Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1050 AM EST Tue Nov 18 2025

Day 1
Valid 16Z Tue Nov 18 2025 - 12Z Wed Nov 19 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS WESTERN
ARIZONA, FAR SOUTHEASTERN CALIFORNIA, AND FAR SOUTHEASTERN
NEVADA...

...Southwest U.S...

16Z Update: Current radar and IR satellite composite continues to
indicate a broad area of diffluence within the eastern periphery of
a closed ULL situated just off the CA coast. Radar indications and
observational ground truth pinpoint some instances of locally
moderate to occasionally heavy rain this morning across western AZ,
alignment within the greatest MUCAPE and deepest moisture presence
in the proximity of 0.9-1.1" PWATs. Both 12z RAOB soundings out of
KVEF (Las Vegas, NV) and K1Y7 (Yuma, AZ) indicate relatively deep
saturation within the profile and PWATs well within the 90-99th 
percentile overall for the time of year. These indicators are 
textbook for more efficient rainfall prospects, especially with the
additional instability maxima expected later during prime 
differential heating. 

A more defined shortwave over Baja is currently progressing north-
northeastward with an indication of greater ascent being initiated
across southwest AZ at this hours as scattered convective pulses 
begin materializing to the east of the Colorado River Basin. This 
has been well-forecast within the CAMs on the hourly outputs, and 
should maintain some persistence the next several hours with the 
convective footprint shifting north within deep uni-directional 
steering flow positioned along that eastern flank of the upper- 
level circulation. This would put areas further north between 
Prescott, AZ to Lake Havasu City, extending into the southern half
of Clark county in NV during the peak of the late-morning and 
afternoon convective threat. Highest HREF neighborhood 
probabilities for >2" of precip align within that general zone with
the highest probs >60% situated between Prescott and Lake Havasu 
City. Considering the nature of the precip likely exhibiting more
convective schemes and the lower FFG presence in that area of the
Southwest CONUS, there is enough to warrant a continuation of the
SLGT risk from previous forecast with a broader MRGL encompassing
the lower deserts in AZ/NV/CA, including major metros of Phoenix
and Las Vegas proper. 

Kleebauer

..Previous Discussion..

Upper difluence will become quite pronounced over the Lower
Colorado River Valley today downstream of a strong mid-level trough
approaching from California. Meanwhile, pockets of surface heating,
near 50s F surface dewpoints, and cold air aloft will all combined
to support one or two focused bands of showers and thunderstorms
especially across western Arizona. These storms will migrate
quickly to the north given seasonably strong wind fields aloft for
steering. However, pronounced backbuilding should develop toward
the source region of nearly 1000 J/kg SBCAPE (across
southwestern/west-central Arizona) that should result in potential
for several hours of heavy rainfall. While local rates may not
exceed 0.5 inch/hr on a widespread basis, multiple hours of
convective training potential will support at least a few instance
of flash flooding as rainfall totals exceed 2 inches in several
areas from Mohave to Yavapai Counties. A Slight Risk has been
maintained for this outlook due to the aforementioned trends.

A few areas of heavier rainfall may also occur across southeastern
California during the afternoon and evening as the large/cold upper
trough moves into that area. Buoyancy should be a bit weaker
compared to farther east, although recent rainfall and relatively
wet soils suggests at least modest/isolated excessive runoff
potential. Marginal Risk remains in place for this scenario.

Cook

...Ohio Valley...

16Z Update: Little change necessary in the previous forecast as the
prospects for flash flood remain within the lower threshold of a
MRGL risk, but still enough of a signal to warrant a general
continuation. CAMs are still in agreement on some convective
potential later today with the aid of some regional ascent stemming
from the passage of a relatively strong shortwave trough migrating
out of the Midwest through the northern Ohio Valley. Surface low
across the Mid-Mississippi Valley will shift eastward with a warm
front arcing over the Ohio River Valley with modest instability
focused along and south of the front. General convergence pattern
within proxy of the front will be the best area of interest for
some heavier convective cores which aligns well with the higher HREF
prob fields for >1" and >2" potential with modest signals for even
>3" located over northern KY into southern OH. The missing piece in
all this is a greater instability maxima, or any in that regard
with the signal leaning pretty mundane in that department for the
period. This should subdue the overall setup to warrant only
isolated flash flood instances with more of the rain leaning the
beneficial side of the proverbial impact coin. In any case, there's
just enough of a signal for a cell or two to potentially cause
problems over an area where 1 and 3-hr FFG's running between 
1-1.5", matching the prospect rate in the forecast. This led to a
general continuity of the previous MRGL with some trimming on the
western edge to match trends in guidance. 

Kleebauer

..Previous Discussion.. 

Farther east across the Ohio Valley, models are all indicative of
scattered, west-to-east oriented convection that should gradually
propagate southeastward through much of the forecast period.
Substantial differences exists, however, with respect to peak
precip maxima - Nam solutions tend to favor higher precipitation
totals across southern Ohio/northern West Virginia on the nose of
confluent 850mb flow in that area. Other solutions and higher-res
guidance/CAMs point to more backbuilding and training across
Kentucky (and perhaps as far west as the MS/OH River Confluence).
The spread in solutions has necessitated broad expansions of the
inherited Marginal Risk across the region, although the likelihood
of widespread training and or greater-than-isolated flash flood
potential seems a bit on the low side. Modest PW values (around
1-1.25 inch) and instability (especially with eastward extent) also
support the notion of relatively isolated excessive runoff issues.

Cook


Day 2
Valid 12Z Wed Nov 19 2025 - 12Z Thu Nov 20 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTH CENTRAL TEXAS TO A PORTION OF THE ARKANSAS RIVER VALLEY AND
OVER THE SOUTHWEST...

Maintained continuity across both Marginal Risk areas but with
minor adjustments based on latest model runs.

The upper level trough moving across the Southwest US continued to
trend somewhat slower than in earlier model runs...allowing for
more coverage of scattered showers and embedded thunderstorms are
likely to persist into Wednesday across these areas. Overall it
looks like less of a focus than the Day 1 period but nonetheless
still enough of a convective signal to suggest localized flash
flooding could continue to be a concern.

Farther east...low level moisture will start to be drawn northward
with dewpoints in the 60s by Wednesday afternoon across portions of
Texas into eastern Oklahoma and western Arkansas. That should help
feed the development of showers and a few thunderstorms during the
latter half of the period...moreso from late Wednesday night into early
Thursday morning. Model guidance continued to show most-unstable
CAPE values pushing 1000 J per kg and precipitable water values in
excess of 1.5 inches becomes established along and south of a front
from parts of south-central Texas into eastern Oklahoma and
western Arkansas. Based on the GEFS...the precipitable water axis
is roughly between 2 and 2.5 standardized anomalies above
climatology for this time of year. Based on that
combination...suspect locally heavy rainfall rates are possible
leading to localized flash flooding concerns. Deterministic QPF
values remain fairly modest...but with the event spanning into
Thursday with broader areal coverage and an increasing risk of
excessive rainfall...kept the Marginal risk on the chance that the
event begins a little earlier than shown by the current model runs.

Bann


Day 3
Valid 12Z Thu Nov 20 2025 - 12Z Fri Nov 21 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN PLAINS...

...SOUTHERN PLAINS...

The flow aloft backs over portions of Texas and Oklahoma and
becomes increasingly diffluent Thursday morning as an upper trough
makes its way from the southern Great Basin into and across the
southern Rockies. This configuration aloft with increasingly moist
flow in the low levels ahead of a surface cold front will foster
an environment producing excessive rainfall by means of intense
rainfall rates by individual storms and by training of cells/repeat
convection. The axis of highest precipitable water
values...generally in the 1.5 to 1.75 inch range...sets up over
the eastern half of Texas into southeast Oklahoma and western
Arkansas...is roughly 2.5 standardized anomalies above climatology
for this time of year feeding into storms that form during the
afternoon and evening. Localized rainfall rates in excess of an
inch per hour are expected with such a set up that results in
flooding or problems from run off in regions of poor drainage.
Model deterministic QPF is greatest over portions of Oklahoma into
portions of western Arkansas...although there is some potential for
higher rainfall amounts developing over parts of central Texas
based on hints from the models for cell training Thursday night.

...CALIFORNIA...
Maintained a Marginal Risk area introduced by the WPC Medium Range
desk over portions of coastal Southern California as the next area
of low pressure produces another round of on-shore flow of moisture
and associated rainfall Thursday into early Friday. Rainfall
amounts at this point look to be pretty modest but will be falling
on areas where antecedent conditions have been wet for the better
part of 5 days already. That may result in additional localized
flooding on Thursday or Thursday night.

Bann


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt