Graphic for MPD #0064

Mesoscale Precipitation Discussion 0064
NWS Weather Prediction Center College Park MD
1215 AM EST Mon Feb 19 2024

Areas affected...Central and Southern California

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

Valid 190515Z - 191600Z

SUMMARY...Heavy rainfall arriving across the coastal ranges of
central to southern CA, including portions of the Transverse
Range, will couple with wet antecedent conditions to result in
areas of flooding and flash flooding.

DISCUSSION...An atmospheric river of Pacific moisture focused
around the southeast flank of an upper-level trough offshore of
the West Coast will be focusing a heavy to excessive rainfall
threat overnight and into Monday morning across the coastal ranges
of central to southern CA.

The latest CIRA-ALPW data products in conjunction with GOES-W WV
satellite imagery show a relatively concentrated axis of mid-level
moisture already streaming inland across the terrain with the axis
of deeper layer moisture transport extending well southwest and
offshore of southern CA. IVT magnitudes nosing toward the Santa
Lucia Range and extending south down toward the Santa Ynez Range
are already approaching 500 kg/m/s, and localized rainfall rates
across these areas have been approaching a 0.50"/hour over the
last couple of hours.

The latest GOES-W IR satellite imagery shows some cloud top
cooling noted offshore of Monterey, San Luis Obispo, and Santa
Barbara County with a more focused batch of heavier showers
quickly approaching and this was confirmed by a 0253Z GPM/GMI
overpass which showed multiple bands of broken convection and
heavier rainfall rates. This activity is being aided by a
combination of some offshore instability and also the nose of an
increasing southwest low-level jet which is yielding an axis of
stronger low-level speed convergence.

A cold front offshore of the West Coast will be gradually settling
southeast overnight and will be crossing over the coastal ranges.
However, the front will also be slowing down as it becomes more
aligned with the deeper layer steering flow. This coupled with
strengthening IVT magnitudes approaching 750 kg/m/s in the 06Z to
09Z time frame in association with the low-level jet should focus
enhanced rainfall rates into the orographically favored terrain
along the coast. Given the orthogonal orientation of the deeper
layer IVT axis, the rates should be maximized and occasionally
capable of reaching 0.75" to 1.0"/hour, and especially with any
more organized convective elements that advance inland.

Rainfall amounts overnight and through Monday morning are expected
to be on the order of 3 to 6 inches, with isolated heavier amounts
possible. The heaviest rains should gradually focus over the
Transverse Range in particular, with Santa Barbara County and
eventually Ventura County likely to see enhanced rainfall rates
and corresponding totals with this event.

Given the sensitive/wet antecedent conditions on the ground
already, these rains with this latest atmospheric river event are
likely to cause areas of flooding, including a threat for flash
flooding. Debris flow and landslide activity will also be a
notable concern at least locally as these heavier rains arrive.

Orrison

ATTN...WFO...HNX...LOX...MTR...

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

LAT...LON   36942093 36072004 35261962 34751858 34381845
            34011895 34201973 34282032 34552069 35322112
            35582139 36172184 36632192 36822169
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Last Updated: 1218 AM EST Mon Feb 19 2024