Upland, California Weather Forecast Discussion

426
FXUS66 KSGX 210351
AFDSGX
Area Forecast Discussion
National Weather Service San Diego CA
851 PM PDT Sat Sep 20 2025
.SYNOPSIS...
Low clouds will spread into the far western valleys tonight.
Tropical moisture returns Sunday 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.
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...
Evening update...
Water vapor imagery shows dry southwest flow ahead of an upper
level low off the Central California coast and tropical moisture
over central and southern Baja lifting north. Some of this
moisture will become entrained in the southwest flow and moves
into So Cal on Sunday as the upper low drops southward. This will
bring scattered light to locally moderate showers on Sunday. There
is little to no elevated instability, which will limit both
rainfall rates and thunderstorm potential. NBM has thunderstorm
chances below 5% across most of the forecast area, locally up to
8% over the southern San Diego County mountains Sunday afternoon.
This plume of moisture shifts south and east Sunday night into
Monday morning with the more widespread precipitation coming to an
end. Some mid-level moisture in weak easterly flow lingers into
Monday afternoon for a slight (15-25%) chance of showers and
thunderstorms over the San Diego County mountains Monday
afternoon.
No forecast changes this evening, and Tue- Wed remain highly
questionable as any precipitation/thunder chances are very
sensitive to the track of the cutoff low as it meanders off the CA
coast.
Previous discussion...
The destabilizing environment will wreak havoc on our marine
inversion so that coastal low clouds incrementally decrease each
night and morning. The upper low seems to cut off and deepen
Tuesday, adding a little more energy to the system and also
concentrating and deepening the moisture column. That`s when
chances of measurable rain and instability increase, enough for
deeper convection and a small chance of thunderstorms. But this is
when the range of outcomes widens. In a new development, many
ensembles now do not bring the plume directly into our region
Tuesday, but rather a glancing blow as the main plume hits to our
west. The chances and forecast amounts of rainfall have come down
a little for the three days.
Finally, the moisture pushes out to the north on Wednesday,
leaving only a slight chance of showers and thunderstorms in the
mountains and desert of San Bernardino County. Most solutions
weaken the cutoff low over the west, north of SoCal, to bring dry
weather. With a weak trough over the West, high temperatures will
continue mild, a few degrees below average, through the end of the
week. In the absence of the tropical moisture, the marine layer
should be able to tepidly rebound and rebuild and bring more
coastal clouds.
&&
.AVIATION...
210230Z....Coast/Valleys...Low clouds based 600-1000 ft MSL have
pushed a few miles inland along much of the SD county coastline.
These clouds will slowly push inland to around 5 miles in Orange
County, reaching around KSNA, and about 10 miles in SD County,
reaching to the I-15 corridor, by 12z. Scatter out 15-17z to the
coast. SHRA will begin to move into the region generally after 12z,
with more widespread SHRA after 15z. Locally lower VIS and CIGs with
any shower.
Mountains/Deserts...VFR conditions through early Sunday. Mid and
high level clouds AOA 10,000 ft MSL start to build overnight. 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 Sunday and again
Monday which could briefly lower visibly. Slight chance for a few
thunderstorms over the waters Monday night into Tuesday evening. Any
storm could briefly produce lightning, gusty erratic winds, and
choppy seas. Otherwise, no additional hazardous marine conditions
are expected through Wednesday.
&&
.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...SS/MM
AVIATION/MARINE/BEACHES...Westerink

NWS SGX Office Area Forecast Discussion