Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
849 PM EDT Fri Aug 28 2026
Day 1
Valid 01Z Sat Aug 29 2026 - 12Z Sat Aug 29 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE COASTAL
CAROLINAS INTO SOUTHEAST VIRGINIA AND SOUTHERN GEORGIA...
...01Z Update...
All of the inherited Marginals were adjusted to account for the
latest trends in Doppler Radar, as well as the latest 18Z HREF
guidance. In the West, an impressive IVT for late August (>99th
climatological percentile values per NAEFS from Southern CA to
central WY) is likely to continue scattered showers throughout the
West. Some embedded storms are possible, but upslope enhancement
within an air-mass containing >90th percentile PWs could still
support some localized flash flooding. In the Southwest, storms
there are more thunderstorms available, but the storms are more
diurnally driven and will dissipate across much of the region after
06Z.
Farther east, the Slight Risk remains in place over coastal areas
of the southeast and a little farther inland towards central SC.
18Z HREF guidance shows pockets of low-to-moderate chances (20-40%)
for 12-hour rainfall totals >3". With some storms already
contributing to flash flooding in parts of southern GA and northern
FL, and storms likely to continue into tonight along the coast, the
Slight still seems reasonable, but did trim off the northern most
portion that was over southeast VA. Lastly, in the Midwest, a fast
moving 500mb vort max will be responsible for developing scattered
thunderstorms overnight. 18Z HREF probabilities do show some low-
to-moderate chance probabilities (30-50%) for localized rainfall
totals >2".
Mullinax
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
849 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
849 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
849 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
849 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
