Frost Advisory 2, NWS San Diego, CA

Area Forecast Discussion
National Weather Service San Diego CA
911 PM PST Sun Jan 12 2025

.SYNOPSIS...
Santa Ana winds and critical fire weather conditions for the
coastal slopes of the mountains and adjacent foothills and valleys
through the Wednesday. Cooler with a return of onshore flow
Thursday through the weekend, along with a chance of light
precipitation late Friday through the weekend.

&&

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

Evening update...
The brief lull in Santa Ana winds is ongoing this evening as
surface pressure gradients weaken, currently at -1.4 mb SAN-DAG
and -6.5 mb SAN-TPH. As a result, wind gusts are generally
between 25-35 mph in the wind prone passes and canyons. A short
wave dropping south through the Great Basin and splitting tonight
will bring a resurgence of Santa Ana winds, peaking Monday morning
into the early afternoon with gusts up to 45 mph in the wind prone
locations.

Previous discussion...
Sfc pressure gradients will remain offshore through Wednesday and
humidity will remain low. The dry airmass and lack of cloud cover
will enable efficient radiation cooling overnight so that low
temps in the wind-sheltered valley locations will fall into the
30s for the next few days. Lows in the 20s can be expected in
the High Desert and in the teens and in higher elevation mountain
locations. Highs today for inland areas will be 5 to 10 degrees
below average, with slightly above average temperatures expected
near the coast. Slow warming through mid-week before cooler
conditions develop for later in the week and next weekend.

For Monday through Wednesday...Most ensemble solutions indicate
the development of a closed upper low retrograding from WA/OR to a
position about 850 miles southwest of San Diego by Tuesday night,
forming a Rex Block with a high north of the low. Meanwhile, sfc
high pressure in the Great Basin will maintain offshore pressure
gradients and Santa Ana conditions for SoCal. See the Fire Weather
section below for more information on the dry, windy conditions.

The majority of solutions show the closed low to the southwest
beginning to move northeast over SoCal by late Thursday, becoming
an open wave over AZ on Friday afternoon as it becomes absorbed in
the mean westerly flow. Most ensemble solutions show slight
chances for mostly light precipitation in SoCal on Sat and Sun.
Since most of the available moisture will likely be concentrated
in the boundary layer, any precipitation will be light and
restricted to the mountain slopes and areas west to the coast.
Numerical model guidance tends to struggle with resolving the
evolution of closed low-latitude lows in this part of the world
so there is significant spread in the model solutions and
consequently much uncertainty in the forecast beyond Thursday.

&&

.AVIATION...
130500Z...Mostly clear skies will continue through Monday evening
with unrestricted VIS. Gusty north to northeast gusts 20-35 kt will
continue at times in the mountains and parts of inland Orange County
and the Inland Empire overnight, with gusts increasing to 25-40 kt
locally Mon/Mon evening.

&&

.MARINE...
Local wind gusts from the northeast could approach 20 kt at times
from Monday night through Wednesday. Otherwise, no hazardous marine
conditions are expected through Friday.

&&

.BEACHES...
Astronomical high tides of 6-7 feet may produce minor tidal overflow
at susceptible low-lying beach parking lots and boardwalks Monday
morning.

&&

.FIRE WEATHER...
Northeast wind gusts of 25-35 mph this evening will increase to
30-45 mph early Monday morning along the coastal slopes of the
San Bernardino mountains into the Inland Empire and on the Santa
Ana mountains into portions of inland Orange County. Poor humidity
recovery tonight for the wind prone areas and foothills, peaking
at only 20-35%.

Another round of Santa Ana winds is expected for Monday through
Wednesday. Winds will become more easterly Monday afternoon,
increasing across the Riverside and San Diego County mountains
and adjacent valleys. Winds are currently forecast to peak
Tuesday and Wednesday, with gusts of 45 to 55 mph expected.
Relative humidity will remain around 10 to 15 percent each
afternoon inland with poor overnight recovery in the wind prone
areas and thermal belts. Red Flag Warnings remain in effect for
the San Bernardino Mountains, Inland Empire, San Gorgonio Pass,
Santa Ana Mountains, and Inland Orange County through 6 PM
Wednesday. A Red Flag Warning will be in effect for the Riverside
and San Diego County Mountains as well as the San Diego County
Valleys from 4 PM Monday through 6 PM Wednesday.

Offshore winds will weaken significantly on Thursday and sea
breezes could develop Thursday afternoon, with minor improvements
in humidity, especially for Orange County and wester parts of the
San Diego County Valleys. Onshore flow will increase on Friday,
bringing cooler weather and a more significant increase in
humidity. There remains a slight (15 to 30 percent) chance of
light precipitation west of the mountains, though the forecast has
already begun to trend drier. There is the potential for a return
of dry offshore winds early next week but uncertainty is high at
this time.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Red Flag Warning until 6 PM PST Wednesday for Orange County
     Inland Areas-San Bernardino County Mountains-Including The
     Mountain Top And Front Country Ranger Districts Of The San
     Bernardino National Forest-San Bernardino and Riverside
     County Valleys  -The Inland Empire-San Gorgonio Pass Near
     Banning-Santa Ana Mountains-Including The Trabuco Ranger
     District of the Cleveland National Forest.

     Red Flag Warning from 4 PM Monday to 6 PM PST Wednesday for
     Riverside County Mountains-Including The San Jacinto Ranger
     District Of The San Bernardino National Forest-San Diego
     County Inland Valleys-San Diego County Mountains-Including
     The Palomar And Descanso Ranger Districts of the Cleveland
     National Forest.

PZ...None.

&&

PUBLIC...SS/PG
FIRE WEATHER...SS
AVIATION/MARINE/BEACHES...Maxwell