Rancho Cucamonga, California Weather Forecast Discussion

708
FXUS66 KSGX 041646
AFDSGX
Area Forecast Discussion
National Weather Service San Diego CA
946 AM PDT Sat Oct 4 2025
.SYNOPSIS...
Weak offshore winds this morning will give way to the typical sea
breeze west of the mountains in the afternoon. Temperatures this
afternoon will be warmer than yesterday outside of the immediate
coast and low deserts, but still near to below the daily average.
Daily marine layer clouds will likely make a comeback early Sunday
morning, with more widespread low clouds early next week. Little
change in temperatures through Monday, followed by 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, bringing a slight chance of showers, mostly over the
mountains.
&&
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...
.SHORT TERM...
The upper level low that brought cooler temperatures and breezy
winds to the region yesterday has moved to the northeast into the
Intermountain West this morning, leaving us in broad, weak
troughing. As a result, winds are calm outside of a few typically
windy gaps like the Banning Pass into the northern Coachella
Valley and through the In-Ko-Pah Gorge just east of the San Diego
County line, which are still seeing gusts to 20-30 mph. Elsewhere,
the movement of the low has resulted in weak offshore (generally
north to northeast) flow. This is contributing to the mostly clear
skies in the area, outside of a scattering deck around 1500 feet
in the more sheltered Inland Empire and northern Orange County.
More typical, mild SoCal weather is expected today into early next
week. Winds will shift to the west starting at the coast early
this afternoon as the sea breeze develops, then move inland into
the late afternoon. High temperatures today will be a few degrees
(2-4 F) above yesterday`s outside of the immediate coast and low
deserts, where temperatures will be about the same as yesterday to
even a tad cooler. Despite this mild warmup, temperatures remain
near (along the coast) to 5 to 10 degrees below the daily average
temperature.
A broad trough remains over the western U.S. into early next
week, and a shortwave digging down the western side of the trough
will develop into a weak low off the south-central California
coast. This will result in a very subtle cooling trend Sunday
through Monday for most of southwest California, but in practice
temperatures will still be very similar to today. However, it
will shift the flow to a more onshore regime, which will bring
nightly and morning low clouds back to our coastal areas. Low
clouds will struggle to form most of tonight, but an eddy
circulation developing early Sunday morning will moisten things up
enough to get at least patchy low clouds along the San Diego and
likely northern Orange County coastline. Low clouds Sunday and
Monday nights will be earlier to develop and more widespread.
&&
.LONG TERM...
The weak closed low will open into the broader trough and move to
the northeast late Tuesday into Wednesday as the west end of a
broad high over the southern U.S. nudges into the area. This will
result in a warming trend Tuesday through the mid to latter half
of next week. Meanwhile, a large low pressure system will drop
south out of the Gulf of Alaska and make its way down the West
Coast. Simultaneously, a developing tropical system south of Baja
California will gradually make its way northwest through next
week, likely leading to some amount of remnant moisture moving
into Southern California by late Thursday/Friday.
There is significant differences in model output for where the
low from the northwest will set up, as well as the track of the
tropical system along with how much moisture from it will advect
into the area, leading to a low confidence forecast for both
temperatures and possible convective rainfall for the latter part
of next week. For now, the forecast is for near zero chances for
rainfall increasing to a small chance (10-15%) by late Thursday,
then with slight chances (15-20%) over the mountains and parts of
the deserts and less than 15% chances elsewhere Friday and
Saturday. Moisture would be fairly high-based, resulting in those
higher chances over the mountains, but if we get strong enough
forcing from 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...
041600Z....Coasts/Valleys...Patchy low clouds with bases 1500-3000
ft MSL will scatter or clear by 17Z. Patches of low clouds may
develop after 03Z Sun. Less than 30% chance for cigs at coastal TAF
sites before 12Z Sun. Any clouds that develop will be patchy in
coverage with bases 1500-2500 ft MSL.
Mountains/Deserts...VFR conditions will prevail into Sun.
&&
.MARINE...
No hazardous marine conditions are expected through Wednesday
morning.
&&
.SGX WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&
$$
PUBLIC...CSP
AVIATION/MARINE...CO

NWS SGX Office Area Forecast Discussion