FOUS30 KWBC 250824
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
424 AM EDT Sat Oct 25 2025

Day 1
Valid 12Z Sat Oct 25 2025 - 12Z Sun Oct 26 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF 
SOUTHEAST TEXAS INTO CENTRAL ARKANSAS AND WESTERN MISSISSIPPI...

...Previous Discussion...

Multiple rounds of heavy showers and thunderstorms are expected to
impact the broader northwest Gulf Coast region this period, with
one initial axis of on-going convection moving across parts of
eastern/southeastern Texas and advancing across western Louisiana 
and southern Arkansas. Convection presumably is interacting with a
40 kt southerly low-level jet off the western Gulf. Precipitable 
water values at or above 1.75 inches and MUCAPE values locally of 
2000 J/kg will support high rainfall rates potentially exceeding 2 
inches/hour with the stronger storms.

Additional organized convection is expected later today/tonight
with the arrival of an upper- level jet streak in the subtropical 
stream which interacts with the southern stream closed low/trough 
axis crossing the southern Plains. Even though antecedent 
conditions are quite dry, the concern for multiple rounds of 
convection which combined may yield some 3 to 5+ inch totals, will 
probably be enough to support a threat for scattered areas of flash
flooding. The more sensitive urbanized locations will be at 
greatest risk for impacts. 

...Pacific Northwest and Northwest California...

Multiple rounds of moisture transport into the coastal ranges will
continue as an atmospheric river activity continues...with greatest
rates/accumulation from later this morning into the afternoon. 
Locally a few additional inches of rain will be possible for the 
windward slopes of the terrain, but given the early-season and 
relatively modest nature of this atmospheric river activity, any 
flooding concerns are expected to be isolated at best. 

Bann


Day 2
Valid 12Z Sun Oct 26 2025 - 12Z Mon Oct 27 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID-AND LOWER-MISSISSIPPI VALLEY, TENNESSEE VALLEY, SOUTHERN 
APPALACHIANS AND SOUTHEAST...

Maintained the Marginal risk area already hoisted for portions of
the southern United States on Sunday. Thunderstorms capable of
locally intense downpours remain possible across the immediate Gulf
coast early as southerly flow continues to tap Gulf moisture and
advect it northward...potentially as far north as the Tennessee
Valley. A front making its ways eastward should help focus
convective activity early in the period...but also help shunt it
eastward with time as surface high pressure builds in from the
west. Isolated cells have the potential to produce rainfall rates
in excess of an inch per hour prior to frontal passage. Farther
north...concern is more for quarter to half inch rain rates within
a weakly destabilized environment (200-400J/Kg MUCAPE). PWATs of 
1-2" may support any isolated instances of excessive rainfall/flash
flooding within the marginal risk area. Models are trending toward
a higher qpf solution along the Central Gulf Coast, but their
continued to be enough uncertainty in the 24/00Z guidance to
preclude any upgrades at this time.

Bann


Day 3
Valid 12Z Mon Oct 27 2025 - 12Z Tue Oct 28 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS A PORTION
OF THE SOUTHEAST UNITED STATES...

Low pressure moving across the southern United States on day 2 
will begin to take a more northeastward trajectory. Given less 
favorable low level flow and a more stable atmosphere...the risk of
excessive rainfall will be diminished. Even so...the 25/00Z suite 
of numerical guidance does show spotty convection with isolated 
bulls-eye amounts of 2 inches or greater mainly in the Carolinas. 
The NAM was far and away the most aggressive with its amounts and 
was not given much consideration given the placement of the 
heaviest rainfall in a region where stable air was banked up 
against the east aspect of the terrain in response to the ridging 
of surface high pressure to the north. While initially easy to 
downplay the spotty nature of the model QPF...the machine learning 
first guess algorithm did point the area as a marginal risk of 
excessive roughly co-located with the WPC deterministic QPF. As a 
result...we opted to shrink the Marginal issued on Day 4 and place 
it closer to the southern portion of the Appalachians.

Bann
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