AWUS01 KWNH 181228
FFGMPD
AZZ000-CAZ000-181826-

Mesoscale Precipitation Discussion 1231
NWS Weather Prediction Center College Park MD
727 AM EST Tue Nov 18 2025

Areas affected...portions of western AZ

Concerning...Heavy rainfall...Flash flooding possible

Valid 181226Z - 181826Z

Summary...An area of heavy rain with embedded thunderstorms should
continue across portions of western AZ over the next several
hours. Hourly rain amounts to 1.5" and local 3" totals are
possible, which would continue isolated to widely scattered flash
flood concerns.

Discussion...An upper level low is sliding southeast down the CA
coast past Lompoc. It has led to a broad area of divergence aloft
across the Southwest. Precipitable water values of 1"+ are
indicated in RAP forecasts, with some GPS values across Las Vegas
and Phoenix in the 0.8-0.9" range. When combined with 1000-500
hPa thickness values of 5550 meters, the atmosphere is saturated
which has led to efficient heavy rainfall across portions of
southwest AZ this morning, with radar indications of 1"+ in an
hour at times. MU CAPE of 500-1000 J/kg exists downstream from
the Gulf of CA northward across southwest AZ. Effective bulk
shear of 25 kts is helping to organize the convective elements.

A shortwave noted on water vapor imagery across southern CA could
cause some backing of the low level flow in the short term,
potentially shifting the heavy rain area somewhat westward with
time. RAP guidance fields indicate expansion of the instability
pool northeast with time, which could lead to some northeast
shift. When taken together, the heavy rain footprint could
broaden with time. Hourly rain amounts to 1.5" and local totals
to 3" are possible where cells backbuild and train. Burn scar
locations, arroyos/dry washes, box canyons, and urban areas should
be most sensitive to this rainfall. Isolated to widely scattered
flash flood concerns should continue into the day today.

Roth
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