Graphic for MPD #0046

Mesoscale Precipitation Discussion 0046
NWS Weather Prediction Center College Park MD
1030 AM EST Sun Feb 04 2024

Areas affected...Central to Southern CA

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

Valid 041530Z - 050000Z

SUMMARY...A strong atmospheric river continues to overspread
central CA, with heavy rainfall expected to continue and gradually
shift south down into southern CA later today. Runoff problems and
areas of flash flooding will become a concern over the next
several hours as rainfall rates continue to increase.

DISCUSSION...The latest GOES-W IR satellite imagery shows a strong
low center offshore of central CA with an area of deep Pacific
moisture and associated rainfall seen continuing to surge off to
the northeast and inland across the central CA coastal ranges. The
low center continues to deepen and the IVT magnitudes offshore
continue to increase around the eastern flank of the low center as
very strong low to mid-level southerly flow foster IVT values
approaching 1000 kg/m/s.

Some of these higher IVT magnitudes will be impacting the Bay Area
and especially areas farther south down the coast including the
Santa Cruz and Santa Lucia mountains. Already there are offshore
rainfall rates on the order of 0.50" to 0.75"/hour as evidenced by
recent offshore microwave passes, and the CMORPH2 data shows these
heavier rates encroaching on the central CA coastal ranges.
Strengthening upslope flow into the windward slopes of the higher
terrain will likely enhance these rates further over the next 3 to
6 hours, with some potential for rates to reach 1"/hour
occasionally.

Strong warm air advection and related IVT will be also be surging
inland into the Sacramento Valley and northern portions of the San
Joaquin Valley and this will allow for heavier rainfall rates that
may occasionally reach a 0.25"/hour to spread across these areas
over the next several hours.

Farther down the coast, there will be growing concerns for the
Transverse Range of southern CA where the moist deeper layer flow
and axis of stronger IVT will become increasingly aligned
orthogonal to the terrain. This will foster increasingly heavy
rainfall rates with a combination of deeper layer
forcing/divergent flow aloft ahead of the offshore low
center/trough, but also enhanced moisture convergence and
orographic ascent.

Rainfall rates by early to mid-afternoon across the Transverse
Range are forecast to increase easily into the 0.50" to 1"/hour
range, with a risk for some convective elements that may yield
rainfall rates as high as 1" to 1.25"/hour.

Going through 00Z (4PM PST), rainfall totals of 3 to 6 inches are
expected for the coastal ranges of central to southern CA, and
extending as far south as Santa Barbara and Ventura Counties. This
will drive increasing runoff concerns which will include urban and
small stream flooding, debris flow/landslide activity, and a
threat of flash flooding for any more sensitive burn scar
locations.

Additionally heavy to significant rainfall, and high-impact flash
flooding will be a concern especially after 00Z (4PM PST) for the
Transverse Range and areas closer into the Los Angeles Basin, and
additional MPDs will be issued accordingly later today to address
this.

Orrison

ATTN...WFO...EKA...HNX...LOX...MTR...STO...

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

LAT...LON   40392248 39852184 38492151 37772123 36942070
            36302010 35411972 34931854 34101849 34422057
            35532140 36372217 37622280 38682351 39622341
            40242310
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Last Updated: 1032 AM EST Sun Feb 04 2024