Mesoscale Precipitation Discussion 1029
NWS Weather Prediction Center College Park MD
213 PM EST Thu Nov 29 2018

Areas affected...SRN CA

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

Valid 291911Z - 300011Z

Summary...A cold front moving through San Diego and vigorous cold
core low overspreading the remainder of California will maintain
unsettled weather with locally heavy downpours through afternoon.
Flash flooding is possible.

Discussion...Near 19Z, GOES water vapor imagery (both east and
west satellites) and even WSR-88D mosaics showed an upper low that
was impressively wrapped up in the Sacramento Valley, with broad
500-mb height anomalies of 2.0 to 2.5 standard deviations below
climatology. The trailing surface cold front had produced a
strongly-forced narrow precipitation band over the Los Angeles
basin and adjacent mountains to the north and east between 16-19Z,
resulting in a couple of mesonet observations of 1.0 to 1.5 or
greater inches of rain in a short time. The synoptic moist
conveyor associated with the parent weather system will continue
to press through southern California, with onshore flow increasing
during and following frontal passage. Locally heavy rainfall -
enhanced by onshore flow - is possible through about 22Z, judging
largely by the 12Z WRF-ARW forecast which was handling most
aspects of this event fairly well. Additional rainfall of 0.50 to
1.00 inches is expected near San Diego and adjacent mountains,
with potential for flash flooding being more pronounced near burn
scars.

Farther up the coast, the broad cold core aloft and ample surface
heating will maintain a steep lapse rate environment and a showery
/ convective precipitation regime all along the coast from
Monterey to Los Angeles through late afternoon. Strong deep layer
flow will keep individual convective cells moving at 30 to 40
knots. This will limit residence time over any given location, but
with precipitable water remaining above normal at around 0.75
inches and MUCAPE analyzed greater than 500 J/kg most locations -
instantaneous heavy rain rates may lead to flooding or flash
flooding. Total new rainfall amounts will be further enhanced by
terrain influence / onshore flow from roughly Monterey down to
near Santa Barbara, with less onshore enhancement between Santa
Barbara and Los Angeles. The WRF-ARW and recent HRRR runs produce
local 1-inch-plus amounts in the discussion area through 00Z.

Burke

ATTN...WFO...HNX...LOX...MTR...PSR...SGX...STO...

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

LAT...LON   37412156 35732014 34911929 34201770 34011697
            33671671 33021619 32421602 32221725 33281814
            34362091 36322216 37212221