FOUS30 KWBC 160808
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
408 AM EDT Tue Sep 16 2025

Day 2 Valid 12Z Wed Sep 17 2025 - 12Z Thu Sep 18 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE
NORTHERN AND CENTRAL PLAINS, SOUTHERN HIGH PLAINS, AND SOUTHEAST FLORIDA...

...Northern and Central Plains...

A strong mid-level shortwave trough will eject into the WY Front
Range and eventually close off across NE generating a maturing lee
cyclone across the Central High Plains. The setup will likely cause
quite a robust deformation signature on the northwest flank of the
cyclone with ensemble mean QPF placement situated over western NE
up through western SD into southern ND, arcing back into the nose 
of the warm- conveyor belt (WCB) on the northeast side of the 
system. This is a classic mid- latitude lee cyclogenesis setup with
a strong jet coupling leading to broad large scale forcing and 
widespread precip across the Plains. Eastern side of the cyclone 
will exhibit the best convective potential given the modest theta_E
advection pattern poleward within the WCB leading to scattered
thunderstorm potential from the Southern Plains to as far north as
the Northern Plains and adjacent Upper Midwest (Western MN).
Meanwhile, the deformation axis across NE/SD/ND will provide a 
solid precip field with moderate to locally heavy rates for several
hrs. located in that northwest side of the low. Totals >2" are 
forecast within the means across western NE into the Dakotas with 
scattered elevated QPF across the Missouri River basin located in 
eastern SD and NE down through KS and far western OK. This setup 
is conducive for at least some isolated to scattered flash flood 
prospects considering the broad synoptic scale evolution. A very
broad MRGL exists across much of the Northern and Central Plains
with a cull located over NE and northwest KS where the low center
will be present and drier air entrains into the region Wednesday
evening and beyond as the low intensifies and approaches occlusion
phase by Thursday AM. 

...Southern High Plains...

Very little change in the previous forecast as run to run
continuity has allowed for negligible adjustments within the
previous MRGL risk.

A maturing lee side low to the north will help drag a trailing 
cold front through the Central and Southern High Plains on 
Wednesday. The front will slip south and southwest once down near 
the latitude of the OK Panhandle, leading to a more textbook 
backdoor frontal progression as it enters into the northern TX 
Panhandle and northeast NM. Flow begins to run more parallel to the
boundary in these zones when assessing forecast soundings from 
relevant global deterministic. The key in the setup is a relatively
solid isentropic ascent pattern in a small zone bordering 
CO/OK/NM/TX with cross-section views over the area displaying a 
classic sloped low-level surface FGEN with strong omega correlating
to rapid ascent as the front progresses through the area. Locally 
enhanced corridor of precip will transpire along and in wake of the
front with local maxima of 1.5" to as much as 3" plausible during 
the event. This could be enough for an isolated flash flood threat,
especially in those smaller urban areas situated across the four 
state intersection. 

...Southwest...

Another period of isolated to scattered convection is anticipated
over the terrain encompassing southern AZ up through the central
and eastern portions of NM with the primary threat being heavy 
rainfall situated over more complex terrain and burn scar remnants.
Assessment of forecast soundings and hodographs indicate relatively
slower cell motions, a classic characteristic for flash flood
prospects over the desert southwest. Guidance reflects local 1-2"
totals scattered across the region extending from southeast AZ into
NM with some of the better signals focused within those mountain
ranges like the Huachucas and Sacramento's. This signal garnered
enough attention to warrant an addition of a MRGL to account for
the localized threat. 

...Southeast Florida...

Stalled front with an enhanced moisture return across South FL
will lead to a heightened threat for flash flooding over the urban
centers of southeast FL. The setup is historically favorable for 
flash flood concerns over the urban zones of southeast FL with 
PWATs hovering between 2-2.3", a solid 1/1.5 deviations above 
normal. CAMs guidance has recently come into range for at least the
first half of the period with a mixture of results ranging from
very little precip to multiple heavy convective cores impacting the
zone between PBI down to MFL. Neighborhood probs for >2" are pretty
high (>60%) across a large chunk of that above corridor, but
there's still a precipitous drop off in the probs for >3" with a
very low EAS signal accompanying. At this juncture, the prospects
for heavy rainfall seem to be more isolated in nature, but the
environment is leaning favorable for any cells to potentially drop
appreciable amounts of rain in a short period of time. The heaviest
rainfall totals among guidance seem relegated to the FL Keys, an
area that historically is difficult for flash flood prospects just
due to the sandier soils and much smaller land footprint available.
The previous MRGL risk was relatively maintained given the setup
and the chance for the signal to grow, but for now remains more
within the lower end of the risk threshold.

Kleebauer


Day 3  Valid 12Z Thu Sep 18 2025 - 12Z Fri Sep 19 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE NORTHERN PLAINS, MISSISSIPPI RIVER VALLEY, SOUTHWEST U.S.,
AND SOUTHEAST FLORIDA...

...Central U.S...

Broad, occluding cyclone over the center of the CONUS will lead to
a continuation of scattered convection on the eastern flank of the
disturbance with a continued axis of deformation over parts of the
Dakotas. Flash flood potential will be relatively modest
considering the cyclone undergoing a deteriorating phase by the D3
juncture. Ensemble means are generally between 0.5-1" within the
convective zone in the decaying warm-conveyor belt (WCB) with 1-2"
located within the axis of deformation. The best flash flood risks
will likely coincide in the convective elements just due to the
nature of enhanced rates, so overall we're looking at a mid-grade
MRGL risk for the period, but will assess if there needs to be any
targeted upgrades as we get closer to the period. For now, the
broad MRGL inherited will suffice. 

...Southwest...

Moisture advection from the northeast edge of tropical cyclone
Mario off the California Baja will make its presence known across
Southern CA into adjacent NV/AZ by Thursday morning leading to an
increasing threat of scattered to widespread convective initiation
during peak diurnal instability. As PWATs surge to 2-3 deviations
above normal by Thursday afternoon and beyond across SoCal, the
environment will be conducive for several convective cores to
produce fairly heavy rainfall potential, especially over the
terrain focused from Big Sur, south into the Transverse and
Peninsular Ranges. The threat for thunderstorms will exist over the
SoCal deserts, an area very prone to flash flooding due to the high
runoff capabilities and multitude of dry arroyos littered across
the region. The northern periphery will likely make its presence
into southern NV, another area prone to flash flooding due to
similar issues and added complex topography scattered about the
area. Totals of 1+" will be common with a likelihood of a few 2+"
totals considering the environment likely in place. A MRGL risk is
currently forecast, but would not be surprised to see an upgrade in
future forecasts, especially over portions of SoCal. 

...Southeast Florida...

A rinse and repeat setup carrying over from D2 will lead to a
continued threat of heavy convection across much of South Florida.
Frontal alignment still indicates a scattered to widespread
convective posture across southeast FL where the urban corridor
will be under threat for more heavy rainfall and flash flood
prospects. The greatest threat will lie within that zone from
Melbourne down to Miami with some of the ensemble means greatest
near West Palm Beach and Port St. Lucie. A MRGL risk exists once
again for the defined area above between MLB to MFL. 

Kleebauer
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