Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
409 AM EDT Wed Sep 2 2026

Day 1
Valid 12Z Wed Sep 02 2026 - 12Z Thu Sep 03 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHEAST TEXAS...

...Eastern Texas and Southwest Louisiana...

Raining/backbuilding with high rainfall rates from tropical
cyclone Edouard are ongoing although there should be some
weakening by late in the morning, but periodic backbuilding
clusters will remain a possibility. Another uptick late Wednesday
night into Thursday morning likely as the low level vorticity
should remain fairly well defined. PWs of 2-2.25 inches will
support continued heavy rain with hourly rates of 2-3 inches/hour.
There is potential for a locally enhanced inflow band on its south
side which could prolong and enhance the rain across areas that
receive heavy rain. The Moderate Risk was expanded to include
Freeston, Anderson and Henderson counties as well more of Polk
county. The eastern boundary of the Slight Risk was adjusted
eastward a bit to reflect the latest QPF guidance and WPC forecast.

...Northern Mid Atlantic & Southern New England...

A stronger shortwave will interact with the slow-moving warm front
over the Northeast to trigger scattered to widespread showers and
thunderstorms across the region. There continues to be ample
support of training of clusters of thunderstorms with 1-2"/hr rain
rates. The guidance focuses this heaviest rainfall across portions
of Upstate New York, and while soil moisture is generally below
normal according to NASA SPoRT, this should support up to 3" of
rain in some areas leading to scattered instances of flash
flooding, with more isolated responses elsewhere.

...Florida...

The tropical moisture plume remains entrenched across Florida,
reflected by PW anomalies of +1.5 sigma stretching across most of
the state. Both coasts could experience slow moving storms with
2+"/hr rain rates, as well as across the inland portions of the
peninsula where storm mergers/collisions can occur. A Marginal Risk
was maintained for this period.

...Arizona...

Monsoonal moisture will return to southern Arizona to trigger
diurnal convection. Locally excessive rainfall may be possible
therefore a Marginal Risk is in effect.

Campbell


Day 2
Valid 12Z Thu Sep 03 2026 - 12Z Fri Sep 04 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHERN ARIZONA AND EXTREME SOUTHWEST NEW MEXICO...

...Southwest...

A Slight Risk remains in effect across southern Arizona and into
New Mexico for this period. An uptick in moisture should result in
increased convective coverage and a scattered flash flood risk.
This region will continue to be monitored over the coming days.

...Texas, Gulf Coast and Florida...

The remnant vort center of Edouard should be less defined by this
time, but still present over east Texas with lingering tropical
rain over sensitive soils. It could still help focus some
organized convective clusters, and thus an isolated flash flood
risk could linger into Thursday. Elsewhere onshore flow within the
pooled high PW values and stalled boundary could trigger convection
along the coastline and across the Florida Peninsula leading to
isolated areas of flash flooding/ponding. A Marginal Risk spans
from eastern Texas to Florida.

...Upper Midwest to Mid-Atlantic...

While the details on where the heaviest rain will occur there
continues to be a signal for one or more rounds of organized
convection to track trough portions of this area through the day
into Thursday night. Parts of this region have had recent rains to
lower FFG and increase sensitivity to additional heavy rainfall. A
Marginal Risk was maintained from Minnesota/Iowa to Del-Mar-VA and
New Jersey.

...California into the Intermountain West...

A Marginal Risk continues from northern California into Idaho.
During this period a strong synoptic system will bring an axis of
heavy rainfall to the region. Not much instability, but just enough
showing up in model forecasts to suggest some potential for
localized higher rates and an isolated flash flood potential.

Campbell/Chenard


Day 3
Valid 12Z Fri Sep 04 2026 - 12Z Sat Sep 05 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FRIDAY ACROSS
PORTIONS OF ARIZONA...

...Northwest/Northern Rockies...

Marginal Risk area is in effect across portions of the interior
Northwest, particularly the area surrounding the northern Idaho
Panhandle. This area will be much closer to the upper level trough,
so forcing should be much better than in the Southwest. However,
moisture and instability will be much lower, resulting in more
likelihood of stratiform rainfall. Despite this, the rainfall
should still have modest amounts of instability to work with in a
generally dry, and therefore flash flood prone, area of the
country.

...Southwest/Four Corners/Central-Southern Rockies...

The signal for the return of heavy rain areas across the Southwest
into the Four Corners and central-southern Rockies remains strong
GEFS and ECENS PWAT anomalies increase back to 2-2.5 standard
deviations above normal with the axis of best monsoonal moisture
influx shifting slightly west. Still remains to be seen how much
influence the eventual tropical system that ultimately pulls west
of Baja will have in terms of the upper level moisture (potential
uptick in the PW anomaly). A Slight Risk was maintained for Arizona
while a broad Marginal covers southeast California the Four Corner.

...Upper Midwest/Ohio Valley/Central Appalachians...

Expect more additional organized/widespread convection downwind
of the upper ridge axis. Precipitable water values are high enough
with sufficient instability that embedded cells could produce
local downpours and isolated flooding. This will likely start over
eastern portions of the Upper Midwest, where the upper jet streak
to the northeast is expected to strengthen as the pivoting northern
stream trough impinges on the broad-scale ridge to the south. Have
essentially traced the Marginal Risk area southeast along the mean
850-300 mb thickness fields, where the upper level forcing
along/ahead of the surface front will be most supportive of more
organized activity. A Marginal Risk is in effect from Wisconsin to
West Virginia.

Campbell/Hurley


Day 4 and Day 5

Valid 12Z Sat Sep 05 2026 - 12Z Mon Sep 07 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL SATURDAY AND
SUNDAY ACROSS PARTS OF THE SOUTHWEST...

A deep upper low over the Pacific Northwest working in tandem with
a strong ridge centered over the central U.S. will direct
southwesterly flow into the southwest U.S. both Saturday and
Sunday. Deterministic and ensemble guidance continue to advertise
an impressive influx of moisture over the region, featuring PWs
surging in excess of 150% of normal on Saturday and 200% of normal
on Sunday. Models are somewhat split on whether or not that
moisture can translate to significant QPF, and no doubt that likely
has to do with the degree of influence from Tropical Storm Marie.
With that being said, many of the models that keep Marie on a more
northwesterly and eventually westerly track away from southern
California still show a good deal of moisture, which is also likely
monsoonal in nature, being pulled northward from the Pacific.

Given the above, thought it would be prudent to have at least some
cover for the potential for localized flash flooding from any
heavy rain that may develop over the weekend. As a result, decided
to largely keep the MRGL risk inherited from yesterday's Day 5 for
the new Day 4/Saturday ERO with only minor changes. For the new Day
5/Sunday ERO, which will likely be the day with the more
impressive moisture surge, decided to introduce a MRGL risk which
encompasses portions of southern California, southeast Nevada, much
of Utah, and western Arizona. Should the moisture signal diminish
due to even less tropical interaction in the coming days, future
updates may reduce or remove the MRGLs entirely.

Miller


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