855
FXUS66 KSGX 201821
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
1121 AM PDT Sun Sep 20 2026

.SYNOPSIS...
Temperatures remain below normal until Wednesday with a deep
marine layer and perhaps morning drizzle early week. Warmer
weather returns for the end of the week as high pressure builds.

&&

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

The marine layer pushed in efficiently this morning, filling in
the coastal basin and western valleys. High clouds associated with
an upper level low over Central CA will continue to stream into
SoCal through the afternoon. Even as the low marine layer clouds
burn off, these higher clouds will keep temperatures a bit cooler
today as well as decreasing heights thanks to the aforementioned
low. Highs this afternoon will be 2-6 degrees below normal, in the
mid 70s for coastal areas, 80s for inland spots and High Desert,
and 90s for the lower deserts. The cooling trend continues into
Monday as the low deepens to the north, encouraging stronger
southwest flow and a deeper marine layer both Monday and Tuesday.
While moisture isn`t robust, both mornings may feature a deep
enough marine layer to be sufficient for some localized patchy
drizzle.

The low will become absorbed into the larger trough and sweep
northward on Tuesday. Behind it, CA will be in transition between a
strengthening high over Texas and a large Canadian low north of
Washington. The Texas high will attempt to elongate and return the
region to at least weak ridging, but influence from the Canadian low
may limit the heating towards the end of the week. For now, high
temperatures should return to near normal or slightly above,
especially for Thursday/Friday as the ridge becomes compressed by
the low propagating eastward along the Canadian/US border. This
could shift the thermal axis closer to CA and increase
temperatures on Thursday around 10-15 degrees compared to Monday`s
cooler temps. The spread remains wide on high temps later this
week, but confidence is growing on a backdoor trough behind the
main Canadian low that will stretch down to CA sometime over the
weekend, likely cooling temperatures back to normal by Sunday as
the ridge is pushed east.

Models are beginning to show the development of a tropical cyclone
south of Baja late this week. NHC indicates that there is about a
90% chance of development here in the next 48 hours, although there
is no confidence on track until after formation. Early indications
suggest should something form, the elongated high may influence the
propagation of such a system, and we will keep a close eye on any
development here or if there are any impacts associated. Further
information can be found at NHC`s website at nhc.noaa.gov.

&&

.AVIATION...
201800Z...Coast/Valleys...Partially masked on satellite by SCT-BKN
high clouds, lower marine layer clouds (based near 2500ft MSL) that
spread well inland overnight have begun to erode across the inland
valleys. Under the influence of a larger storm system spreading into
California, SCT marine layer clouds look to remain present along the
coasts into the afternoon. Intermittent BKN CIGs remain possible
into the afternoon. Come 00z Monday, expecting these low clouds to
redevelop and surge back inland, filling the entire coastal basin by
09z. Bases will generally lift to around 2500-3000ft MSL, but could
lift above 3000ft overnight. Clouds begin to scatter around 16-18z,
but could take until as late as 21z Monday to fully clear out.

Mountains/Deserts...VFR conditions prevail into Monday. BKN high
clouds AOA 20000 ft MSL becoming SCT to FEW in the afternoon and
overnight.

&&

.MARINE...
No hazardous marine conditions are expected through Friday.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.

&&

$$

PUBLIC...Zuber
AVIATION/MARINE...Munyan

NWS San Diego (SGX) Office