412
FXUS66 KSGX 130426
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
926 PM PDT Mon May 12 2025

.SYNOPSIS...
Tuesday will likely be the coolest day of the week. In addition
to cooler conditions, winds will strengthen over the mountains
and into the deserts tonight, continuing into Wednesday morning.
There are slight chances of light showers on the coastal mountain
slopes and drizzle for the coast and valleys on Tuesday.
Some warming for Wednesday, but temperatures remain slightly
below average into the weekend. The marine layer will maintain a
presence, with some low clouds and fog in the valleys and coastal
areas.

&&

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

This evening...The marine layer is quickly deepening and is
currently about 3,500 ft deep, with low clouds increasing in
coverage from the coast to the mountain slopes. Widespread mid and
high clouds are streaming in from the west with bases above 15,000
ft. There is some instability aloft which is reflected in the
convective elements of the mid-level clouds. The onshore winds
continue to strengthen with the wind-prone locations reporting
gusts of 35-50 mph in the last hour.

From previous discussion...
Cloud coverage will continue to increase tonight as a trough of
low pressure moves across the Pacific Northwest. Tuesday is
expected to be the coolest day of the week, with temperatures
cooling an additional 10 to 15 degrees for inland locations.
Strong and gusty winds will continue over the mountains and into
the deserts through early Wednesday morning. Winds will be
strongest on the desert mountain slopes, through passes, and into
the deserts. Peak gusts of 40 to 55 mph expected in the deserts,
with local gusts closer to 65 through wind prone passes like the
San Gorgonio Pass. On the desert mountain slopes, peak gusts will
be 50 to 60 mph.

As the trough deepens southward into the Great Basin tonight into
Tuesday, the marine layer will deepen. Widespread low clouds can
be expected from the coast to the mountain slopes. The moisture
will be deep enough that when a shortwave moves through the mean
flow on Tuesday morning and when the trough axis passes Tuesday
evening light showers may develop. Any shower development is most
likely on the coastal mountain slopes, with the potential for some
drizzle west of the mountains mainly on Tuesday morning/early
afternoon. Ensemble guidance is showing a 40 to 60 percent chance
of rainfall accumulations reaching or exceeding 0.10" on the
coastal mountain slopes, with a 20 to 40 percent chance of
rainfall accumulations reaching or exceeding 0.20" on the coastal
mountain slopes. For the coast and valleys there is a less than 10
percent chance for accumulations to reach or exceed 0.20". Local
high resolution WRF guidance and HRRR guidance continues to
indicate rainfall rates less than 0.10" per hour Tuesday morning
and evening, with only a 10 percent chance of rainfall rates
exceeding 0.10" per hour on the coastal mountain slopes.

The main trough axis will be to our east by Wednesday, which will
allow for the winds to weaken, dry conditions to return, and for
high temperatures to warm 5 to 10 degrees, locally up to 15 degrees
warmer in the mountains. Most guidance indicates we will maintain an
overall troughing pattern over the western United States for the
remainder of the week, with a ridge of high pressure building over
the Eastern Pacific. The overall troughing pattern will keep high
temperatures just below the seasonal normal, maintain at least night
and morning low cloud coverage, and elevated afternoon sea breezes.
Over the weekend, there becomes some disagreement in the upper level
pattern which will impact whether we get warmer conditions and a
squashed marine layer or increased cloud coverage, winds, and cooler
conditions. For example on Saturday, 40 percent of solutions show a
weak trough passing through the Intermountain West, 40 percent of
solutions show a deeper trough, and 20 percent show ridging over the
West Coast. By Sunday, 50 percent of solutions indicate ridging with
30 percent of solutions indicating a deeper trough.

&&

.AVIATION...
120330Z...Coast/Valleys...BKN low clouds based near 2000 ft MSL with
tops to 3500 ft MSL are quickly filling in near the San Diego Co
coast at this hour. Bases will rise closer to 2,500-3,500 ft
overnight with the coastal basin filling up by 06-07z. Low clouds
will slowly scatter out through Tuesday morning for the Inland
Empire and Orange County, but will likely stay BKN in San Diego
County and along the coastal slopes of the mtns in the afternoon
hours. Low cloud coverage expands once again Tuesday evening, but
may remain SCT for northern areas of the coastal basin overnight.

.Mountains/Deserts...SCT-BKN High clouds AOA 20000ft through
tonight, but otherwise, VFR conditions through Tuesday. Wind gusts
currently 30-40 kts and up to 55 kt along desert slopes and through
mountain passes with STG UDDFS occurring downwind of the desert
slopes. Winds weaken slightly overnight and become more confined to
the desert slopes and passes. Then, strong winds become more
widespread again after 15z Tuesday. BLDU in windier corridors,
primarily in the deserts. Winds finally diminish late Tuesday
night.

&&

.MARINE...
Strong west-northwest winds will continue through Tuesday, with
sustained winds of 15 to 20 kt and gusts to 25 kt across the outer
coastal waters leading to hazardous conditions for small craft,
strongest winds near San Clemente Island. A Small Craft Advisory
remains in effect through late Tuesday night. Winds briefly diminish
late tonight through Tuesday morning, but will strengthen again
Tuesday afternoon and evening. Winds taper off by Wednesday morning.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory until 5 AM PDT Wednesday for Apple and Lucerne
     Valleys-Riverside County Mountains-San Bernardino County
     Mountains-San Diego County Deserts-San Diego County
     Mountains-San Gorgonio Pass near Banning.

PZ...Small Craft Advisory until 11 PM PDT Tuesday for Waters from San
     Mateo Point to the Mexican Border Extending 10 to 60 nm out
     including San Clemente Island.


&&

$$

PUBLIC...PG
AVIATION/MARINE...KW