Mesoscale Precipitation Discussion 0122
NWS Weather Prediction Center College Park MD
725 PM EDT Sun Mar 31 2024

Areas affected...southern CA

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

Valid 312323Z - 010520Z

Summary...A localized flash flood threat will continue across
southern CA through 05Z, from near Los Angeles/San Bernardino into
the San Diego metro areas, with potential for 0.5 to 1 inch of
rain in an hour or less.

Discussion...GOES West visible imagery at 23Z showed a persistent
axis of surface convergence that was located west-east through the
Channel Islands, just north of San Clemente Island before turning
southward and parallel to the coast, roughly 10-20 miles west of
the San Diego County coastline. Recent outflow has been noted to
be pushing across San Clemente Island on visible imagery but some
degree of convergence remained to its north. This feature has been
an active focus for showers and thunderstorms for the past several
hours, located just northeast of a surface low, located roughly
150 miles west of the northern Baja Peninsula via visible
satellite imagery. This area of southern CA was beneath the center
of a mid to upper-level low/trough with weak storm motions, ~5 kt,
using 0-6 km AGL mean layer winds as a proxy for cell motions.

As the southern portion of the parent upper trough slowly edges
southeastward over the next 6 hours, low level onshore flow from
the south to southwest will continue to favor the likelihood of
cells containing heavy rain between the coast and the Peninsular
Ranges over the next few hours. It is unclear the future location
or existence of the surface convergence axis just offshore, but
given its proximity to the coast, it seems reasonable that showers
and embedded thunderstorms will continue to impact coastal
locations of Orange into northern San Diego County in the short
term, focusing near an estimated wave along the boundary west of
NFG within an axis of instability containing a few hundred J/kg of
MLCAPE. Forecasts of MLCAPE from the RAP support values up to ~500
J/kg lingering just offshore of San Diego County through 06Z with
low level onshore flow (weak as it may be) maintaining a fetch
from the instability reservoir onto land. Slow moving to nearly
stalled showers and thunderstorms will have the potential to
produce high rainfall rates of 0.5 to 1 inch in an hour or less,
supporting a lingering but localized flash flood threat into the
night for southern CA. The focus is expected to be in Orange and
San Diego counties, but cannot rule out some activity farther
north into Los Angeles County as well.

Otto

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

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

LAT...LON   34371803 34111742 33731690 33291659 32681644
            32411645 32261692 32391758 33161814 33661886
            34271864