FOUS30 KWBC 180802
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
402 AM EDT Thu Sep 18 2025

Day 1  Valid 12Z Thu Sep 18 2025 - 12Z Fri Sep 19 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF
SOUTHERN CALIFORNIA AND SOUTHWESTERN ARIZONA...

...Southwest U.S...

Remnant moisture from post-tropical cyclone Mario will continue to advect
poleward with California firmly in the cross-hairs for elevated 
moisture (PWATs 2-4 deviations above normal) to move into the 
southern half of the state during the entirety of the D1 period. 
Energy from Mario will lag the initial low to mid-level moisture 
surge, but eventually will enter into the area by the late-morning 
hours today, prompting an uptick in the areal coverage of 
convection over the southern half of the state into the Lower 
Colorado Basin between CA/AZ. The last succession of HREF runs,
including this evenings 00z suite was pretty pronounced in the
indication of precip totals between 2-4" potentially impacting some
of the coastal ranges of CA into the adjacent valley just north of
the Transverse and Peninsular Ranges. There's also a pretty good
indication of scattered to widespread convective development later
this morning, through the afternoon period across the lower deserts
of southeast CA to the neighboring Colorado Basin at the CA/AZ
line. This was the area of greatest change when it came to the
magnitude of the signal when assessing the run to run differences
over the past iterations of the HREF suite. Ensemble bias corrected
QPF was also adjusted upward from its previous forecasts across the
aforementioned area, a testament to the improving signature for 
local flash flood prospects in that general area. HREF neighborhood
probs for >1" across the deserts are upward of 50-80% now with 
70-90+% located within the Transverse and Peninsular Ranges during 
the period, a reflection of the heightened threat for at least 
moderate rainfall, enough to spur high runoff concerns in the 
terrain with several burn scars still present in-of those mountain 
chains. Rates will be sufficient considering the deep moist 
environment brought on by the advection of the tropical moisture 
flux involved with Mario, another reason for the threat to be 
leaning towards the higher side as elevated rates are a significant
driver to flash flooding historically across CA. 

The previous SLGT was expanded a bit to account for the trends in
hi-res guidance and the conditioning of the environment favoring
the higher moisture anomalies being advected further north into CA.
The threat is on the higher end of the SLGT threshold with the
"saving grace" in this entire setup being the more progressive mean
flow helping push everything through in relatively faster time than
what is customary for higher level risks. That still doesn't deter
the fact that scattered flash flood prospects are becoming more
certain in the the areas outlined in the SLGT risk with a potential
for targeted upgrade plausible if conditions warrant. 

...Central U.S...

Current WV satellite depicts a broad upper low (ULL) with the center
positioned over SD with a surface low analyzed almost underneath 
the ULL meaning the confidence in an occlusion process beginning 
soon is very high. Once we see the occlusion phase begin, we'll see
the slow degradation of the prominent deformation field across the
Dakotas over the course of the forecast leading to decreasing 
rates and expected totals compared to what transpired the period 
prior. As a result, flash flooding concerns will be relatively 
isolated given the more stratiform rain process with the convective
threat positioned further east into the Mississippi Valley. 00z 
HREF was not very robust for heavy rain prospects in any one area, 
but there were several locations that will stand to benefit from 
locally heavier convection as the cold front continues to migrate 
eastward and drive a broad surface ascent pattern downstream of the
low. A few stronger mid-level vorticity maxima will help amplify 
the threat in a few areas, mainly in the Missouri and Mid-
Mississippi Valleys later this afternoon as a stronger mid-level 
shortwave continues to propagate into the region from its overnight
trajectory. HREF blended mean QPF distribution outlines this area 
fairly well and matches with the modest >3" probabilities situated 
over eastern OK up into MO. Overall, the entire setup with a slowly
decaying, or steady state low will contribute just another period 
of widely scattered flash flood prospects from the Northern Plains 
over and through the northern two-thirds of the Mississippi Valley.
The previous MRGL was general maintained with some adjustments 
based on the latest hi-res ensemble probs and blended mean QPF footprint.

...Southeast Florida...

PWATs across south FL remain elevated with KEYW and KMFL both
coming in with values of 2.08" and 2.19", respectively meaning the
deep layer moisture forecast the past several days is firmly
entrenched over the region. Several shortwave perturbations will
advect northeast out of the Florida Straits and nearby Cuba with
the presence of a Cuban outflow being picked up in the radar/sat
composite over the past few hours. This is actually well-forecast
via some of the CAMs suite, a likely indicator that the future
trends within the models probably hold merit for a threat of heavy
rain to cross into the Key and the southern Peninsula in the early
morning hours. As the boundary migrates to the northeast, expect
copious showers/storms to gallivant through the area dropping
locally enhanced rainfall with rates between 2-3"/hr pretty much a
given in a few of the convective cores just due to the environment
in place. Models are still in flux on the exact locations of where
heavier rain will occur, but the neighborhood probs for >3" running
between 50-80% and >5" between 25-35% across both the Keys and
southeast FL are sufficient for a continuation of the previous MRGL
risk through the forecast cycle. Will monitor closely for potential
targeted upgrades, but the setup is certainly favoring at least
isolated flash flood potential within those two corridors,
particularly more urban zones due to impervious surface runoff. 

Kleebauer


Day 2  Valid 12Z Fri Sep 19 2025 - 12Z Sat Sep 20 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE
CENTRAL SIERRA NEVADA AND CENTRAL VALLEY OF CALIFORNIA...

...California...

Remnant energy from what is left over from Mario will pivot up into
Central CA early Friday allowing for a continuation of heavy
rainfall prospects over the region as PWATs surge to between 3-4
deviations above normal. Guidance is keying on prevailing upslope
flow and focused ascent within the Central Sierra of CA with a
myriad of thunderstorms lingering in the Central Valley. The
environmental conditions favor locally enhanced convective cores
which could spell problems for those towns in the valleys, as well
as runoff in the terrain. HREF neighborhood probs for >1" are very
high (70-90%) within the above areas with mainly modest >2" probs
centered over the Central Valley and adjacent Sierra foothills.
Considering some overlap of heavy rain from D1 into D2, the threat
seems worthy of a targeted upgrade in these two zones to account
for the continued risk bleeding from period to period. The good
news is the setup looks to wane the second half of the forecast
cycle as energy pushes out of the region and we see a slow
improvement in the moisture anomalies. This would spell the end 
for the threat of flash flooding and much of the rain prospects. 

...Southwest...

Moisture increases across all of the Southwest CONUS will aid in
favorable environmental conditions for locally heavy convective
rainfall during the period of diurnal heating on Friday into early
Saturday. HREF mean QPF is relatively tame overall, but some of the
CAMs are more robust in their depictions of isolated 2+" totals,
especially across the Mogollon Rim and Southeast AZ terrain leading
to a broad MRGL risk maintenance from previous forecast. The 
threat for flash flooding will likely lie as far north as the 
I-70/15 junction and points west over UT/NV with slot canyons and 
dry arroyos as the primary targets for flash flooding as is 
customary in these setups. The threat for a targeted upgrade is on 
the table, but will need to assess the evolution of the mid-level 
vorticity maxima ejecting out of CA to the east over the course of 
time. The most favored areas will likely be those outlined above. 
For now, the MRGL risk will suffice.

...Mississippi Valley to ArkLaTex...

Occluded low over the Upper Midwest will continue to spin with a
dumbbelling mid-level shortwave pivoting southeast out of the
Dakotas allowing for a secondary broad ascent pattern to
materialize downstream over the Mississippi Valley. To the south,
thunderstorms will fire along a decaying front positioned over the
Mid-Mississippi Valley down into the ArkLaTex with ample
instability and moisture to fuel another isolated flash flood
threat when assessing individual CAMs and ensemble bias corrected
outputs. The threat is similar to the previous period, perhaps even
a touch less robust considering the occlusion phase well in
progress by this point in the forecast. Overall, this threat was
deemed enough for a broad MRGL risk to stay in place, but there's
some potential for a targeted upgrade, pending better overlap from
model guidance on any particular area from TX to the Upper Midwest.

...Southeast Florida...

The southeast coast of Florida will be hanging on to the last bit 
of remaining anomalous moisture as the pattern starts to shift away
from persistence of the preceding three periods. The MRGL was 
maintained given some of the guidance hanging on to heavier 
convection right along the coast with modest probabilities for >3" 
lingering from Miami up to Daytona Beach. That was enough to 
maintain continuity of a MRGL for at least one more period.

Kleebauer


Day 3  Valid 12Z Sat Sep 20 2025 - 12Z Sun Sep 21 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL AND SOUTHERN PLAINS INTO THE MISSOURI VALLEY...

...Central U.S...

Shortwave ejection out of the Great Basin/Four Corners area will
migrate into the Central and Southern Plains by Saturday afternoon
providing a more focused large scale ascent pattern during peak
diurnal instability. Ensemble means are relatively modest in their
QPF distribution and magnitude, signaling 1-2" of rainfall
currently in their depictions. Individual deterministic is
certainly a little more robust, but nothing that would promote a
more substantial risk. This correlates well with moisture anomalies
hovering between normal and +1 deviations when assessing the latest
NAEFS meaning the threat is warranted, but well-within the
threshold of a MRGL risk. As a result, kept the previous MRGL risk
and expanded to account for some uncertainty in the spatial
coverage of expected convection by this time frame.

Kleebauer
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