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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FOUS30 KWBC 170758
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
358 AM EDT Wed Sep 17 2025

Day 2 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 occlusion will be taking shape across the Central CONUS with
locally heavy rainfall potential focused within the residual axis
of deformation across the Dakotas, as well as along the progression
of the cold front from MN down into the Southern Plains. Guidance
is all over the place with the exact placement of relevant QPF
maxima, however the ensemble means still emphasize the potential
within these zones referenced above. Expectation is for multiple
storm clusters to aid in the threat with some places likely to see
2-4" inches thanks to the +1 to +2 deviation PWATs situated from OK
up through the Upper Midwest and Northern Plains. Environment
favors rate potential of 1-2"/hr in any cell within the WCB as
general buoyancy should spur several thunderstorms along and ahead
of the front as it migrates into the Mississippi Valley. As we get
closer in time, it's plausible to have a targeted upgrade, or two,
so it will be a period to monitor.

...Southwest U.S...

Remnant moisture from post-tropical cyclone Mario will advect
poleward with California generally in the cross-hairs for elevated
moisture to move into the southern half of the state during the end
of the D1 into D2 time frame. Energy from Mario will lag the
initial low to mid- level moisture surge, but eventually will enter
into the area by the late-morning hrs. Thursday. Models are pretty
consistent in the meridional push of elevated moisture leading to scattered/widespread convection forming across much of SoCal,
especially the lower deserts and terrain focused areas from Big Sur
down into the Transverse and Peninsular ranges. It will be
important to monitor the progression of the remnant mid-level
energy as a more consolidated vorticity maxima would likely cause a
period of enhanced, focused rainfall in-of the coastal terrain
north of Los Angeles. As of now, the setup favors the energy
becoming increasingly sheared with a more scattered convective
depiction in the precip field. 00z ECMWF shows what could transpire
with a more consolidated vorticity maxima with a stripe of heavy
rainfall aligned south to north which would easily necessitate a
risk area higher than the current MRGL in place. With PWATs likely
to surge to 1.7-2.0", rainfall rates >1"/hr are certainly possible,
a threshold that historically has caused localized flooding to some
degree, leading to totals of 1-3" with room for higher if
everything breaks unfavorably for the region. This is something to
monitor as we step closer in time, but for now the threat is deemed
a MRGL with a chance at an upgrade as we move further into the CAMs
window for a better assessment on potential precip maxima.

...Southeast Florida...

General persistence in the pattern across southern FL with elevated
PWATs and streaming mid-level vorticity plaguing the region leading
to scattered bouts of heavy rainfall within any convective
development. The area of interest remains centered on the urban
corridor from Melbourne to Miami just due to the repeated nature of
convection and the lower FFG's aligned within the urbanization
footprint. Models are still everywhere in the placement of the
heaviest precip, some just offshore, and others hitting the Keys
and southern peninsula pretty hard over the course of D1 and D2.
Considering the environmental factors and continuity in the
pattern, maintaining the previous MRGL risk is more than sufficient
for the setup. Little to no change was necessary given the 00z NWP
output.

Kleebauer


Day 3 Valid 12Z Fri Sep 19 2025 - 12Z Sat Sep 20 2025

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

...Northern Plains to Mississippi Valley...

Our occluded low will continue to churn across the northern CONUS
with another round of convection forecast on the eastern flank of
the remnant cyclone. The threat is forecast to wane compared to the
previous periods, but there is still a general MRGL risk for flash
flooding in any cells that develop from the Mid to Upper-
Mississippi Valley. Models are still all over the place in location
and magnitude, but the premise of the environment remaining
favorable for the threat warranted a continuation of the MRGL risk
from previous forecasts.

...Western U.S...

Elevated moisture from remnants of Mario will linger and allow for
scattered to widespread convection to materialize over the
Southwestern U.S. and over portions of the Sierra's of CA. Pending
the mid-level evolution of the energy from Mario bleeding into D3,
a heavier precip threat is possible, but not anticipated at this
time. The setup should be able to adequately shear any mid-level
vorticity leading to a less pronounced setup regionally. Still, the
moisture anomalies and expected instability presence will allow for
at least widely scattered flash flood concerns heading into the end
of the week and early weekend. A MRGL risk remains for the
Southwest and the California Sierra's.

...Southeast Florida...

A chance for lingering heavy rainfall across southeast FL near
Miami remains within the means, enough to warrant a maintenance
of the inherited MRGL risk. It will really come down to finer
mesoscale details and timing of when the incessant shortwave energy
progression vacates the area. For now, there's enough to keep what
was forecast prior, but will assess as we get closer in time. The
area is small in spatial coverage, so there's a chance it is
removed, or expanded once more CAMs windows shed light on the setup.

Kleebauer
$$
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