427
FOUS30 KWBC 050050
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
850 PM EDT Sat Oct 4 2025

Day 1
Valid 01Z Sun Oct 05 2025 - 12Z Sun Oct 05 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL GULF COAST...

Maintained a small Marginal Risk area covering far southeastern
Louisiana and coastal Mississippi. While much of the heavier activity
associated with a slow-moving wave has remained offshore into this
evening, guidance indicates that veering flow will be enough to
nudge both deeper moisture (PWs over 2 inches) and greater
instability further north, raising the threat for increasing rain
rates and accumulations along the coastal regions overnight. Recent
runs of the HRRR have been consistent in showing a signal for
localized heavy amounts moving onshore, while both the 18Z HREF and
the 12Z RRFS indicate that isolated totals over 3 inches are
possible.

Pereira

Day 2
Valid 12Z Sun Oct 05 2025 - 12Z Mon Oct 06 2025

...THERE IS MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL GULF COAST, THE SOUTHEAST COAST, AND THE CENTRAL
PLAINS...

20Z Update: There has been an additional lighter trend in the
guidance for the coastal portions of Georgia, and the majority of
heavier showers in onshore flow is expected to be across eastern
Florida, so the new Marginal Risk area is trimmed back to
Jacksonville. For the Midwest, the Marginal Risk is extended a
little more to the northeast to include northeastern Iowa where a
relatively narrow band of higher QPF ahead of a slow moving cold
front is likely to be, with the potential for some convective
training that may result in a few instances of flooding where cells
persist the longest. All seems on track with the Marginal Risk area
for the central Gulf Coast. /Hamrick

Previous discussion...

...Central Gulf Coast...

A weak disturbance in the western Gulf will likely continue to focus
enhanced convergence and convection along portions of the central
Gulf Coast into Sunday. Heaviest rainfall totals may tend to stay
just offshore where the better instability will reside...although
coastal convergence and instability into the immediate coastline
should allow for some heavier cells to make it onshore. Even our
deterministic QPF, which is a multi model blend, brings two day
rainfall totals as high as 3-5" over far southeast LA and right
around 3" along coastal MS and AL. Given the convective nature of
rainfall and PWs over 2" would expect totals to locally exceed these
amounts, and isolated instances of flash flooding are possible. The
00z REFS probabilities of 8" is over 90% across coastal MS/AL. This
is seemingly an under dispersed ensemble in this case, but
interesting nonetheless, and possibly indicative of a lower
probability worst case scenario. Either way, will need to continue
to monitor trends, as can not rule out the need for a Slight risk
along portions of the immediate coast.

...Southeast Coast...

The model QPF signal continues to trend down across the Southeast
coast, although onshore easterly flow, coastal convergence and PWs
around 2" will support localized instances of heavy rainfall.
Overall the flash flood risk appears pretty low, but an isolated
threat can not be ruled out.

...Central Plains...

Convection will likely expand in coverage Sunday evening across
portions of northeast KS, southeast NE and southwest IA. A stalling
frontal boundary will allow for persistent moisture convergence in
the lower levels coupled with a divergence signature aloft. Thus it
seems probable that rainfall will persist through much of the night,
not moving all that much over that time. Whether we see any flash
flood risk evolve will likely come down to the degree of instability
present. It seems likely that we will initially have enough
instability for deeper convection and 1"+ an hour rainfall. However
most indications are that this instability axis will be rather
narrow and under 1000 j/kg. Thus would expect that instability to
get eroded fairly quickly, resulting in decreasing rainfall rates
overnight. Nonetheless, our latest deterministic QPF has 1-2" of
rain, with 2-3" if you include the rain that continues into Monday.
When accounting for convection resulting in locally heavier amounts,
and the possibility that instability ends up greater than forecast
(which a couple models show) then a low end flash flood risk can not
be ruled out.

Chenard


Day 3
Valid 12Z Mon Oct 06 2025 - 12Z Tue Oct 07 2025

...THERE IS MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS EASTERN
FLORIDA AND THE CENTRAL PLAINS...

...Eastern Florida...

The 00z ECMWF, AIFS and GEFS mean all suggest an increase in
convective coverage over eastern FL on Monday compared to Sunday.
The approach of a subtle mid level wave may help increase low level
easterly moisture transport while also providing a bit more in the
way of upper divergence. Isolated urban flash flooding should thus
remain a threat into Monday, although would anticipate the risk to
stay quite localized.

...Central Plains...

Some convection will likely be ongoing Monday morning across
portions of northeast KS, southeast NE, southwest IA and northwest
MO. The stalled front will remain in the area through the day likely
helping focus additional convection that should only slowly drift
southward. Instability should increase Monday along this front,
however this will coincide with a decrease in moisture transport and
convergence along the front. Thus not really anticipating anything
more than a very localized flash flood risk. The threat is likely a
lower end Marginal risk, but given the slow front and two day
rainfall totals of 2", to perhaps as high as 3-4", across this
corridor, localized flooding is possible. The existing Marginal
Risk has been maintained with this update.

...Ohio Valley...

A Marginal Risk area has been introduced from eastern Arkansas to
southern Indiana. Moist return flow around a surface high off the
East Coast, in tandem with a frontal boundary slowly approaching
from the northwest, will result in a corridor of warm air advection
with scattered to numerous showers and thunderstorms developing.
Dewpoints rising into the upper 60s to lower 70s, in combination
with increasing mixed layer CAPE, will provide enough instability
for higher rainfall rates that may result in some instances of
flooding, mainly in the 18Z Monday-6Z Tuesday time period.

Hamrick/Chenard


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