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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