Mesoscale Precipitation Discussion 0718
NWS Weather Prediction Center College Park MD
241 PM EDT Wed Jul 24 2024

Areas affected...Southern California

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

Valid 241840Z - 250040Z

SUMMARY...Showers and thunderstorms will be initiating and
expanding in coverage over the higher terrain of southern
California including portions of the San Bernadino and San Gabriel
mountains and adjacent areas of the Peninsular Range. There will
be concerns today for slow-moving and locally terrain-anchored
convection that may produce heavy rainfall amounts and some flash
flooding.

DISCUSSION...GOES-W Day Cloud Phase RGB satellite imagery is
showing the early stages of convective initiation taking place
over the higher terrain of southern California and convection is
expected to develop and locally expand in coverage over the next
several hours as strong diurnal heating/boundary layer instability
works in tandem with orographically enhanced differential heating
boundaries and related mesoscale circulations.

SBCAPE values have risen already to locally 1500 to 2500 J/kg,
with areas east of the Peninsular Range including the interior
deserts of southern CA seeing the greatest magnitudes currently.
Much like areas farther north in the Sierra Nevada, the airmass is
quite moist with CIRA-ALPW data showing a rather deep column of
elevated moisture content, and the PWs are locally reaching 2+
standard deviations above normal.

A combination of this moisture and instability near and adjacent
to the terrain along with a favorable set-up for small-scale
terrain-induced circulations should yield slow-moving pockets of
showers and thunderstorms capable of producing heavy rainfall
rates that may reach 1 to 2 inches/hour. The latest hires CAMs are
very likely underdone with the rainfall potential, but some spotty
2 to 3 inch rainfall totals are generally expected where these
slow-moving and locally focused pockets of convection set up.

Isolated to widely scattered instances of flash flooding will be
possible over the higher terrain of southern CA including the San
Gabriel and San Bernadino mountains and the adjacent areas of the
Peninsular Range. Some of the interior deserts where stronger
instability is noted may also see some of this convection occur.
Any burn scar locations in particular will be most susceptible to
enhanced runoff concerns and flash flooding.

Orrison

ATTN...WFO...HNX...LOX...PSR...SGX...VEF...

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

LAT...LON   35441712 35011640 33301548 32691556 32491608
            32631658 33311703 33911783 34331886 34741928
            34951908 34941810 35321766