Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1136 AM EDT Fri Sep 4 2026

Day 1
Valid 16Z Fri Sep 04 2026 - 12Z Sat Sep 05 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE SOUTHWEST,
PORTIONS OF THE GREAT LAKES INTO THE UPPER OHIO VALLEY, AND
CENTRAL APPALACHIANS...

...Southwest...

16z Update:The forecast remains on track for scattered instances
of flash flooding possible this afternoon across portions of
Arizona and New Mexico. Moisture transport from the tropical
Pacific and Gulf of California is supporting PWATS of 1.0-1.5
inches and surface CAPE around 1000-2000 J/Kg that will increase in
the afternoon with daytime heating. Model guidance remains
consistent in showing widely scattered thunderstorms forming with
QPF on the order of a 0.25 of an inch to locally 1-2 inches. The
Slight Risk covers much of the areal model signal for the localized
higher QPF output of 1-2 inches and encompasses much of the areal
15-30% 3hr and 6 hr flash flood exceedance guidance. Favored
terrain such as burn scars, dry washes, and desert soils will be
the most susceptible to localized flash flooding.

Wilder

..Previous Discussion..

A longwave trough over the Pacific Northwest will keep the jet
stream out ahead of it over the monsoon-affected areas of the
Southwest. A Slight Risk of excessive rainfall will therefore
remain in effect for the flood prone areas of southeastern Arizona
and western New Mexico. Convective coverage is unlikely to be
anything noteworthy...rather remaining widely scattered to
scattered. The storms will propagate northeastward, which should
lower the threat for metro Tucson today. Prior days' thunderstorms
in this area are keeping the soil conditions wet enough to promote
any storms' runoff causing localized flash flooding, especially in
low-lying areas, slot canyons, and over and downstream of burn
scars.

Wegman

....Great Lakes to Mid-Atlantic...

16z Update:A Slight Risk of Excessive Rainfall was added across
southern Michigan, northern Ohio, western Pennsylvania, and
northern West Virginia. Rounds of storms may bring as much as 2-3
inches of QPF over areas that have received heavy rainfall this
week. 3 hour flash flood guidance values have been lowered from 1-2
inches across central Ohio to half an inch across eastern Ohio and
western Pennsylvania. Therefore,the corridor of northeast Ohio and
western Pennsylvania are the most susceptible to flash
flooding. Additionally, urban areas such as Detroit, Toledo, and
Pittsburgh will be vulnerable to flash flooding. The broader
Marginal across the southern Mid- Atlantic remains on track this
afternoon as progressive rounds of storms will be capable of
dropping 1-2 inches of rainfall and isolated instances of flash
flooding in mostly urbanized areas.

Wilder

..Previous Discussion..

A large ridge of high pressure over the middle of the country will
steer "ridge-riding" disturbances southeastward from the Great
Lakes to portions of the Mid-Atlantic. Many areas expected to see
storms again today were hit by storms, sometimes multiple times,
yesterday. A Marginal Risk remains over the area since the storms
are likely to be very fast moving, as they have been in recent
days. Due to abundant moisture still in place, however, any storms
will be capable of locally very heavy rainfall. This could cause
flash flooding in urban, low-lying, and other flood-prone areas
today.

Wegman

...Gulf Coast/Central Texas...

16z Update: No changes were made to the ERO outlook as the forecast
remains on track. Satellite shows a mid-level low rotating across
the Gulf of America, streaming locally heavy thunderstorms across
the Central Gulf Coast as of this morning. 1-2 inch hourly rainfall
rates have prompted isolated Flood Advisories for coastal LA/MS,
including a Flash Flood Warning for urban areas around New Orleans.
The potential for isolated flash flooding and water ponding will
continue across much of the Gulf Coast as thunderstorm coverage
increases in the afternoon with daytime heating and sea-breeze
boundary collisions.

For central Texas into the Hill Country, the Marginal Risk was held
on for continuity. Currently, radar shows light showers west of
the I-35 corridor and hourly rainfall rates remain a quarter to
half an inch. Going into the afternoon, there is only a 20-40%
chance of observing hourly rainfall rates that exceed an inch;
meanwhile, hourly flash flood exceedance is rather high at 2-3
inches due to the drier antecedent conditions. Bottom line, any
excessive rainfall, if any, will be on a very isolated basis.

Wilder

..Previous Discussion..

Abundant Gulf moisture remains along all of the Gulf Coast. A
stalled front a bit inland from the Gulf could provide some focus
for organized convection in localized areas. However, the storms
are largely expected to remain disorganized, likely following any
sea breeze fronts. High FFGs due to sandy soils and around average
moisture levels in the top soils should keep any flash flooding
localized to urbanized areas.

Wegman


Day 2
Valid 12Z Sat Sep 05 2026 - 12Z Sun Sep 06 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR EASTERN NORTH
CAROLINA...

...Ohio Valley and the Mid-Atlantic...

A lower-end Slight was introduced with this update for
eastern/coastal North Carolina. A back-door cold front will make
steady progress southward down the Delmarva Peninsula today. Ahead
of the front in eastern North Carolina, multiple rounds of storms
are possible as instability maximizes in the area ahead of the
front. Sea breeze fronts are likely to be the predominant forcing
for any storms during the early to mid-afternoon. Then as the front
approaches in the late afternoon and evening, a second much more
widespread area of storms will impact the area. While soils there
are dry currently, expect a round of storms today to help saturate
the soils ahead of this Day 2/Saturday round. This nudges the area
into the low-end Slight category.

Further west into the Ohio Valley, storms along the western
extension of the front will also push southward with time. The
front will be much slower-moving to the west of the Appalachians.
Any associated storms impacting the Ohio Valley may produce
localized flash flooding in urban areas and other low-lying and
flood prone locales.

...Gulf Coast...

Another round of typical afternoon convection may cause localized
flash flooding in urban and flood prone areas. Once again, most
forcings for the storms will be localized, such as sea breeze or
old outflow boundaries. An abundance of moisture, with PWATs over 2
inches over a large areas will support any storms being capable of
very heavy rainfall. Generally flat terrain with sandy soils should
largely hinder any flooding to any urbanized areas.

...Southwest...

A Marginal Risk covers from southeastern Arizona across much of
New Mexico and into far southern Colorado. Storms Saturday
afternoon should largely be confined to the slopes of mountain
ranges. This should keep any resultant flash flooding localized.

Wegman


Day 3
Valid 12Z Sun Sep 06 2026 - 12Z Mon Sep 07 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE COLORADO RIVER VALLEY FROM THE ARIZONA/NEVADA BORDER NORTHEAST
INTO CENTRAL UTAH...

...West...

Sunday looks to be a particularly active day over much of the West.
Some moisture from the large plume associated with Hurricane Marie
well out into the Pacific will be drawn into the Southwest.
Meanwhile the large longwave trough over the West will capture and
fling a few disturbances around the base and across the West. The
result will be much more widespread convection, especially over the
Slight Risk area from Las Vegas northeast over much of Utah. The
Slight Risk was expanded southwestward to include more flood prone
areas near Las Vegas, the Grand Canyon, and some of the western
National Parks in Utah, including Zion and Bryce Canyon. Slot
Canyons may be a particularly dangerous place to be on Sunday
afternoon with the more widespread convection expected. Additional
expansions and a potential inclusion of a higher end Slight are
probable with future updates given the very favorable pattern.

A slow-moving front along the Canadian border will make for a wet
go over much of northern Montana and into North Dakota, then
spreading south over much of Minnesota. A Marginal Risk remains in
place, but here too it's probable a Slight may be needed with
future updates as the convective evolution in this area becomes
more clear.

...Southeast...

The Marginal Risk was expanded with this update to include some
areas where TS Edouardo just hit, another afternoon with widespread
convection over the Florida Peninsula, and some chance that
convection could spread over North Carolina's southern coast. The
area of heaviest rain could be over much of South Carolina's coast.
A Marginal was left for Charleston and adjacent areas in
coordination with the forecast office there. Better consistency
with convective behavior at the end of a front there may allow for
future upgrades, but for now given the difficulty for rain to cause
flash flooding in portions of the Low Country, the Marginal is
sufficient.

Wegman


Day 4 and Day 5

Valid 12Z Mon Sep 07 2026 - 12Z Wed Sep 09 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE
SOUTHWEST, SOUTHERN TO CENTRAL CALIFORNIA INTO THE SOUTHERN GREAT
BASIN, ACROSS THE UPPER LAKES INTO THE UPPER MISSISSIPPI VALLEY AND
OVER THE SOUTHEAST/NORTH FLORIDA...

...SOUTHWEST,SOUTHERN-CENTRAL CALIFORNIA,SOUTHERN GREAT BASIN....
A mid to upper level trof in the mean will persist across the
western U.S days 4 abnd 5. A lead closed low will be ejecting
northeastward from the northern Great Basin into the Northern
Plains, while additional height falls reform the mid to upper level
trof along the West coast. The net effect will be to maintain an
axis of much above average PW values from Southern CA into the
Southwest day 4 and expanding northward into Central CA and the
Southern Great Basin by day 5 in response to the re- amplifying
West coast upper trof. A marginal risk is maintained day 4 over
eastern AZ into western to central NM/far southern CO in the model
consensus qpf axis. Scattered convection across this region will
support locally heavy rains and isolated runoff issues. With the
expansion of the anomalous PW farther to the north and west day 5
into Central CA and the Southern Great Basin, the marginal risk was
also expanded from day 4, stretching from southwest CO, western NM
into all of AZ, far southern NV and southeast CA. Increasingly
favorable right entrance region jet dynamics on the east side of
the re-amplifying upper trof will support locally heavy rains in
the widespread scattered convection, supporting isolated runoff
issues.

...Northern Plains into the Upper Mississippi Valley/Upper Lakes...
The above mentioned northeast ejection of the western trof from
the Northern Great Basin/Northern Rockies into the Northern Plains
at the beginning of day 4 will support potential for organized
convection along and to the northeast of the northwest to southeast
oriented frontal zone across the Northern Plains. Strengthening
low level southerly flow into this front in an axis of anomalous PW
values, 2-3+ standard deviations above the mean, expected across
this region, supporting potential for heavy rains and isolated
runoff issues day 4. By day 5, the associated surface frontal
boundary is expected to become more southwest to northeast
oriented along the southern edge of the westerlies. At this time
the low level inflow into this front is also expected to become
more parallel to the boundary, supporting potential for a period
of training along the front Tuesday night into early Wednesday.

...Southeast/North Florida...
Widespread scattered convection likely day 4 in the vicinity of
the slowly southward moving frontal boundary across the
Southeast/North Florida. Low confidence in qpf details in the
vicinity of this front, but with PW values 1.75-2.00", locally
heavy rains and isolated runoff issues possible.

Oravec


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt