452
FOUS30 KWBC 010106
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
906 PM EDT Sun Aug 31 2025

Day 1
Valid 01Z Mon Sep 01 2025 - 12Z Mon Sep 01 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHERN NEW
MEXICO THROUGH PARTS OF WESTERN AND CENTRAL TEXAS...

...New Mexico into Texas...

0100 UTC Update -- Based on the latest observational and guidance
trends, have trimmed back on the Slight Risk area, now no longer in
place across much of central and eastern TX. The latest HREF (18Z)
and RRFS (12Z) probabilities for FFG exceedance during the
overnight support the changes, as does recent HRRR runs, current
radar/satellite trends, and mesoanalysis trends.

Hurley

...Central Plains...

1600 UTC Update -- Abundant cloud cover across the central Plains
as well as ongoing shower activity have greatly reduced the
available instability for what very little convection is ongoing.
This light activity is expected to continue through the day and
into tonight. Despite the long duration, it`s unlikely that except
in very isolated instances, that there will be much embedded heavy
rainfall across the area. Further, the rain that is expected is
likely to fall in areas that have been very dry in recent days,
displaced from where the heavier rains have occurred further west.
Thus, it appears that the flooding threat has waned down well into
the Marginal category. Thus, the Slight Risk is downgraded with
this update.

...East-Central Florida...

0100 UTC Update -- Have scaled back on what was a fairly broad
Marginal Risk across much of the FL Peninsula to one that is
focused across central and east-central portions of the peninsula,
including the much of the Orlando metro area and the Space Coast.
CAM trends of late have indicated spikes of 3-5+ inch totals during
the overnight, including recent HRRRs and 12Z ARWs, while also the
RRFS (albeit a much smaller heavy rainfall footprint). Aligned the
Marginal Risk area to best fit a blend of the latest HREF and RRFS
probabilities for FFG exceedance. However, suspect the heaviest
rain will remain offshore, given that the onshore/easterly low-
level flow opposes the mean 850-300 mb flow, thereby maintaining
the west-northwest (offshore) Corfidi vectors at a healthy
15-20kts through the overnight.

Hurley

Day 2
Valid 12Z Mon Sep 01 2025 - 12Z Tue Sep 02 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHERN TIER, PLAINS, EAST COAST OF FLORIDA, AND TENNESSEE
VALLEY...

...2030Z Update...

A stalled out frontal system and monsoonal moisture will persist
across southern Arizona, New Mexico, and Texas on Monday. Coverage
of convection in these areas is expected to be more isolated, and
therefore without good forcing, are likely to keep flash flooding
to isolated instances. The Marginal was trimmed on the north side a
bit as the front sags southward. Signals for heavy rain have
dropped on the western side of the Mogollon Rim of Arizona, so the
Marginal there has been trimmed, while the guidance remains steady
state across New Mexico, where the Marginal is unchanged.

The Marginal was trimmed along much of the Gulf Coast east of
Houston, as the front causing the storms should be far enough south
of the Gulf Coast that the rainfall threat is minimal. Easterly
onshore flow into Florida required the Marginal for the east coast
to remain, while the center and west coast of Florida were removed
with this update.

The Marginal Risk area across the Tennessee Valley was also trimmed
a bit as the heavier rain in this area is more likely on Day
3/Tuesday. Antecedent dry conditions along the Appalachians should
preclude any flash flooding. Better coverage of convection should
stay further west into central Tennessee and northern Alabama. A
vort max tracking east across Tennessee will be the forcing
mechanism for any convection across Middle Tennessee on Monday.

Finally, the Marginal in the Plains was trimmed a bit on the north
side out of Minnesota, northwest Iowa and far southeast South
Dakota, while it remains over the central Plains. Overall, as
today, instability and abundant cloud cover should limit the
flooding threat, but prior rains from previous days in the form of
more saturated soils could support an isolated flash flooding risk.

Wegman


Day 3
Valid 12Z Tue Sep 02 2025 - 12Z Wed Sep 03 2025

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE MIDDLE TENNESSEE VALLEY...

...2030Z Update...

A Slight Risk upgrade was introduced with this update for portions
of the Middle Tennessee Valley. Remnant convection from the
overnight period on Monday night will persist into Tuesday as a
reinforcing shortwave digs southward and reaches Middle Tennessee
by Tuesday night. A weak, but still significant low level jet will
advect Gulf moisture northeastward across the Deep South and into
Tennessee on Tuesday, which will keep a steady supply of fuel in
place for any convection. Thus, expect a long-duration rain event,
that while much of it will be light, the LLJ advecting moisture
into Tennessee and various different sources of forcing will
support some storms becoming strong and able to produce heavy rain.
The storms will also be slow-moving, which will lengthen the amount
of time any area sees heavy rain.

Elsewhere, the stalled out front across Florida will support more
widespread convection Tuesday afternoon across much of the
Peninsula, while the Panhandle dries out a bit from prior days`
rains. A second shortwave and associated front will produce areas
of heavy rain across portions of the upper Midwest. The area may
also need to be monitored for a possible future Slight Risk upgrade
as well, depending on how slow the front tracks and how much
convection can form and ride along the front. Finally, the
monsoonal moisture should shift west on Tuesday into southern
California, so lingering moisture into New Mexico and Arizona will
likely require upslope support to produce showers and thunderstorms
capable of heavy rain, so the Marginal Risk in that area was
trimmed to follow the continued signal in the guidance and upslope
support.

Wegman

...Previous Discussion...

A Marginal Risk area was maintained for the Mid-South as shortwave
energy and a weak low traversing the cold front may trigger
convection capable of higher rainfall rates over the region.
Meandering frontal boundaries within an unstable environment will
support heavy rainfall and localized flash flooding potential for
Florida, therefore the Marginal Risk area was kept for this period.

A cold front will drop into the Northern Plains and Upper Midwest
ahead of the strong reinforcing upper trough. Convection is
expected fire up along and ahead of the frontal boundary, with
potential for heavy rainfall. A Marginal Risk area is in effect for
the eastern Dakotas, northern Iowa, Minnesota, northern Wisconsin,
and portions of the UP of Michigan.

Meanwhile, monsoonal moisture is forecast to flow into the
southwestern U.S. underneath the mean upper ridge for scattered
showers and thunderstorms. A Marginal Risk span from southern
Colorado/New Mexico westward to eastern California.

Campbell


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