936
FOUS30 KWBC 091516
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1116 AM EDT Tue Sep 9 2025

Day 1
Valid 16Z Tue Sep 09 2025 - 12Z Wed Sep 10 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN FLORIDA PENINSULA...

16Z Excessive Rainfall Discussion...
Decided on an upgrade to a Slight Risk across portions of the
southern Florida Peninsula given the overlap of lowered Flash Flood
Guidance from recent heavy rainfall as well as the impervious
landscape of the urban corridor with the the pooling of deep
moisture along/near a quasi-stationary front. At the same time...an
area of low pressure over the western Gulf will remain nearly
stationary through the period will keep a flow of tropical
moisture continuing across the state. With peak afternoon heating,
the clashing of the tropical air mass moving north and the front
over the northern part of the state will cause numerous showers and
storms to form. Numerous other forcings such as cold pools and sea
breezes will also force storms to form, as it won`t take much
given the amount of moisture in the area. As on most days, despite
the moisture, the forcings being so numerous will result in
competition between them resulting in a chaotic thunderstorm regime
across the Peninsula. The lack of organization should preclude
most flash flooding threat, as well as high FFGs. However, with
PWATs running at or above the 90th percentile for this point in
September and CAMS showing rainfall amounts the begin to challenge
the Flash Flood Guidance (2 to 4 inches in general and only 1 to 2
inches in spots) across the far southern peninsula..,opted for the
upgrade. Elsewhere across the peninsula that are less
hydrologically sensitive...localized/isolated flash flooding may
occur


...Upper Midwest...

A stationary front set up along the south shore of Lake Superior
will continue to focus additional showers and thunderstorms ahead
of an upper level shortwave approaching from the west. Due to
recent and ongoing rainfall, soils across the western U.P. and
northern Wisconsin are near saturation, so additional rain
should largely convert to runoff. Many of the CAMs show showers
and thunderstorms training over this area into this afternoon,
showing multiple hours of repeating showers and thunderstorms.
Expect isolated instances of flash flooding as a result of this
weather pattern.

...Pacific Northwest...

A large trough and embedded cutoff low will move into the coast of
Oregon today, then drift southeast over northern California.
Numerous small shortwaves will rotate around this low, acting as
local foci for additional training rainfall. The storms will move
over portions of the mountains characterized by frequent large burn
scars from large wildfires in recent years. Rainfall over these
burn scars could cause localized instances of flash flooding. The
Marginal Risk inherited is largely unchanged. Uncertainty is high
as to how most of the rain will cause flash flooding given very
high FFGs west of the Cascades.

Bann/Wegman

Day 2
Valid 12Z Wed Sep 10 2025 - 12Z Thu Sep 11 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE PACIFIC NORTHWEST AND THE FLORIDA PENINSULA...

...Pacific Northwest...

The upper level low over western Oregon will remain nearly
stationary on Wednesday as the trough becomes positively tilted,
and a strong shortwave rides north along the eastern periphery of
the low. The storms will be embedded in an air mass that is up to 3
sigma above normal for this time of year, with PWATs at or around
1 inch. Since the greatest forcing will be with the northbound-
moving shortwave, the heaviest rain is likely across much of
eastern Oregon, in areas where FFGs are very low, due to normally
dry conditions. Thus, while there`s a fair amount of uncertainty as
to precipitation intensity and duration, the fact that the heaviest
rains will be moving over flood sensitive areas should result in
isolated to perhaps even widely scattered instances of flash
flooding. A Slight Risk may be needed over portions of Oregon with
future updates.

...Florida Peninsula...

A cold front, which had been stationary over the northern part of
the state on Tuesday, will continue to slowly press southward
across the northern half of the Florida Peninsula on Wednesday.
Meanwhile, the stationary surface low will remain over the western
Gulf. Thus, once again, the clashing of the southerly flow around
the Gulf low, and the front will cause numerous showers and
thunderstorms to form Wednesday afternoon as on Tuesday afternoon,
albeit a bit further south down the Peninsula. Otherwise, the
storms will remain disorganized and in clusters, so the flood risk
will remain isolated due to this lack of organization.

Wegman

Day 3
Valid 12Z Thu Sep 11 2025 - 12Z Fri Sep 12 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE PACIFIC NORTHWEST, WESTERN COLORADO AND FAR NORTHWESTERN NEW
MEXICO AND MUCH OF THE FLORIDA PENINSULA...

...Pacific Northwest...

The slow-moving upper level low will continue to drift eastward
across the Intermountain West on Thursday. Numerous shortwaves and
vort maxes will move through the flow while moisture levels across
this region remain between 2 and 3 sigma above normal. Thus, expect
an area of showers and storms to develop across Oregon and Idaho
through the day Thursday, then spread into Montana Thursday night,
allowing more areas into the potential for flash flooding. The
Marginal Risk was expanded northeastward to account for this fast
flow around the slow-moving upper low. Depending on where the
high-resolution guidance ends up over the next couple days, enough
rainfall may fall on the southwestward facing slopes to possibly
need a Slight Risk upgrade with future updates.

...Colorado/New Mexico...

Portions of western Colorado and New Mexico will be in the most
favorable divergence region around the upper level low parked over
the Pacific Northwest. The guidance that simulates the radar at the
Day 3 time range, like the RRFS, suggests showers and few storms
will track northeastward across western Colorado starting in the
afternoon, peaking in coverage and intensity Thursday afternoon,
then gradually subsiding overnight. There is some uncertainty as to
how quickly the rain dies out. Regardless, much of the Marginal
Risk area has soils that are at or drier than average. This should
help to mitigate the flooding risk a bit, except in the slot
canyons where soil moisture levels are far less relevant.

...Florida Peninsula...

The front over the northern part of the state will continue to sag
southward across the central part of the state on Thursday. Showers
and storms will once again develop along and ahead of the front in
the abundant tropical moisture that will persist over the southern
part of the state. The air mass will be characterized by PWATs
above 2.25 inches in some areas. Thus, any storms that form will
still have access to ample moisture needed to produce heavy rain.
Since the front will be the primary forcing for the storms, they
will likely form along the front, then most southward down the
Peninsula Thursday afternoon. This should result in enough of a
progressive movement to limit the flash flooding potential
somewhat, but given the multiple days of heavy rain, the flooding
threat may be gradually increasing with time due to the saturated
soils. It`s not out of the realm of possibility that a Slight Risk
upgrade may be needed with future updates.

Wegman


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