High Surf Advisory 4, NWS San Diego, CA

Area Forecast Discussion
National Weather Service San Diego CA
936 AM PST Mon Feb 20 2023

.SYNOPSIS...
Dry conditions and relatively warm temperatures persist through
today. Cooling spreads inland on Tuesday with strong onshore winds
ramping up in the afternoon and evening. The remainder of the week
will feature a very active weather pattern with periods of strong
and gusty winds, showers with locally heavy rainfall and snowfall,
and low snow levels.

&&

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

Tranquil weather persists just through today with highs topping
out several degrees warmer than yesterday. A very strong storm
system then drops southward into the western US, spreading much
cooler temperatures and very strong west winds across the region.
Highs on Tuesday will be anywhere from 4 to 11 degrees cooler than
today’s, mainly across areas west of the mountains. EPS wind
guidance suggests 700mb winds of 55-70kts and 850mb winds of
40-50kts from the west as the storm system strengthens. This will
favor very strong winds over the mountains and desert foothills
where potential for triple digit wind gust readings exists.

Very strong winds will spread inland from the coastal waters as
well, with EPS guidance favoring 6-hourly maximum wind gusts
between 40 and 50 mph at KSAN early Wednesday morning. The NBM
forecast gust of 55 mph at the San Diego airport before dawn on
Wednesday would tie for the strongest wind gust from the west, set
in January 1988.

Precipitation impacts begin Tuesday evening generally across the
coastal mountain slopes. Snow levels beginning near 6000 feet
Tuesday evening will quickly fall to 2000 to 2500 feet on
Wednesday, so most precipitation in the mountains on Wednesday
will fall as snow. This will also impact many mountain passes,
such as the Cajon Pass, the San Gorgonio Pass, as well as the
Ortega Highway through the Santa Ana Mountains. A few locations
will see snow levels briefly drop below 2000 feet, namely in the
Inland Empire.

As implied by the low snow levels, much colder air will be
ushered into Southern California with the passage of this first
system. Highs Wednesday through Friday in coastal locations will
be confined to the lower and middle 50s, with widespread highs in
the 40s across much of the inland valleys. Even the lower deserts
will remain rather cool, with a few isolated locations barely
reaching the 60 degree mark for a high. This equates to high
temperatures across much of Southern California running anywhere
from 12 to 20 degrees cooler than seasonal normals. Several
record low maximum temperatures will be challenged later this
week.

Another low diving southward along the US West Coast on Thursday
will keep persistent precipitation chances in the forecast through
the weekend. While some mountain areas will see persistent showery
weather through much of the week ahead and into the weekend, the
two “waves” of precipitation look to be late Tuesday-early
Wednesday, with a stronger wave Thursday-Friday. The exact end
timing of any lighter precipitation in Southern California remains
rather uncertain. Needless to say, the potential for several
inches of rainfall in lower elevations remains valid. See the
Hydrology section below for more details.

&&

.AVIATION...
201615Z...SCT-BKN with bases 8000-10,000 FT MSL into this afternoon
before diminishing. Patchy low clouds with bases 1500-2000 FT MSL,
mainly across San Diego County 12z-17z Tue.

&&

.MARINE...
Locally gusty northwest winds approaching 20 kt will be possible for
several hours this afternoon across the outer coastal waters before
diminishing this evening. Strong northwest winds will develop
Tuesday afternoon, with high-end gales Tuesday evening through
Wednesday. Peak wind gusts could exceed 45 kt in the outer waters,
with winds peaking overnight Tuesday into early Wednesday. Seas are
expected to reach 10 to 18 feet, briefly exceeding 20 feet in the
outer waters. These will generate particularly hazardous boating
conditions. Strong northwest winds will begin to subside Wednesday
evening, weakening further on Thursday, though locally gusty winds
could lead to hazardous conditions at times Thursday through
Saturday.

&&

.BEACHES...
Gales over the coastal waters will generate a large wind swell of
very short period Tuesday night through Wednesday. Sloppy sets of 6
to 8 feet will produce dangerous swimming conditions.

&&

.HYDROLOGY...
For Tuesday night into next weekend, cold low pressure systems from
the northwest are expected to bring periods of showers with low snow
levels. Snow levels are expected to fall to 2000 to 2500 feet Wednesday,
then generally fluctuate between 3000 and 3500 feet for Thursday and
Friday, rising to 3500 to 4500 feet on Saturday. The timing and locations
of potentially heavier precipitation are less certain, but more likely
on Thursday and Friday. Rainfall through next weekend could range from
2 inches or more at the coast to 5 inches of more of liquid equivalent
in the mountains with 1 inch or more in portions of the deserts. With
the slower model solutions, showers could linger into Saturday and
possibly Sunday. This will likely lead to rising water levels on various
rivers and creeks in the region, as well as increasing potential for
urban and street flooding during the heavier rainfall Thursday and
Friday. This is also reflected in the latest forecasts from the CNRFC,
which support the San Diego River at Fashion Valley approaching and
eventually exceeding monitor stage by Friday evening. Similar
rises may occur on the Santa Margarita River as well, so stay
tuned for additional updates.

&&

.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 10 PM Tuesday to 8 PM PST Wednesday for
     San Diego County Coastal Areas.

     Wind Advisory from 10 PM Tuesday to 6 PM PST Wednesday for
     Orange County Inland Areas-San Bernardino and Riverside
     County Valleys-The Inland Empire-San Diego County Valleys.

     Winter Storm Watch from Tuesday evening through Saturday
     afternoon for Riverside County Mountains-San Bernardino
     County Mountains-San Diego County Mountains.

     High Wind Warning from 1 PM Tuesday to 1 PM PST Wednesday for
     Apple and Lucerne Valleys.

     High Wind Warning from 4 PM Tuesday to 6 PM PST Wednesday for
     Coachella Valley-San Diego County Deserts-San Gorgonio Pass
     Near Banning.

     Wind Advisory from 10 PM Tuesday to 8 PM PST Wednesday for
     Orange County Coastal Areas.

PZ...Gale Watch from Tuesday afternoon through Wednesday afternoon
     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/HYDROLOGY...Adams
AVIATION/MARINE/BEACHES...EA