AWUS01 KWNH 011742
FFGMPD
AZZ000-NVZ000-CAZ000-012330-

Mesoscale Precipitation Discussion 1018
NWS Weather Prediction Center College Park MD
141 PM EDT Fri Sep 01 2023

Areas affected...Lower Colorado River Valley...

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

Valid 011745Z - 012330Z

SUMMARY...
Thunderstorms developing along the Gulf of California are growing
upscale into southwest Arizona. Additional showers and
thunderstorms will develop over southeast California, with cell
mergers likely into this afternoon, resulting in localized flash
flooding.

DISCUSSION...
A deep longwave trough is rapidly digging southward down the
California coast. GOES-W WV suite denotes a compact mesoscale
shortwave trough over the northern Gulf of California lifting
northward causing a cluster of strong thunderstorms to develop
from the Gulf of California northward into extreme southern
Arizona. These two features are likely to merge near/over the
Lower Colorado River Valley, causing strong thunderstorms to
develop with daytime heating. The storms will be capable of heavy
rainfall, which is likely to result in flash flooding over the
desert. PWATs of 1.5 to 2 inches being advected north out of the
Gulf of California are contributing to PWAT anomalies around 2.5
sigma above normal in this area. Solid insolation this morning has
resulted in increasingly unstable environment with MLCAPEs of
1500-2000 J/kg available for stronger updrafts and moisture
loading into the downdrafts. This should support widespread shower
and thunderstorm development across this region, especially as the
two features merge.

Convection may initially develop along the mountains associated
with the trough, but are likely to propagate northeastward
following the moisture influx and instability which will rapidly
increase through the late morning with daytime heating. Resultant
flash flooding is most likely in any urban areas impacted, such as
Lake Havasu and Yuma as well as across impervious ground surfaces,
allowing increased runoff and likely scattered incidents of flash
flooding, given suggestion of 1-1.25" totals in 15-30 minutes (as
suggested by the HRRR).

Wegman

ATTN...WFO...FGZ...PSR...SGX...TWC...VEF...

ATTN...RFC...CBRFC...CNRFC...NWC...

LAT...LON 35331547 35261418 34291306 33001208 32221154
31591171 32401458 32591603 34051647

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