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CAZ000-040200-

Mesoscale Precipitation Discussion 0004
NWS Weather Prediction Center College Park MD
1251 PM EST Sat Jan 03 2026

Areas affected...Southern CA

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

Valid 031750Z - 040200Z

SUMMARY...Excessive Rainfall rates around 1"/hr will likely lead
to additional flash flooding along the Coastal Range and Topatopa
Mountains today where soils are sensitive due to recent heavy
rainfall. Burn scars in the area are also prone to flash flooding
and debris flows. Localized urban-induced flash flooding may occur
in the suburbs north and west of Los Angeles

DISCUSSION...Doppler Radar shows a narrow band of
moderate-to-heavy rain oriented SW-to-NE over the Santa Ynez and
San Rafael Mountains. A narrow squall line ahead of the cold front
is also analyzed just west of Santa Barbara where a couple rain
gauges southeast of Solvang have reported ~1"/hr rainfall totals.
Aside from the squall, most 1-hour rainfall observations show
>0.5" totals as the band of heavy rain marches east towards Santa
Barbara. Some 3-hr totals have reached 1.75", which also coincides
with 1-hr averaged rainfall rates just over 0.5". This ribbon of
rainfall is embedded just ahead of an approaching cold front with
the warm sector approaching the Santa Barbara Channel. As the warm
front collides with the western Transverse Ranges, a sliver of
100-250 J/kg of MUCAPE will become available within a highly
saturated atmosphere. Latest RAP forecast soundings near Oxnard
show PWATs approaching 1.25" (above the 97.5 climatological
percentile) and a fully saturated warm-cloud layer that is 9,000ft
deep. Latest 12Z HREF probabilistic guidance does show moderate
chances (40-60%) for 6-hour rainfall totals >3" over the Topatopa
Mountains in Ventura County with similar probabilities along the
Santa Ynez just north of Santa Barbara.

As the frontal system moves east, so will the shield of heavy
rainfall as it heads for the L.A. metro area. However, PWATs
approaching 1.25" will retreat over off the coast and 850mb winds
south of L.A. will tend to be weaker than the >30kt winds farther
west. Therefore, rainfall rates should struggle to get much higher
than 0.5"/hr except for the mountain ranges to the north and west
where upslope enhancement will play a big role. In terms of
impacts, the Santa Ynez, San Rafael, and Topatopa are of greatest
concern given their lingering high soil moisture content (>95th
soil moisture percentile in sfc-100 cm layer) and burn scars in
these ranges. Debris flows and rock slides cannot be ruled out,
particularly within burn scars. As for the urbanized communities,
recent heavy rainfall has made soils sensitive and the greater
concentration in hydrophobic surfaces naturally provides some
flash flood concerns. Given the decreasing rates this afternoon,
however, any flash flooding would generally be ponding on roads
that could pose a hydroplane threat for motorists. In summary, the
mountains north and west of L.A. are likely to witness additional
flash flooding today, although localized flash flooding in more
urbanized settings north and west of L.A. cannot be fully ruled
out.

Mullinax

...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=
DZ3fjg!-7HaO-2_1JAsI4qeaTRyryPz2LAxCyYFSoB0xzT6DK0BMumMO9tgiqjxmTRaGq3lfeD3=
Kt9uO69SZ7BkF6l3r30Lzy4$  for graphic product...

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

ATTN...RFC...RSA...NWC...

LAT...LON   35201980 34861950 34741920 34761878 34731841=20
            34471830 34191839 33971867 34151933 34281964=20
            34331992 34402043 34712047 35142022=20

=3D =3D =3D
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