Mesoscale Precipitation Discussion 0044
NWS Weather Prediction Center College Park MD
1021 AM EST Thu Feb 01 2024

Areas affected...Southern California

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

Valid 011521Z - 012102Z

Summary...Steady heavy rainfall continues to translate
southeastward across Southern California ahead of an approaching
cold front. Rainfall rates upwards of .50-.75"/hr within this band
could lead to some flash flooding through early afternoon,
particularly atop vulnerable urban zones and burn scars.

Discussion...Regional radar across Southern California this
morning depicts a narrow band of heavy stratiform rainfall
streaming inland as a plume of atmospheric river moisture migrates
southeastward. Over the last two hours, this activity lead to
observed rainfall rates between .50-.90"/hr and elevated FLASH
CREST responses focused in favorable upslope areas of the San
Gabriel and Sierra Madre mountains. More recent reports in the
western L.A metro highlight hourly rates upwards of .50-.73"/hr
which corroborate hourly radar estimates from KSOX and KVTX. 

Over the next several hours, this axis of heavy rainfall will
maintain intensity with .50-.75"/hr rates as it steadily shifts to
the southeast supported by 500-600 kg/m/s IVT, strong low-level
moisture convergence along the front, and very favorable synoptic
ascent (strong left exit forcing and diffluent flow) tied to a
digging 170 kt jet-streak.

Accordingly, the 12z HREF maintains a high 60-70% probability of
hourly rainfall rates exceeding .50"/hr through this morning, with
focused 40-50% probabilities .75"/hr noted along the coastline
where favorable upslope enhancement occurs through 21z. While the
progressive nature of this band precludes a more robust flash
flood threat, the enhanced rates could lead to a few instances of
flash flooding, as 1 hour FFGs hover around .50-1"/hr across
Southern California. Vulnerable areas including urban zones and
burn scars will exacerbate the risk of scattered flash flooding.

Asherman

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

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

LAT...LON   34321825 34131769 33931668 32971648 32551686
            32721764 33271791 33601855 34111868