Mesoscale Precipitation Discussion 0121
NWS Weather Prediction Center College Park MD
546 PM EDT Sun Mar 31 2024

Areas affected...western Transverse Ranges of southern California

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

Valid 312143Z - 010330Z

Summary...A few slow moving showers/thunderstorms will be capable
of producing high rainfall rates of 0.5+ in/hr across western
portions of the Transverse Ranges through ~03Z. Localized flash
flooding will be possible with potentially higher impacts over
areas which received heavy rain over the past 48 hours and/or
sensitive burn scars.

Discussion...GOES West visible imagery at 2130Z showed the
southern and low level portion of a slow moving, vertically
stacked low about 150 miles southwest of Los Angeles. This low was
slowly moving east and in the process of opening up but weak
steering flow was located beneath and northeast of the low center.
Visible imagery and local radar imagery from KVBX showed numerous
showers over the southern Coastal Ranges into the western
Transverse Ranges where the 0-6 km AGL mean wind was from the
north at 5-15 kt. Daytime heating has allowed for the development
of weak MLCAPE, less than 500 J/kg per 21Z SPC mesoanalysis data,
which was supporting ongoing shower activity across the region.

An increased risk for flash flooding this afternoon/evening will
come from the fact that steering flow is fairly similar in
direction and magnitude to low level inflow from the north, which
could promote some slow moving and/or training cores of heavy
rain. The weak instability and modest moisture (PWATs 0.5 to 0.8
inches) should limit updraft strength but isolated thunder could
still occur over the next few hours. The slow movement of heavy
rain cores could allow for rainfall rates to exceed 0.5 inches in
an hour or less. Due to heavy rain which has impacted Santa
Barbara and Ventura counties over the past 2 days, with over 5
inches reported in portions of the Santa Ynez and San Rafael
Mountains, sensitivity to flooding has increased in addition to
already sensitive burn scar regions. Localized rainfall totals
through 03Z of 1-2 inches will be possible, but with much of the
rainfall falling over a relatively short period of time. A
reduction of any lingering instability after sunset should reduce
the threat for flash flooding but conditions will need to be
monitored for a lingering threat beyond 03Z.

Otto

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

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

LAT...LON   35362017 34871943 34731904 34451880 34161879
            33981905 34131948 34221991 34332048 34592073
            35022086 35312060