FOUS30 KWBC 100804
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
404 AM EDT Wed Sep 10 2025

Day 1
Valid 12Z Wed Sep 10 2025 - 12Z Thu Sep 11 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF 
THE URBAN I-95 CORRIDOR OF SOUTH FLORIDA...

...Florida...

A stationary front draped across north Florida will remain in place
as abundant tropical moisture, characterized by PWATs over 2
inches, streams across the state and along that front. Within that
moisture plume over much of the southern half of Florida, numerous
showers and thunderstorms are likely to form with peak heating 
this afternoon. As on previous days, despite the front to the 
north, over most of the peninsula, there are likely to be many 
different sources of forcing for the storms. Without a coherent 
source allowing the storms to organize, it's likely the storms will
remain individual or form into small clusters. While there will be
plenty of moisture for the storms to be capable of heavy rainfall
with any stronger core, without organization it appears unlikely 
that for most of the peninsula, there will be more than isolated
instances of flash flooding.

Across the urban I-95 corridor, however, multiple prior days'
rainfall has thoroughly saturated any soils. Thus, when heavy rain
occurs with the afternoon and evening convection, there is more
likely to be widely scattered instances of flash flooding. Thus,
the Slight Risk area inherited remains unchanged with this update.

...Pacific Northwest...

A large upper level low and associated trough covering much of the
West Coast will gradually drift east/inland across the
Intermountain West today. Shortwave energy rotating around the low,
interacting with the instability caused by the cold air aloft and
unusually high amounts of atmospheric moisture (anomalies to 2.5
sigma above normal) will allow several clusters of showers and a
few storms capable of heavy rainfall to form. Upslope enhancement
may also locally increase rainfall rates as any clusters of showers
and storms move through. While some minor tweaks were made to the
Marginal, especially in northern California, the forecast remains
largely the same.

Wegman


Day 2
Valid 12Z Thu Sep 11 2025 - 12Z Fri Sep 12 2025

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

...Florida...

The stationary front plaguing Florida for well over a week will
begin to move as a cold front on Thursday, as a reinforcing shot of
cooler and drier air pushes south across the Southeast. As the
front pushes south down the Peninsula, the front should act as a
source of forcing for the storms, allowing for greater organization
and greater areal rainfall across South Florida as compared with
previous days when the forcing was far less defined. Add in the
saturated soils across south Florida, and the area where flooding
could be more frequent than isolated instances expands north up the
I-95 corridor towards the advancing front. Thus, the Slight was
expanded north accordingly.

...Pacific Northwest into the Northern Plains...

A plume of anomalously high moisture in place from the Pacific
Northwest to the Northern Plains will support scattered showers and
thunderstorms forming once again along a corridor from eastern
Oregon through much of northern Montana. A separate cluster of
storms may sneak south across the Canadian border and into far
northeastern North Dakota and northwestern Minnesota as well. Due
to the potential for heavy rain in all of these areas, a Marginal
Risk is in place and has been expanded eastward to cover the areas
most likely to see isolated instances of flash flooding from heavy
rain from these features. 

...Western Colorado and Far Northwestern New Mexico...

Southerly flow on the western edge of a low-level jet across the
Plains will transport Gulf moisture into the region on Thursday.
Meanwhile the large trough out west will become positively tilted
with time. This will allow an 80 kt jet southwesterly jet streak to
form, which will increase the divergence aloft across this area.
Expect upslope into the high peaks of southwestern Colorado to
locally enhance rainfall totals, thereby increasing the flash 
flooding risk. Soil moisture measurements from NASA Sport show much
of this area to be below average/drier than normal. Thus, expect
much of the rainfall Thursday in this region to work to saturate
the soils in anticipation of Day 3/Friday's bigger rainfall event.

Wegman


Day 3
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 NORTHWESTERN NEW MEXICO 
INTO SOUTHWESTERN COLORADO...

...Northwestern New Mexico into Southwestern Colorado...

A positively tilted longwave trough centered over much of the
Intermountain West on Friday will become more neutrally tilted on
Friday as a strong shortwave trough causes the southern half of the
trough to begin to race eastward. The result will be even more
enhanced upper level divergence out ahead of the trough over New
Mexico and Colorado. Atmospheric moisture will be largely unchanged
in this area from Thursday. With peak heating Friday afternoon,
numerous showers and thunderstorms will form over nearly all of the
High Plains. However, coverage and intensity will be much higher
in the Slight Risk area in Colorado and New Mexico. The area is
likely to be the same one hard hit from the Day 2/Thursday period,
so expect soil moisture amounts to be higher and FFGs to be lower
over much of the area by Friday's event. Given all of these
ingredients in place and in coordination with GJT/Grand Junction,
CO forecast office, a Slight Risk upgrade was introduced with this update.

...South Florida...

The cold front across central Florida at the start of the day
Friday will make slower, but still incremental progress in its
southward movement down the peninsula. The plume of tropical
moisture across south Florida ahead of the front will also still be
in place. Thus, as is similar to Thursday over the middle of the
state, the axis of heaviest moisture will simply shift southward to
far south Florida on Friday. The inherited Slight Risk area was
expanded west over much of the Everglades and portions of southwest
Florida with this update, to follow more closely to where the axis
of heaviest rainfall is likely to set up. Given the focus will
shift into the more vulnerable and flood prone urban areas of south
Florida, its entirely possible that a higher end Slight risk will
need to be issued for the Ft. Lauderdale to Miami corridor with
future updates.

...Northern High Plains...

Similar to Day 2/Thursday, anomalous moisture will remain in place
across much of Montana and portions of adjacent states again on
Friday. The upper level low and trough will move overhead or just
south of the region on Friday. As the cold air aloft peaks in depth
on Friday, numerous showers and thunderstorms are likely to
redevelop again on Friday afternoon with peak heating and maximum
instability. The axis of heaviest rain from central Idaho through
northwestern Montana will also be largely unchanged and unmoving
from Thursday, meaning these areas will likely have a second day of
showers and storms capable of heavy rain and flooding again on
Friday. Thus, here too a Slight Risk upgrade may be needed with
future updates. This will largely be determined by how much rain
falls on Thursday in this area, which will prime the soils for
Friday's round of rain.

Wegman
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