FOUS30 KWBC 020818
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
418 AM EDT Tue Sep 2 2025

Day 1
Valid 12Z Tue Sep 02 2025 - 12Z Wed Sep 03 2025

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHERN CALIFORNIA AND SOUTHERN ARIZONA...

...Southwest...

There will be a nearly continuous anomalous surge in monsoonal 
moisture into the Sierra Nevada due to the position of the upper 
ridge axis to the east and a weak upper low along the central
California coast to the west. PW anomalies of 2 to 3 sigma are 
forecast from the lower Colorado River up trough the Sierra Nevada
and even into southern Oregon by 00Z Wed. Impressive CAPE will also
be present across southern Arizona and southern California, showing
support for heavy rainfall especially over flashy prone areas. 
This environment has the potential for more than just isolated 
areas of flash flooding, therefore a Slight Risks were raised for
southern Arizona and southern California.

...Mid-South through Tennessee Valley...

Remnant boundaries from ongoing convection today combined with
additional lift from a digging trough from the central U.S. should
allow convective activity to be in similar areas (and west) from 
yesterday's coverage. However, moisture is limited to around 1.5".
This uptick will allow for some greater rates and motion should 
remain slow overall, but overall the rainfall magnitude continues a
decreasing trend. A Marginal Risk remains in effect from central
Mississippi to Kentucky.

...Upper Midwest...

During this period a cold front is expected to traverse across the
Dakotas and encounter pooled moisture with a PW of 1.25", which is
1 sigma above normal. Diurnally enhanced convection is expected 
fire up along and ahead of the frontal boundary and will have the
potential to produce heavy rainfall for Minnesota, northern Iowa,
Wisconsin and the U.P. of Michigan. A Marginal Risk was kept for this area.

Campbell/Jackson


Day 2
Valid 12Z Wed Sep 03 2025 - 12Z Thu Sep 04 2025

...THERE IS A SLIGHT RISK FOR PORTIONS OF SOUTHERN CALIFORNIA AND
SOUTHER ARIZONA...

...Southwest...

Anomalous monsoonal moisture will continue to affect northward 
from the lower Colorado Valley through the Sierra Nevada and much 
of Nevada. Diurnally- driven activity can be expected again over 
the Sierra Nevada which may be locally excessive, as well as much 
of Nevada through western Arizona and southwest Utah. A broad 
Marginal Risk cover much of Arizona,southeast Utah, southern 
Nevada, and eastern/southern California. The presence of strong 
CAPE embedded with the deep column of moisture, especially across 
southern Arizona and across the Transverse and Peninsular Ranges of
southern California will be conducive for heavy rainfall and 
scattered areas of flash flooding. These locals will be vulnerable 
to the high rainfall rates, especially in the flashy prone 
locations. A Slight Risk was raised in coordination with the local 
forecast offices.

...Central U.S...

Elevated moisture ahead of the strong cold front sweeping down the
Plains will trigger heavy rainfall again. Enough focus is present
to warrant maintenance of the Marginal Risk around the central
Great Lakes though timing is in question. While much of the 
activity should be progressive, there is a repeating threat on the 
southern end of activity along the southern Kansas/Missouri 
border- a Marginal Risk was carried on for this period.

...Florida...

Enhanced rainfall expected to develop within the very moist airmass
along the stalled frontal boundary. Individuals cells will focus
heavy rainfall over the southern peninsula and the Keys thus
keeping an elevated threat for flash flooding.

Campbell


Day 3
Valid 12Z Thu Sep 04 2025 - 12Z Fri Sep 05 2025

...THERE IS A MARGINAL RISK AREA FOR PORTIONS OF FLORIDA,
SOUTHWEST/GREAT BASIN, AND THE NORTHEAST...

...Florida...

The environment will remain nearly the same as the Day 2 period,
thus convection will persist. Heavy rain will be possible therefore
the Marginal Risk was maintained southern Florida and expanded 
southward to cover the Keys.

...Northeast...

The cold front passing through the Lower Great Lakes and into the
Northeast will be fairly progressive, however there are signals 
for higher QPF to focus across central and northern portions of New
York. Parts of the region have been drier with some areas with 
notable drought conditions, which will limit the potential for 
impacts from heavy rainfall. The highest amounts are expected for 
the more elevated locations which will increase the threat for 
flooding concerns. A Marginal Risk area was maintained from the 
previous Day 4 but with portions of western and eastern new York 
trimmed out given the D0/D1 drought conditions.

...Southwest, California and southern Great Basin...

The monsoonal moisture will continue to advect into the 
southwestern U.S. underneath the mean upper ridge, promoting 
scattered showers and thunderstorms. Moisture anomalies are
expected to be the greatest for portions of California and Nevada.
A broad Marginal Risk encompasses eastern California, most of
Nevada, southeast Utah, Arizona and west/central New Mexico.

Campbell
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