749
FXUS66 KSGX 090350
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
850 PM PDT Mon Sep 8 2025

.SYNOPSIS...
A low pressure system will bring gradually cooler weather this
week, with temperatures mostly below normal through the week.
This system will also boost winds in parts of the mountains and
deserts during afternoons and evenings. The marine layer will
rebuild to provide increasing nocturnal low cloudiness west of the
mountains.

&&

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

The first inklings of marine layer low clouds are forming along
the central San Diego County coast, with a much larger bank of low
clouds moving southwards east of San Clemente Isle, ahead of a
distinct cold frontal cloud band over the eastern Pacific. This
kind of sight on satellite is usually a pretty good indication
that our relatively cloud-free nights are over for now. Indeed, we
expect low clouds to be deeper and much more uniform tonight than
previous nights, moving inland slightly beyond the I-15 corridor
and leading to some lowered visibility there by the wee hours of
Tuesday morning. Clouds will also hang out over land longer than
previous days tomorrow, likely not clearing to the beaches until
the late morning.

Besides leading to more and long-lived marine layer low clouds,
low pressure settling into the area will lead to noticeably cooler
temperatures Tuesday, as well as gusty winds inland. Expect
tomorrow`s afternoon temperatures to be 3 to 10 degrees lower than
today`s (more dramatic inland and less so near the coasts), a
trend that will continue into mid-week.

In terms of winds, stronger winds started to surface today in our
breezier areas, with peak gusts 35-40 mph through the Banning
Pass (except for the typically very windy gap east of Bonnie Bell,
which gusted to an overachieving 50 mph today), 35 to just under
40 mph in the southern San Diego desert, and up to 46 mph at the
Cuyamaca Mountain station at 6496 feet reported this afternoon.
These stronger winds will be more widespread over the mountains,
desert foothills, and deserts Tuesday afternoon and evening as
stronger winds aloft mix down to the surface. Expect to be
battling head winds driving eastwards down I-10 through the
Banning Pass, or on I-8 through Walker Canyon coming down from the
mountains into the desert or through the canyons on our border
with Imperial County. For more details on this, see the previous
discussion below or see the Wind Advisory on our website.

&&

.PREV DISCUSSION...issued 234 PM PDT Mon Sep 8 2025...
This afternoon, near average temperatures prevail with some low
clouds at the immediate coast and locally strong west winds along
mountain crests and passes. The marine layer will deepen tonight
with a broad deck of low clouds moving slightly beyond the I-15
corridor in SD County and covering all of lower Orange county with
some clouds spilling into the Inland Empire through the mountain
gaps. Fog and low visibility will develop overnight for valleys
around I-15 and along the furthest inland extent of the clouds.

By Wednesday morning the cloud deck should fill the entire coastal
basin, maybe flush into the foothills. We should have a similar
coverage for Thursday. With the deep marine layer we could get a
very thick cloud deck, which would produce drizzle those days.
However, the cooler air aloft would weaken the marine inversion
greatly, and could prevent the cloud from thickening enough for
drizzle.

With the trough moving southward toward Southern California,
afternoon west winds will be strong, especially Tuesday
afternoon. The usual wind-prone mountain passes and adjacent
desert slopes will get the strongest winds each day with gusts
mainly up to 45-55 mph and locally up to 60 mph for the San
Gorgonio Pass.

Temperatures today will be near normal, but decrease each day into
Thursday, likely our coolest day this week. Max temps Thursday will
be in the 70s and low 80s west of the mountains. It should be around
80 in the high desert, low 90s in the low desert and 50s and 60s in
the mountains. These temps are generally 10-15 degrees below normal
inland, even as much as 20 degrees below normal in the mountains. It
might be time to dust off the jackets. The trough over the West
begins to weaken and lift out Friday through the weekend, so the
reverse trend is expected: warmer weather, a shallower marine layer,
and weaker winds. Still, forecast temps this weekend don`t quite
reach normal. Monsoon moisture and thunderstorms will become a
distant memory as dry westerly flow prevails this entire week.


&&

.AVIATION...
090315Z...Coast/Valleys...Low clouds based 700-1200 ft MSL will
become widespread along the coast and reach up to 25 miles inland
overnight. Patchy FG and reduced VIS 0-5SM along elevated coastal
terrain and western valleys. 20% chance for cigs at KONT after 12z,
if cigs develop they will likely be under 1000 ft. Scatter out and
improved vis 17-19z. Low clouds are expected to redevelop along the
coast around 00Z Wed.

Mountains/Deserts...West winds with gusts 30-40 kt will continue
through 12Z Tue. Winds will increase after 21Z Tue with gusts 35-45
kt, locally up to 55 kt for mountain passes through Wed AM. VFR
conditions will prevail through Tuesday.

&&

.MARINE...
Northwest winds gusting to 20 kt near San Clemente Island in the
afternoon and evenings through Wednesday.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory from 2 PM Tuesday to 1 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...None.

&&

$$

PUBLIC...CSP
AVIATION/MARINE...CO

NWS Tucson (SGX) Office