Mesoscale Precipitation Discussion 1031
NWS Weather Prediction Center College Park MD
735 PM EST Thu Nov 29 2018

Areas affected...Southern California

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

Valid 300035Z - 300400Z

Summary...Moisture and instability ahead of a cold long wave
trough will support low topped convection with hourly rainfall
rates near one-half inch. These rates could pose a flash flood
threat, especially over fresh burn scar areas.

Discussion...Regional radars across southern CA showed clusters of
convection ahead of a long wave trough extending down from
northern CA. The convection is forming in an area of RAP analyzed
250-500 J/KG of MLCAPE (the 30/00z KNKX showed 350 J/KG of SBCAPE,
with precipitable water values near 0.75 inches). The most recent
MRMS hourly rainfall produce showed rates generally between 0.25
and 0.50 inches, which is consistent with the precipitable water
values.

The convection is moving eastward ahead of the long wave trough,
and occasionally clusters of low topped storms train. Training
could result in hourly rainfall rates reaching near 0.50 inches,
but much of the high resolution guidance (such as the most recent
HRRR and the 12z WRF ARW2) suggest hourly rainfall rates closer to
0.25 inches.

As the long wave trough moves eastward, warming mid level
temperatures and downward motion behind the trough should tend to
allow the convection to diminish, especially after 30/04z.
However, RAP and HRRR soundings showed the instability remaining
in place through at least 30/04z.

Based on the above, it appear as though the best threat for
convection is this evening, with a gradual weakening trend toward
30/04z. The greatest threat is where cells train over new burn
scars, with the risk of flash flooding outside of these areas
deemed fairly low.

Hayes

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

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

LAT...LON   35562045 35311775 34621650 33791617 32711667
            32851725 33321752 34341986 34432055 35112096