950
FXUS66 KSGX 150524
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
924 PM PST Thu Nov 14 2024

.SYNOPSIS...
An upper level trough will bring cooler and windy weather with
scattered showers and mountain snow later tonight through
Saturday. Subtle warming and drying will occur by Sunday and
Monday. Another trough digging through the Great Basin will
generate Santa Ana Winds and warmer weather west of the mountains
Tuesday through Thursday.

&&

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

Water vapor imagery shows an upper level trough amplifying just
off the coast of California. As this trough approaches, low clouds
have been quick to develop across So Cal this evening, already
extending into portions of the inland valleys. The 00Z KNKX
sounding shows the marine inversion based near 1700 ft MSL, but
this should rapidly rise overnight as the trough progresses
eastward. Isolated light showers may begin as early as Midnight,
increasing in coverage Friday morning as a weak cold front sweeps
through. Showers will taper off by Friday evening for the northern
areas, but persist across San Diego County into Saturday afternoon
as flow turns more northwesterly. Rainfall totals still look to
be around a few hundredths to two-tenths inch for the coasts and
Inland Empire, up to one-third inch for the San Diego County
valleys and foothills, one-quarter to three-quarters inch for the
mountains, and 0.05 inch or less for the deserts. Snow levels will
begin near 5500-6000 ft tonight, dropping to 5000-5500 ft Friday
morning, and as low as 4000-4500 ft Friday night as precipitation
begins to taper off. A Winter Weather Advisory remains in effect
for the San Bernardino and Riverside County Mountains above 5500
ft.

In addition to the precipitation, gusty west winds will develop
across the mountains and deserts tonight, peaking along the desert
mountain slopes and below the passes Friday morning, then
weakening slightly but becoming more widespread Friday afternoon.
Strongest gusts will be around 40-50 mph for the mountains and
deserts, with isolated gusts to 60 mph on remote desert mountain
slopes. Winds gradually weaken Friday evening into Saturday
morning. Otherwise significant cooling on Friday with high
temperatures 10-20 degrees below normal. A secondary low drops
down and closes off over southeast CA into northern Baja on
Saturday, maintaining cool weather. As this low tracks
southeastward, northerly flow aloft and weak offshore surface
pressure gradients will produce a round of weak Santa Ana Winds on
Sunday for drier weather and warming west of the mountains. A weak
but dry trough moves by to the north on Monday for minor cooling
and a return of onshore flow.

The picture becomes a bit blurry beyond Monday as ensemble spread
become massive with respect to the evolution of the trough once it
passes. About 50% of the ensemble space, mostly consisting of
members of the EC, shows a deep low digging into the Four Corners
region, while the remaining guidance has a much lower amplitude
and more progressive wave moving into the Rockies/High Plains by
Tuesday afternoon. This has major implications for the strength of
Santa Ana Winds across So Cal. The CW3E West-WRF ensemble shows a
80% probability of at least a weak event, and a 40% chance of at
least a moderate event. If the less amplified/more progressive
wave scenario occurs, winds would be much weaker with warmer
weather. If the deeper/slower trough scenario occurs, we`re more
likely to have at least a moderate Santa Ana evening and cooler
weather, especially for the mountains and deserts Tuesday and
Wednesday. By Thursday, Ensemble consensus is for an upper ridge
somewhere across the Western US for warmer and continued dry
weather.


&&

.AVIATION...
150330Z...Low clouds have started to develop in parts of coastal
Orange and northern San Diego counties this evening. These will
become more widespread and move into coastal sites by 07Z and into
inland valleys through 11-15Z. Bases are currently around 1500-2000
ft MSL but will rise to 2500-3000 ft MSL overnight. Isolated -SHRA
developing around 07Z this evening becoming more widespread after 12-
13Z. Local VIS reductions 4-6 SM and lower CIGs 1200-2000 ft MSL in
showers. Hit-and-miss showers continue through the forecast period
but will gradually taper off north of San Diego County Friday late
afternoon into the evening.

West to northwest winds continue to strengthen overnight with areas
of winds 25-30 kt over mountains and deserts. Moderate to strong
updrafts over the mountains, with mountains becoming obscured below
6000 ft MSL in clouds and showers.

&&

.MARINE...
Northwest winds associated Pacific weather system will continue to
strengthen overnight, bringing steep, wind-driven seas and hazardous
marine conditions. A Small Craft Advisory is in effect this through
late Saturday night. Winds 15-20 knots with gusts to 25 knots in the
outer coastal waters this evening will extend into the nearshore
waters overnight. Winds peak Friday afternoon at 20 to 25 knots with
gusts to 30 knots before gradually diminishing late Friday into
Saturday. Intermittent gusts to about 35 knots are also possible
(40% chance) near San Clemente Island Friday. Strong winds will
bring steep, wind-driven seas late Thursday through late Friday
night, peaking at 6 to 9 feet Friday. Winds weaken Saturday morning
as seas diminish Saturday into Sunday.

&&

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


&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Winter Weather Advisory until 10 PM PST Friday for Riverside
     County Mountains-San Bernardino County Mountains.

PZ...Small Craft Advisory until 4 AM PST Saturday 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
AVIATION/MARINE...CSP