Baldy Mesa, California Weather Forecast Discussion

046
FXUS66 KSGX 211041
AFDSGX
Area Forecast Discussion
National Weather Service San Diego CA
341 AM PDT Sun Sep 21 2025
.SYNOPSIS...
Low clouds will spread into the far western valleys tonight.
Subtropical moisture returns today with scattered light showers,
most widespread across the mountains and San Diego County. Small
chances of showers continue Monday mainly over the mountains,
increasing for Tuesday and possibly into Wednesday, along with a
slight chance of thunderstorms. Drier weather arrives Thursday and
through the following weekend. Temperatures will be mild this
coming week, with decreasing coastal low clouds.
&&
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...
Today through Tuesday...
Low clouds with the marine layer are continuing to fill in over the
inner SoCal bight waters, with some of these clouds penetrating into
the inland areas of San Diego and Orange counties. This will
continue to advance further eastward throughout the night and
eventually make it back into portions of the IE. There will also
likely be some areas of patchy fog this morning, especially within
some of the wind-sheltered valleys. High clouds correctly south over
Mexico will continue to pass overhead throughout the day. This may
allow some areas to become overcast and could help to keep
temperatures from getting as warm as they are projected to be. It
will be slightly cooler today than it was yesterday, regardless of
the high clouds passing over, and this is because the region will
begin to become influenced by an upper level trough upstream. This
feature is currently over the eastern Pacific waters west of
California, and continue to deepen. Winds will become rather gusty
in the afternoon for the areas to gap flow winds and also within
the mountain passes and canyons. By tomorrow, this will become
cutoff, with the upper level low associated with this positioned
directly to the west-southwest of San Diego at roughly 500 nm
(given the consensus of the deterministic models) and closing off
with heights of around 577 dm. This will allow for some of the
subtropical moisture to be advected up over the region from the
southwest. However, given that there will be very weak instability
with this, it is more likely that any precip will be more
stratiform in nature and in elongated bands extending from
southwest to northeast, and only amount to generally light precip
with amounts of less than a tenth of an inch for most locations.
By Tuesday, the low will continue to meander and slowly transition
towards Socal, which will allow for a better chance of more
widespread precipitation. There could also be a few embedded
thunderstorms over the mountains, due to orographic lifting, as well
as possibly over the desert where there is increased instability.
I(f this occurs, then there could be some areas which receive
locally higher rainfall amounts of possibly greater than a quarter
of an inch. Given where the position of the U/L low will be, and all
of the CWA being within the relatively unstable side, the best
chance for showers and thunderstorms (some possibly being on the
heavy side) during the afternoon on Tuesday. The best instability
that CAMS are displaying with mixed layer CAPE is roughly around
1100 J/kg over the Palomar area during the late afternoon on
Tuesday, which will be the more desirable location for a
thunderstorm to develop if the instability was correctly resolved
(by the NAM 3km). Temperatures will also remain on the cooler side
with the cut-off low over the region, and even slightly below the
seasonal average for this time of year, especially for the inland
areas and deserts. Amounts will generally be less than a tenth,
although, there could be higher amounts along the west-facing slopes
of the foothills, and over the mountains, where there could be
amounts ranging from 0.10-0.25 inches. A storm may also result in
locally higher amounts. The marine layer will also be disrupted by
this, and will attempt to reform and move back inland by much later
in the night.
Wednesday through Saturday...
By Wednesday, the U/L low is going to begin to propagate more
towards the north, and this will allow for a bit of a `lull` in
terms of the chances of precipitation as the region becomes more
"dry-slotted", and temperatures will also be slightly warmer as
well. The U/L low will then begin to transition slowly towards the
southeast as the trough deepens on Thursday. Since the low will be
to the northeast, and predominantly on the more stable side, there
will only be a very slight chance of showers, and possibly a
thunderstorm, on the backside of this low. By Friday, there appears
to be more division in agreement with the deterministic models, and
it will all depend on where the cut-off low decides to transition
which will determine how much of a chance of showers and
thunderstorms there will still be with us on Friday, and going into
Saturday, for the mountains during the afternoon hours. High
temperatures will remain on the cooler side as overall troughing
remains persistent over the region going into the following weekend,
and ensembles also confirm this, with the relatively cooler and
drier conditions locked in through then.
&&
.AVIATION...
210830Z....Coast/Valleys...Low clouds based 600-1000 ft MSL have
reached to the I-15 corridor in SD County and several miles inland
into Orange County, with patchy coverage. VIS 3-6SM, locally lower
where terrain and clouds intersect as well as some inland valleys.
Scatter out 15-17z to the coast.
-SHRA/-RA will begin to move into the region generally after 12z,
with coverage of -RA increasing after 15z. Periods of light rain are
expected to continue through early Monday, though more often off
than on for any given area. Locally lower VIS and CIGs with any
SHRA/RA.
Clouds will generally be around 10,000 ft MSL with low marine layer
clouds struggling to develop Sunday night into Monday. Cigs below
3,000 ft MSL will be few and far between, around 20% coverage along
coastal areas with similar bases as tonight.
.Mountains/Deserts...Mid and high level clouds AOA 10,000 ft are
moving into the region from the south. SHRA move into the region
generally after 15z, with brief lowering of VIS and CIGs with any
shower.
&&
.MARINE...Rain showers possible for much of the day today and again
Tuesday which could briefly lower visibly. Slight chance (5-10%) for
a stray thunderstorm over the waters today and Tuesday. Any storm
could briefly produce lightning, gusty erratic winds, and choppy
seas. Otherwise, no additional hazardous marine conditions are
expected through Thursday.
&&
.Beaches...
Thunderstorm chances return early Tuesday, peaking Tuesday
afternoon, and may prevail into early Wednesday.
&&
.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...Stewey
AVIATION/MARINE/BEACHES...Westerink

NWS SGX Office Area Forecast Discussion