Mesoscale Precipitation Discussion 0070
NWS Weather Prediction Center College Park MD
410 AM EST Tue Feb 20 2024

Areas affected...Portions of the Transverse and Peninsular Ranges
including the Los Angeles Basin

Concerning...Heavy rainfall...Flash flooding likely

Valid 200910Z - 202110Z

SUMMARY...The tail-end of an atmospheric river continues to impact
areas of southern CA, and some uptick in rainfall concentration
and intensity is expected going through the morning and midday
hours. Given the extremely sensitive conditions on the ground
locally, any additional heavy rain is likely to foster more runoff
problems and flash flooding.

DISCUSSION...The latest GOES-W WV suite shows a well-defined
subtropical moisture fetch associated with the tail-end of an
atmospheric river impacting southern CA along with proximity of a
stalled out frontal zone.

Shortwave energy offshore of southern CA situated near and
adjacent to 30N 130W will be advancing off to the east over the
next 12 hours and this will be interacting with the deeper layer
moisture axis and nearby front for what should be a gradual
increase in shower activity offshore of the Transverse and
Peninsular Ranges.

This activity heading through the morning hours will overspread
the coastal terrain, including the Los Angeles Basin, and with an
uptick in forcing and stronger IVT magnitudes, the rainfall rates
should also increase. IVT magnitudes by 12Z are forecast to rise
to as much as 500 kg/m/s across portions of Los Angeles and Orange
County, and adjacent areas to the south into San Diego County will
also see increasing moisture transport that will facilitate a
threat of heavier shower activity.

A combination of a generally orthogonal orientation of the IVT
axis relative to the terrain along with the subtropical origins of
the moisture transport should favor rather efficiently high
rainfall rates that should increase to occasionally over a
0.50"/hour.

The 00Z/06Z HREF guidance suggests additional rainfall totals of
as much as 2 to 4 inches with isolated heavier amounts possible
for the southwest-facing slopes of the San Gabriel and San
Bernadino mountains by midday on Tuesday. Areas farther back up
the coast involving eastern portions of the Santa Ynez mountains
which have already been hard hit by this most recent atmospheric
river event will also see locally heavy rains of 1 to 3 inches.

Given the extreme sensitivities locally on the ground, these
additional rains will favor more runoff problems and a likelihood
of additional flash flooding which will include more debris flow
and landslide activity.

Orrison

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

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

LAT...LON   34811972 34791864 34541791 34431717 34111669
            33681664 32941668 32761704 32971732 33371768
            33591797 33871842 33931879 34111936 34341998
            34652003