Mesoscale Precipitation Discussion 1232
NWS Weather Prediction Center College Park MD
203 PM EST Tue Nov 18 2025

Areas affected...Western Arizona, Southeast California and
Southern Nevada

Concerning...Heavy rainfall...Flash flooding likely

Valid 181903Z - 190103Z

SUMMARY...Flash flood risk increasing this afternoon into the
evening across the Colorado River basin between southeast
California, southern Nevada, and western Arizona. Heavy rainfall
potential existing through sunset leading to potential night time
flash flooding concerns.

DISCUSSION...Current radar/IR satellite composite indicates a
brief lull initiating over portions of southwest AZ and
southeastern CA. Meanwhile, a steady progression of a stronger
convective pulse over far northeast Baja will make headway to the
north as it migrates within the broad deep layer steering flow out
of the south-southwest. This is due to the proximity of the 
persistent ULL presence off the southern CA coast with smaller
shortwave perturbations emanating around the southern and eastern
periphery of the upper-level circulation. Hi-res guidance,
especially the last several HRRR iterations have been manifesting
this very scenario of a brief lull, followed by the advection of
the next stronger mid-level perturbation exiting out of Baja and
shifting north along the adjacent Colorado River basin between
AZ/CA/NV. This will coincide within a field of relatively buoyant
air situated within the river basin, arcing back into southern NV
along the warm/cold conveyor belt transition along the northern
side of the surface low  analyzed over the southern tip of NV.

SBCAPE between 1000-1500 J/kg is analyzed across much of southwest
AZ with an extension of 500-1000 J/kg located along the northern
extent of the Colorado River basin between the three-state
intersection. This setup is poised to advect further north with a
greater SBCAPE alignment focused over the southern tip of NV down
through far-southeast CA and much of western AZ. The combination
of suitable vorticity advection and modest instability over an
area of +2 to +3 sigma PWATs will assist in a re-invigoration of
area convection with cells capable of 0.5-1.25"/hr at peak
intensity, enough to favor some  scattered flash flood prospects
within the confines of northern Yuma, La Paz, Mojave counties in
AZ, southern Clark county in NV, and far-eastern San Bernardino
county in CA. Some training could occur with cells materializing
on a north-south alignment for areas east of the Colorado River
given the deep layer flow remaining fairly uni-directional. This
could exacerbate flash flood concerns locally, leading to the
threat running closer to the flash flooding likely prospect,
especially within those western AZ counties.

Kleebauer

ATTN...WFO...FGZ...PSR...VEF...

ATTN...RFC...RSA...STR...NWC...

LAT...LON   35831459 35801375 35291312 34391280 33221314
            33161409 33711453 34251494 34881526 35531530