Upland, California Weather Forecast Discussion

902
FXUS66 KSGX 040957
AFDSGX
Area Forecast Discussion
National Weather Service San Diego CA
257 AM PDT Sat Oct 4 2025
.SYNOPSIS...
Gusty west winds in the mountains and deserts will weaken through
the morning. Weak offshore winds and warming west of the
mountains Saturday. Another brief cool down for early next week,
with a gradual warming trend into the middle/latter part of next
week. There is the potential for tropical moisture to move into
the area late in the week with a slight chance of showers over the
mountains.
&&
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...
Morning water vapor imagery shows the upper level trough axis
moving into the Colorado River Valley. West winds are gusting to
around 25-40 mph on the desert mountain slopes and through the
passes, locally up to 50 mph below the San Gorgonio Pass. Winds
will continue to weaken through the morning as the trough moves
off to the east and surface pressure gradients relax and turn
weakly offshore to the north. The 00Z KNKX sounding had a very
weak marine inversion, and as a result low cloud coverage remains
very patchy. Any low clouds will clear by mid morning. Otherwise
warmer today for most areas, except the lower deserts where highs
will be a couple degrees lower than Friday.
A weak coastal eddy is forecast to develop tonight, which would
help with low cloud development over the coastal areas. Weak
positively-tilted troughing remains across the Western US on
Sunday, then a closed low develops and retrogrades off the Central
CA coast on Monday, then opens up and moves off to the northeast
as another low drops out of the Gulf of Alaska and digs off the
West Coast. Overall not much change for Sunday through Tuesday
with highs around 3-6 degrees below normal and night and morning
low clouds over the coastal areas and western valleys.
By Wednesday evening ensembles really begin to diverge on the
placement of the low as it drops down off the Pac NW coast. Most
EC members have a track closer to the coast, the GEFS is more
offshore, and the GEPS is a mix of the two. Not only will this
have implications for high temperature forecasts for the latter
half of the week, it will greatly influence where tropical
moisture tracks and how much forcing will coexist with it. The
track of the tropical system itself also varies greatly. Needless
to say, there is a lot of uncertainty as we head into the latter
half of the week. For now have slight (15-20%) chances of showers
over the mountains and parts of the deserts Fri-Sat as moisture is
fairly high based, but if we get strong enough forcing (troughing
nearby), showers west of the mountains would not be out of the
question, as indicated by a handful of ensemble members. Currently
instability appears too weak to include any thunderstorms in the
forecast.
&&
.AVIATION...
040930Z....Coasts/Valleys...Patchy low clouds based 2500-3500 ft
will remain scattered and intermittent through this morning. Any
clouds that remain after sunrise should scatter out between 15-17Z.
Low clouds will struggle to redevelop Saturday night. Less than 30%
chance for cigs at coastal TAF sites before 12Z Sun. Any clouds that
develop should have similar bases and remain confined to coasts and
western valleys.
Mountains/Deserts...Elevated west winds will continue to subside
this morning. Otherwise, VFR conditions prevailing through Saturday
night.
&&
.MARINE...
Winds and seas will continue to gradually diminish through this
morning. Otherwise, no hazardous marine conditions are expected
through Wednesday morning.
&&
.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.
&&
.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory until 5 AM PDT early this morning for San Diego
     County Deserts-San Gorgonio Pass near Banning.
PZ...None.
&&
$$
PUBLIC...SS
AVIATION/MARINE...KW

NWS SGX Office Area Forecast Discussion