Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
441 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
441 PM EDT Fri Aug 28 2026
Day 2
Valid 12Z Sat Aug 29 2026 - 12Z Sun Aug 30 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CENTRAL AND SOUTHERN ARIZONA...
...2030z Update...
...Southwest, Great Basin, and the Rockies...
For Saturday (day 2), a Slight Risk Excessive Rainfall Outlook was
added for portions of central and southern Arizona, including
parts of the Flagstaff, Phoenix, and Tucson CWAs after
collaboration. Moisture should increase due to an increase in
integrated vapor transport from the Pacific ahead of an upper-
trough and subtropical jet. Heavy rainfall and instances of
scattered flash flooding will be supported with PWATS in the
1.5-1.75 inch range. Additionally,forecast CAPE around 1000-1500
J/Kg will support afternoon thunderstorms. The Slight Risk
generally covers moderate HREF probabilities of locally heavy
rainfall totals greater than 2 inches, as well as 20-30% 3 hour
flash flood guidance exceedance in central Arizona and 40-60% 3
hour flash flood guidance exceedance across Tucson's CWA.
A Marginal Risk of excessive rainfall remains for surrounding areas
of the Great Basin and Rockies as above average moisture for the
monsoon fuels diurnally driven widely scattered to scattered
convection across Nevada, New Mexico, and the Colorado Front Range.
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. Additionally, areas
farther north in the Great Basin and Rockies from Utah to Wyoming
and Montana may see some locally heavy rainfall as anomalous
moisture streams in from the Pacific ahead of a potent upper-level
trough. Model guidance is indicating heavier QPF totals of 1-1.5
inches across northern Utah, eastern Idaho, and western Wyoming.
Sensitive terrain would be favored for flash flooding.
...Southeast and Gulf Coast...
The forecast remains on track from the previous guidance. Across
the Carolinas and into the eastern Appalachians, a stalled front
and diurnal heating will support afternoon thunderstorms
development capable of producing localized rainfall totals of 2-3
inches with a 20% chance for exceeding FFG exceedance according to
the afternoon HREF. Closer to the Gulf Coast, a combination of a
moist, tropical environment and a mid-level low /surface trough
may bring locally heavy rainfall and some flash flood concern as
well.
...Previous Discussion...
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.
...Northern Plains into the Upper Midwest...
Model guidance continues to indicate rounds of storms that form
overnight in the Plains to head eastward towards Minnesota and
Wisconsin. Generally, anomalous moisture and sufficient
instability with lift from a shortwave should bring a general 1-2
inches of rain with embedded locally heavy rain rates that may
exceed flash flood guidance. Renewed convection is possible ahead
of an upper-level trough Saturday afternoon and evening across
eastern Montana and western North Dakota that may also bring a
localized flash flood threat.
...Previous Discussion...
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.
Wilder/Campbell
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
441 PM EDT Fri Aug 28 2026
Day 3
Valid 12Z Sun Aug 30 2026 - 12Z Mon Aug 31 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
ARIZONA AND SOUTHERN UTAH...
...Southwest/Great Basin/Intermountain West...
A Slight Risk has been added to portions of south-central to
north-central Arizona, as well as south-central Utah. Similar to
day 2, a persistent Monsoon environment characterized by PWATS in
the 1.5-1.75 inch range and CAPE around 1000-1500 J/Kg, as well as
increasing synoptic support as an upper-trough enters the
Southwest, will support increasing thunderstorm coverage and
periods of heavy rainfall from Arizona to southern Utah. Increasing
coverage of storms is expected into the Deserts compared to day 2,
particularly in central Arizona around the Phoenix vicinity.
Elsewhere, a Marginal Risk of excessive rainfall remains for above
average moisture fueling diurnally driven widely scattered
convection across 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.
Wilder/Campbell
...Previous Discussion...
..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 an 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" are possible. At this time, Marginal
Risk areas remain in effect. However, there may be a need for a
Slight Risk somewhere between the Great Lakes and Northeast.
Campbell/Roth
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
441 PM EDT Fri Aug 28 2026
Day 4 and Day 5
Valid 12Z Mon Aug 31 2026 - 12Z Wed Sep 02 2026
...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS SOUTHERN AND
CENTRAL ARIZONA...
...Southwest/Four Corners/Eastern Great Basin/Rockies...
On Day 4 (Aug 31), an upper trough approaching the West Coast will
work together with a heat dome over the East to produce a
diffluent mid-upper level wind field over the Southwest. The
evening of Aug 31, NAEFS shows a strengthening IVT originating out
of the tropical East Pacific that will direct >90th climatological
IVT values over the Desert Southwest. The region will also see
soils continue to saturate with multiple days worth of rainfall
leading up to Aug 31. Given the unusually strong syoptic-scale
forcing, increasing IVT and lingering 90th climatological
percentile PWs, and increasingly sensitive soils, a Slight Risk was
introduced for portions of central and southeast AZ this forecast
update.
On Day 5 (Sept 1), a longwave trough with a closed 500 mb low (H5
height anomalies around 2 standard deviations below normal) will
be positioned off the Pacific NW coast. The upper trough will help
tap into deep-layer moisture and direct the moisture farther north
across the Western U.S.. With many of these regions having dealt
with daily rounds of heavy showers and thunderstorms, and a
farther northward push of anomalous moisture via the upper trough,
the inherited northward expansion of the Marginal Risk area still
looked reasonable as of this update. 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 (Aug 31), 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 remains fairly high at the moment. However, PWs
above the 90th climatological percentile will revolve around the
northern flank of the ridge and reach the Northeast by evening.
With favorable synoptic-scale support and increasing moisture
aloft, there is support for at least localized areas of excessive
rainfall across much of NY, northern PA, and as far east as the
I-95 corridor from northern NJ to southern New England.
By Day 5 (Sept 1), 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
scattered thunderstorms will be likely 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. It should be noted that NBM QPF is approaching 1" near the
Tug Hill, which is a signal that could support a potential Slight
Risk upgrade. Especially if soils grow more saturated from rounds
of storms the previous day.
...Eastern Gulf Coast...
The inherited Marginal Risk on Day 4 (Aug 31) remains in place with
only minor adjustments westward to include more of eastern LA. Shortwave
energy moving westward underneath the closed 500 mb High will be
the focus 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.
Mullinax/Hurley
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
441 PM EDT Fri Aug 28 2026
Day 4 and Day 5
Valid 12Z Mon Aug 31 2026 - 12Z Wed Sep 02 2026
...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS SOUTHERN AND
CENTRAL ARIZONA...
...Southwest/Four Corners/Eastern Great Basin/Rockies...
On Day 4 (Aug 31), an upper trough approaching the West Coast will
work together with a heat dome over the East to produce a
diffluent mid-upper level wind field over the Southwest. The
evening of Aug 31, NAEFS shows a strengthening IVT originating out
of the tropical East Pacific that will direct >90th climatological
IVT values over the Desert Southwest. The region will also see
soils continue to saturate with multiple days worth of rainfall
leading up to Aug 31. Given the unusually strong syoptic-scale
forcing, increasing IVT and lingering 90th climatological
percentile PWs, and increasingly sensitive soils, a Slight Risk was
introduced for portions of central and southeast AZ this forecast
update.
On Day 5 (Sept 1), a longwave trough with a closed 500 mb low (H5
height anomalies around 2 standard deviations below normal) will
be positioned off the Pacific NW coast. The upper trough will help
tap into deep-layer moisture and direct the moisture farther north
across the Western U.S.. With many of these regions having dealt
with daily rounds of heavy showers and thunderstorms, and a
farther northward push of anomalous moisture via the upper trough,
the inherited northward expansion of the Marginal Risk area still
looked reasonable as of this update. 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 (Aug 31), 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 remains fairly high at the moment. However, PWs
above the 90th climatological percentile will revolve around the
northern flank of the ridge and reach the Northeast by evening.
With favorable synoptic-scale support and increasing moisture
aloft, there is support for at least localized areas of excessive
rainfall across much of NY, northern PA, and as far east as the
I-95 corridor from northern NJ to southern New England.
By Day 5 (Sept 1), 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
scattered thunderstorms will be likely 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. It should be noted that NBM QPF is approaching 1" near the
Tug Hill, which is a signal that could support a potential Slight
Risk upgrade. Especially if soils grow more saturated from rounds
of storms the previous day.
...Eastern Gulf Coast...
The inherited Marginal Risk on Day 4 (Aug 31) remains in place with
only minor adjustments westward to include more of eastern LA. Shortwave
energy moving westward underneath the closed 500 mb High will be
the focus 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.
Mullinax/Hurley
