Mesoscale Precipitation Discussion 1040
NWS Weather Prediction Center College Park MD
448 PM EST Thu Dec 06 2018

Areas affected...Coastal Southern California

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

Valid 062147Z - 070347Z

Summary...Showers and scattered thunderstorms will continue to
develop off the CA coast south of LA and shift inland ahead of a
surface low located just south of the Channel Islands this
afternoon. Rain rates up to an inch per hour are expected to
continue which may cause additional flash flooding and debris
flows, particularly in recent burn scar areas.

Discussion...A combination of forcing from a low pressure system
centered just south of San Clemente Island off the southern CA
coast, PW of 0.9 to 1.1 inches per 21z GPS data, MUCAPE of 250-750
J/kg per the RAP, and bulk shear of 25 kt (also from the RAP) will
continue to produce bands of heavy rain. Rainfall of 0.83" in an
hour (and 3" in three hours) ending around 21Z was reported in
Corona Del Mar in 50 dBZ echoes from KSOX. Reflectivity of this
magnitude persists in a band drifting down the Orange County coast
from a low level forcing max ahead of the low and crossing the
occluded front.

The crest of terrain from the Orange/Riverside county border
across San Diego County is strait to the southeast, maintaining
topographical enhancement at the same angle as activity moves in
from the SSW into this evening. Therefore, rain rates of up to one
inch per hour will maintain a flash flood/debris flow threat until
the activity moves through. Of particular note is the ongoing
threat to the Holy Fire burn scar from current activity and later
from the 2017 Lilac fire burn scar inland from Oceanside.

Recent HRRR runs have initialized with this reflectivity and
support heavy rain across the San Diego metro area through at
least 06Z. Should this threat persist, an updated discussion may
be required this evening.

Jackson

ATTN...WFO...SGX...

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

LAT...LON   33691776 33581738 33411699 32921651 32591623
            32481630 32421682 32471723 33471794 33581795