Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
347 PM EDT Wed Sep 17 2025
Day 2
Valid 12Z Thu Sep 18 2025 - 12Z Fri Sep 19 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF
SOUTHERN CALIFORNIA...
...2030Z Update...
...Southwest...
In coordination with SGX/San Diego, CA; LOX/Oxnard, CA; VEF/Las
Vegas, NV; PSR/Phoenix, AZ; and HNX/Hanford, CA forecast offices, a
Slight Risk upgrade has been issued for much of Southern
California and the Arizona side of the Colorado River. The mid-
level remnants of Mario are making their way north up the coast.
They are advecting abundant tropical moisture along with them, with
PWATs well over 1.5 inches at and along the coast. This is at least
4 sigma above normal for this time of year. Thus, this amount of
moisture is highly anomalous for this part of the country. In
addition to the moisture, significant instability will also be
present, with a rough average of the CAMs suggesting there will be
over 1,000 J/kg of CAPE in much of southern California Thursday
afternoon. Finally, the mid-level circulation center, while fully
detached from the now dissipated low level circulation of Mario,
will itself remain strong enough to provide adequate forcing for
shower and thunderstorm activity across much of southern
California. Expect any thunderstorm activity to be capable of 1
inch per hour rainfall rates, locally higher in favorable terrain
areas.
Working against any flooding will be the fast storm motion and
potential for cloud cover to stabilize the lower atmosphere. Clouds
will increase along and ahead of the low center, and where clouds
are, the sun will be blocked, which will limit the daytime heating
necessary to locally increase instability values. There is high
uncertainty as to how much cloud cover there will be in between the
storms. More cloud cover will mean lower chances for flash
flooding. Further, with the circulation of Mario continuing to
track north, expect any storms over California to be moving along
Thursday afternoon. This should in most cases limit the potential
for flooding. However, conditions are also favorable for training
of storms, so while no one storm is likely to stall out over a
given area, the combination of multiple storms appears likely. This
event appears to be a low-confidence but high impact event. This
means while the chance of flooding in any one area remains low,
there will likely be multiple instances where heavy rain from
training storms impacts a given area, resulting in flooding.
...Plains...
No significant changes were made to the Marginal Risk area across
the Plains. A slow-moving front may provide the forcing for more
widespread storms from eastern Oklahoma north into the Dakotas, but
the signal for heavy rain is low given the potential for the storms
are moving quickly or that there is less coverage of storms.
Meanwhile into eastern ND, the maximum of QPF is likely from a
long-duration stratiform rain, which should only cause isolated
flooding impacts.
Wegman
...Previous Discussion...
...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 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
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