Flooding and Heavy Snow in California, March 14 to 15, 2024

Area Forecast Discussion
National Weather Service San Diego CA
925 PM PDT Wed Mar 13 2024

.SYNOPSIS...
Strong and locally damaging Santa Ana winds will develop late
tonight and peak Thursday morning, with the strongest winds
occurring across a corridor from the San Bernardino County
Mountains through coastal Orange County. Winds will begin to
diminish Thursday night, but periods of breezy offshore winds will
continue into Friday. Showers and isolated thunderstorms will be
possible across the mountains each afternoon Thursday through
Sunday, with the highest chances developing on Friday, when cells
could drift into the valleys and deserts. Warmer and drier by
early next week.

&&

.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...

.Evening update...
Areas of low clouds are redeveloping over the San Diego County
Valleys and into the foothills this evening as a roughly 4000 ft
deep marine layer remains in place. Otherwise high clouds were
drifting south across the region ahead of an upper low that is
currently moving south through eastern Nevada. Surface pressure
gradients are trending stronger offshore to the Great Basin,
currently around -5.7 mb SAN-TPH, but are still weakly onshore to
the local deserts.

As the aforementioned low drops further south tonight and a vort
max moves into Southern CA, offshore pressure gradients and winds
aloft will increase, generating strong and potentially damaging
Santa Ana Winds over portions of the region. The strongest north
to northeast winds are expected near the foothills of the San
Bernardino Mountains and locally into the Inland Empire where
mountain waves surface, below the Cajon Pass, and into the Santa
Ana Mountains. The HREF has a 10-30% chance of local gusts in
excess of 70 mph in these areas. Isolated gusts may be as high as
90 mph along the most wind prone ridges and mountain slopes. For
the Riverside and San Diego County Mountains and adjacent
foothills, given the large northerly component of the winds,
gusts will be much weaker, generally around 25-45 mph. Winds peak
Thursday morning before slowly weakening through the day.

The other interesting tidbit for Thursday is the chance of showers
over the mountains. Areas further north where Santa Ana winds are
occurring will probably be too dry, but there may be just enough
moisture ahead of the short wave in San Diego County during the
afternoon for at least a slight chance of showers. Given the cold
air advection moving into the deserts and mountains, snow levels
will actually be quite low - possibly below 4000 ft in San Diego
County. If any showers develop, there is the risk of snowfall
along portions of I-8 from Pine Valley eastward. For now any
precipitation looks light, but hazardous could still be hazardous
through this area even without much or any accumulating snow.

Previous discussion...
As the low pressure system parks over the Southwest, mid-level
moisture will wrap around the periphery, leading to chances of
showers and isolated thunderstorms each afternoon Friday through
Sunday. The highest chances will occur over the mountains, but
cells may be able to drift into the eastern valleys and deserts.
The 12z ECMWF deterministic run shows the deepest column of mid-
level moisture on Friday, which will result in the greatest
chances for precipitation. Widespread accumulations are not
expected at this time, but weak steering flow aloft will lead to
slow storm movement, so locally heavy pockets of rain will be
possible. Lesser chances will exist by Saturday and Sunday as
upper-level winds and moisture profiles become less supportive of
convective development.

The low looks to slowly weaken and meander east starting on Sunday
before being overtaken by a building ridge across the West Coast.
Chances for measurable precipitation will fall below 10 percent
for Monday and Tuesday. High temperatures will warm for Sunday
through Tuesday with Tuesday high temperatures around 5 degrees
above average.

&&

.AVIATION...
140430Z...Coast/Valleys...SCT-BKN clouds will continue in San Diego
County through 12Z Thu with bases 1700-2500 ft MSL and tops to 5000
ft MSL, with local terrain obscurations. Most vis will remain
unrestricted through Thu evening. Areas of NE winds 25-45 kt with
local gusts to 70 kt will occur 09Z-23Z Thu, especially below Cajon
Pass and other locations in the Inland Empire and Orange County
below canyons and in the foothills, with STG UDDFS. Most gusts will
remain below 30 kt in San Diego County.

Mountains/Deserts...Local terrain obscurations will occur through
11Z Thu mainly on the west slopes of the San Diego County mountains
due to lingering low clouds. Otherwise, mostly clear skies with
unrestricted VIS will prevail through Thu evening. Areas of N-NE
winds 25-45 kt with local gusts to 70 kt will occur 08Z-23Z Thu over
the mountains with strongest winds on the south slopes of the San
Gabriel/San Bernardino Mountains, with STG UDDFS. Local wind gusts
25-45 kt will occur 08Z-20Z Thu in the deserts as well.

&&

.MARINE...Northeast winds 15 to 25 kt with gusts to 30 kt will occur
over the coastal waters Thursday, mainly between around 6 AM and 3
PM when the Small Craft Advisory is in effect. The strongest winds
will be from San Mateo Point to San Clemente Island, resulting in
hazardous boating conditions. Some choppy seas will occur, though
mostly below 6 feet. Winds will switch to the northwest and weaken
Thu mid afternoon.

&&

.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...High Wind Warning from midnight tonight to 6 PM PDT Thursday for
     Orange County Coastal Areas-Orange County Inland Areas-San
     Bernardino County Mountains-San Bernardino and Riverside
     County Valleys-The Inland Empire-Santa Ana Mountains and
     Foothills.

     Wind Advisory from 2 AM to 2 PM PDT Thursday for Coachella
     Valley-Riverside County Mountains-San Gorgonio Pass Near
     Banning.

     Wind Advisory from midnight tonight to 6 PM PDT Thursday for
     Apple and Lucerne Valleys.

PZ...Small Craft Advisory from 6 AM to 3 PM PDT Thursday for Coastal
     Waters from San Mateo Point to the Mexican Border and out
     to 30 nm-Waters from San Mateo Point to the Mexican Border
     Extending 30 to 60 nm out including San Clemente Island.

&&

PUBLIC...SS/EA
AVIATION/MARINE...Maxwell