Spring Valley, California Weather Forecast Discussion

275
FXUS66 KSGX 132038
AFDSGX
Area Forecast Discussion
National Weather Service San Diego CA
138 PM PDT Sat Sep 13 2025
.SYNOPSIS...
Near to slightly below normal temperatures through Monday. A slow
warming trend will bring a shallower marine layer and near normal
to above normal temperatures by Wednesday. An isolated
thunderstorm or two will possible for areas west of the mountains
Tuesday into Wednesday morning. Ample monsoonal moisture returns
by Wednesday, with thunderstorm chances mainly in the mountains
each afternoon and evening through the weekend.
&&
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...
Quiet weather the past few days with troughing in place, below
normal temperatures, and a decently deep marine layer all making it
feel like the start of fall. Mid-level water vapor imagery currently
shows the large upper level low is positioned over the Colorado
River Valley with weak ridging on the backside of it, with the
thermal ridge axis over California. Despite the increasing heights,
a secondary shortwave trough off the coast is helping to moderate
our warm up today and tomorrow, with temperatures remaining near or
below normal in the 60s and 70s for inland/coast/mountains and 80s
to 90s for the deserts. Sunday into Monday, this shortwave will
become a closed low and slowly begin to trek northward along the
coast of California as a tropical system to the south pivots to the
northwest. Models are beginning to hint at the possibility of some
influence from this system as it moves west, which may help
initiate some storms earlier in the week than previously expected.
If this occurs, rain or thunderstorms will not be out of the
question, primarily San Diego County and offshore, as early as
Tuesday/Wednesday morning. For now, chances are only around 10-15%.
A lesser pressure gradient will see weaker onshore flow into mid-
week, along with a more shallow marine layer, and temperatures
will warm for much of the inland and coastal areas. Highs look to
peak Wednesday for areas west of the mountains, around 4-7 degrees
above normal, before a slow cool down sets in for the rest of the
week. As mentioned the past few days, monsoonal moisture is
expected to make a return for much of the region by Wednesday,
with increasing thunderstorm chances for Thursday onward.
Currently, the best chances for storms will exist in the mountains
each afternoon and evening, but a few storms may make it into the
deserts and valley areas. Any storms that manager to develop will
be accompanied by brief, heavy rain, gusty and erratic winds, and
frequent lightning. Rain rates exceeding 0.50"/hr will be
possible, with the best chances for the San Diego and Riverside
Mountains. Along with the thunderstorm threat, the uptick in
moisture across the deserts and mountains should limit heating
each day, with more seasonable temperatures expected next week.
Highs generally will range from the mid 90s to low 100s for the
deserts, and 70s for the mountains.
Models continue to struggle with the upper level pattern late this
week, although there seems to be general support in the long-range
of persistent monsoonal moisture as multiple tropical systems move
westward, and a cutoff low separates from the larger trough over the
PacNW, setting up just offshore. While confidence is low in the
model guidance this far out, there seems to be pretty good alignment
on such a pattern that would support afternoon convection and cooler
temperatures into the latter half of September. After a abnormally
hot September last year and the driest start to the water year, a
cooler start to fall and some much needed moisture would be very
welcome.
&&
.AVIATION...
132030Z...Coast/Valleys...Marine layer clouds have largely scattered
out this afternoon, though SCT-BKN clouds based at 2100-2600ft MSL
are lingering within 10 miles of the coast. KSAN, KCRQ, and KSNA all
have a chance to see occasional BKN CIGs through the afternoon.
Clouds fill back in and push back inland after 02z Sunday,
eventually filling in about 20-25 miles inland. Bases starting
initially near 2000ft MSL will likely settle to 1500-2000ft MSL by
06z Sunday. Clouds could extent into portions of the Inland Empire
by the late morning, with a 40% chance of CIGs at KONT and a 20%
chance at KSBD between 12-16z Sunday morning. Scattering back to the
coasts 16-18z, likely less clouds lingering along the coasts
compared to today.
Mountains/Deserts...Mostly clear and VFR conditions through Sunday
afternoon.
&&
.MARINE...
No hazardous marine conditions are expected through Thursday.
&&
.SGX WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&
$$
PUBLIC...Zuber
AVIATION/MARINE...Munyan

NWS SGX Office Area Forecast Discussion