136
FXUS66 KSGX 191121
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
321 AM PST Sun Jan 19 2025

.SYNOPSIS...
Onshore flow will continue to bring cooler weather and partly
cloudy skies west of the mountains during the nights and mornings
into Monday. Moderate to strong Santa Ana Winds will develop late
Monday and continue through Tuesday evening. A second, weaker
Santa Ana event is possible on Thursday. There is a chance of rain
next Saturday and Sunday.

&&

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

Key Points:

- Moderate to strong and potentially damaging Santa Ana winds
  Monday night through Tuesday evening.

- There is a 50-60 percent chance of a moderate strength Santa Ana
  event on Thursday.

- There is a 20-30 percent chance of measurable precipitation
  next Saturday and Sunday.

This morning...Extensive marine layer low clouds over the
Southern California Bight are very slowly spreading into the
coastal areas and valleys but their progress seems to be somewhat
inhibited by the nocturnal drainage winds. Sfc pressure gradients
remain weakly onshore so the high resolution models show low clouds
increasing in coverage over the coastal areas and valleys early
this morning before clearing around 10 am.

Weak onshore flow will continue through tonight, with marine layer
low clouds returning to the coastal areas and western portions of
the inland valleys tonight into Monday morning. Some patchy fog
may develop on mesas and higher terrain away from the immediate
coast tonight into Monday morning. High temperatures today will
be up to 10 degrees below seasonal averages in coastal areas and
valleys while in the mountains and the upper deserts, high
temperatures will be a few degrees above seasonal averages.

A surface high moving into the Great Basin on Monday will create
moderate to strong offshore sfc pressure gradients. The effects
of the offshore flow will be enhanced by a shortwave trough moving
south in the mean flow aloft. The outcome will be a moderate to
strong Santa Ana event, with strong northeast to east winds
developing Monday afternoon and continuing into Tuesday evening
leading to critical fire weather conditions. Please see our Fire
Weather section below and the latest Red Flag Warning for more
information. This system will bring in cooler and drier air, with
highs in the 60s west of the mountains and lower deserts, and 40s
across the mountains. Nights will continue to be chilly,
especially for wind-sheltered valley locations that may dip into
the upper 20s and lower 30s with patchy frost in the early morning.

As the upper level shortwave trough moves to the east and sfc
pressure gradients weaken on Wednesday, there could be some
humidity recovery in the coastal areas. As we lose the cold air
advection from the Great Basin and high pressure aloft shifts over
SoCal from the west, high temperatures will rise into the 70s west
of the mtns on Wed and remain there through Friday.

Another weaker Santa Ana event could develop for Thursday as a
second shortwave trough moves into the Great Basin with a
following sfc high setting up offshore flow. Mostly due to the
track of the shortwave and the intensity of the sfc high, most
solutions across model platforms indicate that this will be a
significantly weaker event than the Mon/Tue event.

For next weekend, most model solutions show a low pressure system
bringing cooler, cloudy conditions with onshore flow to SoCal.
While the models are in fairly good agreement with respect to the
synoptic pattern, there remains significant differences in the
details so forecast confidence for next weekend is moderate at
best. About a third of ensemble members across model platforms
indicate 20-30 percent chances for measurable precipitation next
Sat and Sun.

&&

.AVIATION...
191030Z...Low clouds have been slowly filling in over the ocean this
morning and should continue to fill eastward and onshore around 12-
14z. Weak offshore flow could limit inland extent and serve to keep
the marine layer still somewhat patchy, but BKN CIGs are expected
with bases around 1000-1500ft MSL. VIS restrictions 4-6SM possible
along the coasts with locally lower VIS within valleys and on
coastal higher terrain down to 1-3SM. Confidence for KSAN is 80-90%
for MVFR CIGs and VIS around 6SM after 12z, and 20% for IFR CIGs.
Any clouds that pushed ashore should clear to the coasts by 18-20z
though a few clouds may stick near the beaches into the afternoon.

Low clouds look to surge back ashore quite early this
afternoon/evening, around 00-03z. Bases may lift some but will still
generally be 1000-2000ft MSL. Clouds should fill well into the San
Diego County valleys and into the Inland Empire by 06-09z. VIS
restrictions will likely be limited to inland valleys and higher
terrain.

NWS San Diego is aware of limited communications of observations
from KTRM since January 8th. We are working with the FAA to address
this issue.

&&

.MARINE...
Strong Santa Ana winds develop Monday night, with gusts up to 25
knots possible over the waters late Monday night into Tuesday
morning. A Small Craft Advisory is now in effect and contains more
details.

&&

.FIRE WEATHER...
Moderate to strong Santa Ana winds event will develop for late
Monday through Tuesday evening. Winds will begin to strengthen on
Monday afternoon, peaking in intensity overnight into Tuesday
morning. Widespread wind gusts of 35-50 mph will occur across
inland valleys west of the mountains, with local gusts over 70 mph
in the favored passes, canyons, and on coastal slopes. Initially,
winds will be northeasterly on Monday, slowly becoming easterly
with the strongest winds favoring the San Diego and Riverside
County mountains and adjacent foothills Tuesday morning. Minimum
relative humidity Monday afternoon will be around 15-20% with very
poor overnight recovery. Extremely dry weather is expected on
Tuesday and Tuesday night with minimum humidity of 5-8% Tuesday
afternoon, potentially remaining in the single digits in the
mountains and foothills into Tuesday night with poor recovery
in most areas. Winds will weaken Tuesday night with weak offshore
flow and dry conditions continuing into Wednesday.

Winds strengthen again on Thursday when there is a 50-60 percent
chance of a moderate Santa Ana event. These winds will possibly
continue into Friday, though weaker. Relative Humidity will remain
in the single digits to mid teens, maintaining near critical to
critical fire weather conditions. Conditions improve greatly
Saturday and Sunday with the return of onshore flow and a 20-30
percent chance of measurable precipitation.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Red Flag Warning from 10 AM Monday to 10 PM PST Tuesday for
     Orange County Inland Areas-Riverside County Mountains-
     Including The San Jacinto Ranger District Of The San
     Bernardino National Forest-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 Diego County Inland Valleys-San Diego County Mountains-
     Including The Palomar And Descanso Ranger Districts of the
     Cleveland National Forest-San Gorgonio Pass Near Banning-
     Santa Ana Mountains-Including The Trabuco Ranger District
     of the Cleveland National Forest.

     High Wind Watch from Monday evening through Tuesday evening for
     San Bernardino County Mountains-San Bernardino and
     Riverside County Valleys-The Inland Empire-San Gorgonio
     Pass Near Banning-Santa Ana Mountains and Foothills.

     High Wind Watch from late Monday night through Tuesday evening
     for Riverside County Mountains-San Diego County Mountains-
     San Diego County Valleys.

PZ...Small Craft Advisory from 10 PM Monday to 10 AM PST Tuesday 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/FIRE WEATHER...PG
AVIATION/MARINE...Munyan