Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
432 PM 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 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt