Mesoscale Precipitation Discussion 0117
NWS Weather Prediction Center College Park MD
453 AM EDT Sat Mar 30 2024

Areas affected...portions of southern California

Concerning...Heavy rainfall

Valid 300851Z - 302051Z

Summary...A landfalling atmospheric river will gradually shift
southeastward across southern California through 21Z.  Areas of
hourly rain rates approaching 1 inch will continue to pose a
flood/flash flood risk during that time.

Discussion...A landfalling atmospheric river continues across
southern California currently.  The greatest integrated water
vapor transport was focused into the Transverse Ranges as of
0830Z, with several areas of 0.25-0.6 inch/hr rain rates observed
and estimated per MRMS from Santa Barbara through Los Angeles
Counties.  Local rain rates approaching 1 inch/hr were also
observed. The atmospheric river was located on the southeastern
periphery of an anomalously deep and slow moving upper low
centered about 150 mi west of San Francisco.

This low and attendant atmospheric river will make slow
southeastward progress today, with heavy rainfall eventually
focusing along the eastern Transverse Ranges, Peninsular Ranges,
and lower-elevation areas near Los Angeles and San Diego.  The
heaviest rainfall (including hourly rates of 0.25-0.9 inches) and
peak IVT will continue to focus along the Transverse Ranges
through the next 4-6 hours, before focusing across the Peninsular
Ranges (and San Diego metro area) between 12-21Z today.  With the
atmospheric river expected to maintain its intensity today,
multiple hours of moderate to heavy rainfall are expected
(especially in terrain-favored areas).  0.25-0.9 inch/hr rain
rates will remain likely where IVT is maximized.  Storm totals of
1.5-2.5 inches (locally higher) are possible through 21Z, which
should continue to foster at least an isolated flood/flash flood
risk especially in low-lying, sensitive areas.

Cook

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

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

LAT...LON   35261923 34991773 34621690 33891616 33121613
            32611627 32521701 32801740 33481812 34161938
            34482032 35052000