327
FXUS66 KSGX 081627
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
827 AM PST Sun Dec 8 2024

.SYNOPSIS...
Low clouds and patchy dense fog are possible along portions of the
immediate coast early this morning. Cooler conditions are expected
today and Monday. Areas of low clouds may linger along the coast
into early Monday. Moderate to locally strong Santa Ana winds in the
mountains and valleys Monday through Wednesday, strongest Tuesday,
with critical fire weather conditions expected. Slight cooling with
less wind and a deeper marine layer will return Thursday through
Saturday.

&&

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

.Morning Update...
The marine layer this morning was just deep enough for areas of
dense fog to form along the coast, including at the San Diego
Airport. Clouds may locally stay near the beaches today, but much
of SoCal should see mostly clear skies today. The marine layer will
deepen slightly into tonight, so low clouds will go further
inland covering the coast and far western valleys into Monday
morning, where foggy conditions will be limited to higher terrain
away from the coast.

The Santa Ana wind event that is poised to occur across the region
from Monday through Wednesday remains on track. Wind speeds have
trended slightly upward as the pressure gradient is expected to be
a bit higher (10-12mb gradient from San Diego to Las Vegas), so
will look into fine tuning the forecast this afternoon. The
timing of the strongest winds continues to be placed on Tuesday
with lowest relative humidity expected on Tuesday and Wednesday.
Please use this time to prepare for fire weather hazards and windy
conditions across the mountains and western valleys.

.Previous Discussion (323 AM Sunday)...

An upper level trough of low pressure will dig south into the
Intermountain West Monday. A corresponding surface high will build
over the Great Basin, generating an offshore pressure gradient for
Southern California. The offshore gradient will generate gusty Santa
Ana winds for the mountains and adjacent valleys in Southern
California Monday through Wednesday. Gusty north to northeast winds
will begin to increase Monday for the San Bernardino and Santa Ana
mountain foothills; below the Cajon Pass into the Inland Empire; and
into portions of inland Orange County Monday. As the upper level low
continues to dig south, winds will become easterly Tuesday with
stronger winds expected in the San Diego and Riverside mountain
foothills into adjacent valleys, and below the San Gorgonio pass
near Banning. Strongest winds are expected on Tuesday. These winds
will also usher in very dry desert air. Gusty winds combined with
very low relative humidity and dry fuels will create critical fire
weather conditions. More information on wind speeds and relative
humidity trends can be found in the  Fire Weather discussion below.

Winds will weaken through the day Wednesday with no offshore wind
concerns currently forecast for the end of the week. For Thursday,
most of the mean ensemble solutions are showing some degree of
troughing near the US West Coast, although there is uncertainty in
the placement and strength of the trough. Even with the uncertainty
in the details of the trough, the ridge should be tampered enough to
allow for cooling and a deeper marine layer. For Friday and
Saturday, most mean ensemble solutions keep troughing near the US
West Coast indicating the gradual cooling and deep marine layer will
maintain until at least Saturday. Although chances remain less than
10 percent, light precipitation is still not out of the question by
next weekend depending on how far south the trough digs.

&&

.AVIATION...
081630Z....Coasts/Valleys...Low clouds (based around 100-300ft MSL)
and areas of fog (with VIS 0-3 SM) remain largely offshore this
morning, with the only patch currently onshore over North Island and
near/just west of KSAN. Clouds and fog could linger along the coast
from La Jolla southward through Imperial Beach, but otherwise, low
clouds should remain offshore through the afternoon, leading to fair
skies with SCT-BKN high clouds.

Much better confidence for more widespread, though could be somewhat
patchy, low clouds this evening. They are expected to move into
coastal sites as early as 01-03z Monday, spreading 10-15 mi inland
by 12z. Bases will likely be slightly higher, around 500-800ft MSL,
but areas of VIS 2-5 SM possible along the coasts and coastal mesas,
with VIS 0-3 SM into the western valleys and where clouds intersect
higher terrain. Any clouds/fog will quickly clear out by 17-18z
Monday.

.Mountains/Deserts...Aside from SCT-BKN high clouds at or above
20,000ft MSL, mostly clear skies and VFR prevail through Monday
morning.

&&

.MARINE...
Areas of low clouds and fog with visibilities around 1-2 NM continue
to linger in the southern and eastern waters this morning, but
should decrease in coverage by this afternoon. Another round of low
clouds/fog is possible tonight into Monday, but should be less dense
over the waters, with better visibility and higher clouds bases.
Increasing offshore winds on Tuesday with gusts up to 20-25kts,
primarily for the northern San Diego County coast.

&&

.FIRE WEATHER...
Moderate to locally strong east to northeast Santa Ana winds are
expected Monday through Wednesday. Winds will be predominately from
the north to northeast Monday before becoming more easterly Monday
night into Tuesday. Winds will peak in strength late Monday night
through Tuesday, and gradually fade through Wednesday. Strongest
winds will occur on mountain foothills, through passes, and into
adjacent inland valleys. Gusts 50 to 65 mph expected through favored
passes and on mountain foothills, a few isolated gusts to 70 mph are
possible Tuesday during the peak of the event in typically wind
prone locations. Gusts of 35 to 45 mph are expected in parts of the
inland valleys and inland Orange County. Relative humidity Monday
afternoon is expected to fall to 15 to 25 percent for inland areas
north of San Diego County. By Tuesday afternoon, relative humidity
values will fall to 5 to 10 percent for inland areas and 15 to 25
percent at the coast. Overnight humidity recovery will be poor, only
increasing to 15 to 30 percent Tuesday night into Wednesday. Low
humidity, strong winds, and dry fuels will lead to critical fire
weather conditions from Monday through Wednesday morning, highest
risk on Tuesday. Wednesday and Thursday will see lighter winds, but
humidity will still lower near 5-20% each afternoon for inland
areas. Humidity recovery is expected to improve by Thursday night.

&&

.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 4 AM PST Wednesday for
     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.

PZ...None.

&&

$$

UPDATE/PUBLIC...APR/CO
AVIATION/MARINE...Munyan