Bonnie Bell, California Weather Forecast Discussion

867
FXUS66 KSGX 080941
AFDSGX
Area Forecast Discussion
National Weather Service San Diego CA
241 AM PDT Mon Sep 8 2025
.SYNOPSIS...
A low pressure system will gradually bring cooler weather this
week and temperatures will be mostly below normal. 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...
A pretty cohesive low cloud deck is gradually approaching the
coast as we play a waiting game to see how much land is covered
before sunrise. Expect some coastal areas to wake up to clouds,
other coastal areas to the sun. Today will be seasonably warm,
down from yesterday as a cooling trend gets underway. A low
pressure trough, rather deep and amplified for this time of year,
hangs off the Pacific NW coast. This will gradually sink southward
down the coast, right over California Tuesday through Thursday.
Accordingly, the marine layer will deepen and coastal cloud
coverage will expand in time and space each day. Tonight into
Tuesday it should reach into western valleys, with some true fog
(low visibility) on the inland edge of the cloud deck. 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. Onshore winds will pick up this afternoon and evening as
the trough makes its move southward. The usual wind-prone mountain
passes and adjacent desert slopes will get the strongest winds
each day. Strongest winds appear in those spots for Tuesday
afternoon and evening, with gusts 50-60 mph. 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
west of the mountains, with only a few spots in the Inland Empire
reaching 80 degrees. It should be around 80 in the high desert and
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...
080930Z...Coast...SKC prevailing currently, but patchy low clouds
will develop by 11Z. 30-40% chance BKN CIGs 400-900 ft MSL at the
coastal TAF sites between 11-15Z. VIS down to 2-5SM possible again
for coastal highlands during this time period wherever clouds
develop. Clear skies prevailing after 17Z. Low clouds with higher
bases return to coastal San Diego County after 03Z Tuesday and
spread northward and inland overnight.
Valleys/Mountains/Deserts...VFR conditions prevailing through
tonight.
&&
.MARINE...
Northwest winds gusting to 20 kts near San Clemente Island in the
afternoon and evening hours on today and Tuesday. Otherwise, no
hazardous marine conditions are expected through Thursday.
&&
.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.
&&
.SGX WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&
$$
PUBLIC...MM
AVIATION/MARINE...KW

NWS SGX Office Area Forecast Discussion