Subj : MESO: SW Heavy Rain/Flood
To   : All
From : Mike Powell
Date : Sat Aug 19 2023 07:33:00

AWUS01 KWNH 191136
FFGMPD
AZZ000-NVZ000-CAZ000-191735-

Mesoscale Precipitation Discussion 0946
NWS Weather Prediction Center College Park MD
736 AM EDT Sat Aug 19 2023

Areas affected...southern/southeastern California, southwestern
Arizona, far southern Nevada

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

Valid 191135Z - 191735Z

Summary...Convection is gradually deepening across the discussion
area this morning, and these trends should continue through at
least 17Z.  1+ inch/hr rain rates are likely to cause flash
flooding in a few spots.

Discussion...Recent radar mosaic imagery depicted an uptick in
coverage of deep convection in areas along and south of I-10 and
east of the Peninsular Ranges this morning.  The storms are in a
weakly forced environment with ample instability and moisture
(1000+ J/kg MLCAPE and 1.5-2 inch PW values) to support deep
convection along with efficient rainfall processes.  Additionally,
forcing for ascent was relatively weak, with deepening convection
tied to 1) weak upper-level speed divergence with a southerly
60-90kt speed max noted at 300mb just west of the discussion area
and 2) weak low-level upslope with easterly flow against the
eastern Transverse and Peninsular Ranges.  Despite fast upper
flow, weaker low- to mid-level flow was allowing for modest storm
motions (5-15 knots), which should eventually enable 1+ inch/hr
rain rates to materialize as storms deepen/mature.

Ongoing convective trends should continue through the morning as
heating/instability develop northward toward southern Nevada and
more of western Arizona.  Weak low-level advection will also
likely aid in increasing moisture into those areas as well. 
Occasional areas of 1+ inch/hr rain rates (and local amounts
exceeding 2 inches) are likely to cause flash flooding -
especially where that rainfall can occur across urban, low-lying,
and/or flood-prone areas.  Convective coverage should continue to
expand both northward and eastward with time.

Cook

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

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

LAT...LON   35741491 35331367 33811289 32731282 31921307 
            32181445 32091587 32571643 33391673 34441698 
            35321631 

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