Graphic for MPD #0047

Mesoscale Precipitation Discussion 0047
NWS Weather Prediction Center College Park MD
400 PM EST Sun Feb 04 2024

Areas affected...Transverse Range and Los Angeles Basin

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

Valid 042100Z - 050900Z

SUMMARY...High-impact rainfall with dangerous and life-threatening
flash flooding is likely across portions of the Transverse Range
and the Los Angeles Basin going through tonight as strong
atmospheric river activity arrives.

DISCUSSION...The latest GOES-W Airmass RGB satellite imagery in
conjunction with dual-pol radar and passive microwave data shows
strong atmospheric river activity offshore of southern CA
advancing inland with locally very heavy rainfall rates. Enhanced
IVT magnitudes continue to gradually slide south down across the
Transverse Range and will be heading into the Los Angeles Basin
this evening as a cold front gradually approaches the region.

Low pressure currently offshore of central CA will be lifting off
to the north through this evening and the overnight hours which
will allow for this trailing cold front and corridor of stronger
IVT values along it to tend to slow its advance down the coast
with time. This will facilitate a degree of atmospheric river
persistence for southern CA that will likely drive high-impact
rainfall and concerns for significant flash flooding heading into
the overnight hours.

IVT magnitudes aiming into the Transverse Range and the Los
Angeles Basin should tend to peak in the 500 to 800 kg/m/s range
over the next 6 to 12 hours which will be driven by strong low to
mid-level southwest flow with 850/700 mb moisture flux anomalies
of +3 to +5 standard deviations above normal. This enhanced
Pacific fetch of moisture will be oriented orthogonal to the
terrain which coupled with large scale forcing/divergent flow
aloft and strong frontal convergence should yield enhanced
rainfall rates.

The 12Z/18Z HREF guidance and recent runs of the HRRR favor very
heavy bands of showers with some stronger convective elements
producing rainfall rates as high as 1" to 1.5"/hour, and the
localized persistence/training of some of these rates should yield
highly excessive rainfall totals. Additional rainfall totals going
through 09Z (4AM PST) may reach 4 to 8 inches, which will bring
some storm totals by later in the night as high as 10+ inches.

The heaviest rains should be over the windward slopes of Santa
Barbara, Ventura, and Los Angeles Counties. However, even areas in
the lower elevations including the Los Angeles Basin are expected
to see high-impact rainfall that may reach 2 to 4 inches by late
tonight.

These rains are expected to result in numerous to widespread
instances of dangerous and life-threatening flash flooding, and
especially with lingering sensitivities in the wake of the most
recent atmospheric river event. Impacts will likely include
significant urban and small stream flooding, debris flows, and
landslides.

Orrison

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

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

LAT...LON   35491919 35201861 34981810 34781724 34481670
            33871658 33641671 33511697 33331749 33501800
            33711861 34031923 34181959 34221996 34452051
            34652070 35132064 35362018
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Last Updated: 400 PM EST Sun Feb 04 2024