and the Northeast outside of northern New England.
By Day 5, the Marginal Risk area broadens a bit (from the Upper
Midwest east through a bit more of the Northeast Region), again
with more upper jet streak energy and associated vort lobes riding
along the flatted outer rim of the upper ridge. While the activity
appears to be active across this corridor, it should also be fairly
transient. This along with the high spread in the guidance QPF
would support capping the ERO risk at Marginal for now.
...Eastern Gulf Coast...
Carried over the Marginal Risk area from the current Day 3 ERO to
the new Day 4 outlook (12Z Mon-12Z Tue), though given the guidance
trends, trimming much of eastern LA that is in the Day 3 outlook.
Shortwave energy moving westward underneath the closed 500 mb High
will be the foci for potentially more numerous (organized)
convective clusters during peak diurnal heating, especially when
interacting with the Atlantic and Gulf sea breezes. By Day 5 (12Z
Tue-12Z Wed), the mid/upper ridge presses closer to the Gulf Coast,
thus lowering the 0-6km bulk shear values and reducing the
convective coverage and intensity. The global QPFs are notably
lower on Day 5 compared to Day 4, and as such we opted to not
include a Marginal on Day 5.
Hurley
