FOUS30 KWBC 110811
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
411 AM EDT Thu Sep 11 2025

Day 1
Valid 12Z Thu Sep 11 2025 - 12Z Fri Sep 12 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTH FLORIDA...

...South Florida...

A quasi stationary front over central Florida will make incremental
southward progress down the Peninsula today. Ahead of this source
of forcing, atmospheric moisture levels across south Florida remain
very high, with PWATs above 2 inches. With afternoon heating,
MUCAPE levels will once again rise to between 2,000 and 3,000 J/kg.
This will support numerous showers and thunderstorms developing
over the southern half of the state, and generally moving southward
through the afternoon. However, as is typical of Florida afternoon
convection, while the overall storm motion may be southward,
localized forcing such as cold pools and sea breezes may make
individual cell motion far more erratic. Given that soils across
much of the Florida Peninsula are fully saturated after weeks of
afternoon convection each and every day, this will still pose a
widely scattered flash flooding risk, particularly along the urban
I-95 corridor. However, given the saturated soils, the threat
extends into much of interior south Florida as well. The inherited
risk areas were trimmed from the north to eliminate most areas that
are already behind the front, and therefore much drier.

...Northwest...

An area of well above average moisture on the northwestern side of
a nearly stationary longwave trough/cutoff low over the
Intermountain West will act as a focus for additional shower and
thunderstorm activity in the typically dry areas from eastern
Oregon northeast through much of Montana. While the longwave trough
will be very slow-moving, the embedded shortwaves and vort maxes
circulating around it will be faster moving, providing additional
forcing and areas of divergence for any shower and thunderstorm
development. Local mountains and ranges will also work to cause
some thunderstorm development. Thus, due to there being several
forcing mechanisms with little to note as to which will be
dominant, expect multiple clusters of storms to develop this
afternoon with peak heating. With plenty of atmospheric moisture
available for those storms, many may be capable of heavy rainfall
which could quickly cause flash flooding, especially in any urban
areas, slot canyons, and narrow river valleys.

...Western Colorado into Northwestern New Mexico...

On the eastern side of the deep longwave positively tilted trough
in the west, increased divergence and lift ahead of that trough,
supported by southerly flow of moderate amounts of Gulf moisture
will support numerous showers and a few storms forming across the
Marginal Risk area today through tonight. The key to the flooding
potential in this area will be the long duration of mostly light
rain but that will be training in this Marginal Risk area from this
afternoon through much of tonight. The Marginal Risk area remains
largely unchanged from inherited.

Wegman


Day 2
Valid 12Z Fri Sep 12 2025 - 12Z Sat Sep 13 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTH FLORIDA AS WELL AS FOR PORTIONS OF SOUTHWESTERN COLORADO INTO
CENTRAL NEW MEXICO...

...South Florida...

The front across South Florida on Friday will make additional
progress southbound down the Peninsula. The portion moving down the
west coast of the peninsula will make better progress southward
compared to the portion of the front along the East Coast. This
should support the front becoming increasingly aligned southwest to
northeast. Since the front will be moving a bit more on Friday,
there should be a bit better coverage of storms across south
Florida, where the Slight Risk area inherited remains largely
unchanged. As the front pushes south, it will shove the bulk of the
tropical moisture into the Florida Straits and into the Bahamas.
This should work to limit storm coverage on future days, as a much
drier and cooler air mass continues to envelop the rest of the state.

...New Mexico and Colorado...

The positively tilted upper level trough from Thursday will begin
to pivot to become neutrally tilted on Friday. This means the
southern end of the trough will get a boost to advance eastward,
while the northern end will largely stay in place. The result will
be a more south to north oriented axis of heavy rainfall,
stretching from the southern end of New Mexico north across much of
western Colorado. Moisture from the Pacific will also be drawn
northward along the New Mexico/Arizona border. This should expand
the rainfall footprint for Friday across much of the southern
portion of New Mexico and southeastern Arizona. Expect Friday to
the wettest day this week for New Mexico and Colorado. 

CAMs guidance suggests the plume of moisture stretching NNE from
southeast Arizona through north-central New Mexico and into 
Colorado will be the focus for additional convection capable of 
producing flash flooding. With better southward extent, the storms 
will be able to tap into some deeper moisture and instability from 
the Pacific on Friday. This will make thunderstorm coverage Friday 
greater than in previous days. Since the whole plume will also be 
lifting north and east, expect a much larger area of rain to impact
the area Friday. Embedded thunderstorms among the mostly lighter 
shower activity will be the foci for any potential flash flooding. 
This will be particularly true in any areas where multiple rounds 
of storms producing heavy rain occur.

...Northwest...

Shortwaves and vorticity maxes rounding the periphery of the
longwave trough will be the foci causing additional convection from
northern Idaho east across much of Montana. The expected rainfall
forecast has decreased the overall rainfall in this area. However,
mountain forcing may offset this in some areas.

Wegman


Day 3
Valid 12Z Sat Sep 13 2025 - 12Z Sun Sep 14 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EASTERN NEW
MEXICO THROUGH SOUTHWESTERN KANSAS, AS WELL AS FOR A PORTION OF THE
URBAN CORRIDOR OF SOUTH FLORIDA...

...New Mexico through Kansas...

As the upper level trough over the Intermountain West becomes more
negatively tilted, a portion of the low level jet out ahead it will
draw deeper Gulf moisture north into the Panhandles and eastern New
Mexico and into southwestern Kansas. This will support a line of
storms that will develop over central New Mexico and then press
eastward into the Slight Risk area. Once over eastern New Mexico
and the Panhandles, the eastward progress of the storms will
diminish greatly, resulting in much slower-moving but still
organized storms. This will be the basis for training, resulting in
widely scattered instances of flash flooding where the storms are
the most persistent.

...South Florida...

The front over South Florida will finally push east and clear all
of the deepest tropical moisture well out into the Atlantic and
Bahamas by Sunday morning. Thus, expect Saturday to be a transition
day in the area...where there will still be storms, particularly
along the urban I-95 corridor from Miami through Palm Beach, but
there will be less coverage over Florida as the moisture is pushed
eastward out into the Atlantic. The newly issued Slight is a low-
confidence one, as once the front clears the state, that should end
the flooding threat. However, given the many days of daily
afternoon convection in the I-95 corridor, soils are sensitive
enough that even the reduced time and coverage of storms could
still cause widely scattered instances of flash flooding. 

...Northern Plains...

Across the Northern Plains, ahead of the neutrally tilted trough
that is becoming negatively tilted, the northern extent of the low
level jet ahead of the trough will draw deeper Gulf moisture
westward across the Dakotas. This should support persistent
convection. However, this area of the Dakotas is fairly difficult
to flood, so despite the higher probability for training and heavy
rain, the area remains in a higher-end Marginal for now. A Slight
Risk upgrade may be needed for portions of the area as guidance
begins to resolve this part of the event across the northern Plains.

Wegman
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