South Fontana, California 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

937
FXUS66 KSGX 301030
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
330 AM PDT Mon Jun 30 2025

.SYNOPSIS...
Continued hot for the deserts, although slightly cooler elsewhere
today. A shallow marine layer will continue to moderate
temperatures near the coast, with low clouds and fog extending 10
to 15 miles inland during the nights and mornings. Temperatures
begin to trend downward tomorrow west of the mountains, then in
the mountains and deserts for Wednesday through the end of the
weekend. During this time, the marine layer deepens somewhat,
extending its influence further inland.

&&

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

Today through tonight...

The subtropical ridge axis is continuing to shift towards the east
ever so slightly. The amplification of the ridge is going to allow
for another hot day for our deserts, with values likely exceeding
the 110F degree mark within the Coachella Valley and for portions of
the lower deserts in San Diego County. There is some increased
moisture in the lower levels due to southeasterly flow east of the
mountains which will allow for heat indices to be elevated. However,
due to there being mostly moderate coverage of HeatRisk, and only a
few areas of major impacts from the heat in less populated areas, an
Excessive Heat Warning has not been issued. Regardless of this, it
is recommended to be extra cautious if planning to be outdoors
within these areas, and be sure to stay hydrated to avoid any heat-
related illnesses. For locations west of the mountains, temperatures
today will be slightly cooler from where they were yesterday, with a
slight enhancement of onshore flow. The marine layer this morning
will continue to erode from east to west. There could be some areas,
especially within the wind-sheltered inland valleys and along the I-
15 corridor, with foggy conditions this morning. Most locations will
clear by 9 AM this morning, despite a few areas along the coast that
could have mostly cloudy skies persisting into the early afternoon.
These low clouds will regather and fill back in around the usual
time later this evening.

Tomorrow through Thursday...

An upper level low is going to continue to form off the coast of
California, nearly due west of Pt. Conception and then propagate
towards the southeast ever so slowly going into the afternoon hours
tomorrow. As it does, it is going to allow for some of the mid-level
subtropical moisture to be brought up and over the CWA. This, along
with some weak instability from the U/L low, could allow for there
to be a few showers and possibly a thunderstorm to develop over the
higher terrain and deserts by later in the afternoon, as a consensus
of the deterministic models have been alluding to in recent model
runs, especially the Canadian. High-res models have not been as
"gung-ho" about this, and have kept any precipitation further east
of the CWA. Regardless, there will be a very slight chance (less
than 10 percent) of a rogue shower and/or storm popping off later
this afternoon over the mountains/deserts.

Some models have also kept a very slight chance of precipitation
over the entire CWA going into Wednesday and Thursday, as the U/L
low continues to move overhead and off to the northeast as it
becomes absorbed back into the longwave trough, although given how
weak the instability is and how moisture-starved this feature is,
confidence was too low to increase PoPs above what the NBM was
already displaying. The trough and associated U/L slowly
transitioning over the region will allow for there to be a very
subtle and gradual cooldown taking place each day throughout this
period of the forecast. The marine layer should deepen and slightly
raise, given the lower heights aloft, but coverage will remain
similar to where it currently is now, with perhaps only slightly
further penetration eastward during the overnight hours due to
heights being more elevated.

Friday through the weekend...

By this period in the forecast, there tends to be more of a
diversion of the models, although ensembles have remained more
consistent in displaying a flattening of the ridge and trough
upstream with a transition to a more zonal longwave pattern going
into the weekend, which will result in the slightly cooler
temperatures (slightly below the seasonal average for this time of
year) and dry conditions remaining fairly persistent through then.

&&

.AVIATION...
301000Z....Coast/Valleys...Low clouds based 800-1200 ft MSL have
finally filled into most of coastal SD County. Expecting Orange
County to follow shortly with clouds pushing up to 15 miles inland
over the next few hours. VIS restrictions (1-4SM) for elevated
western valleys. Patchy FG where clouds intersect terrain.
Scattering back to the coasts by 16-17z with full clearing likely in
the afternoon. Low clouds with slightly higher bases begin to push
inland again after 05z Tuesday. Similar inland extent with the IE
remaining SKC.

With small active fires burning in the Inland Empire, ISLD HZ/FU are
possible with reductions of VIS 3-5SM for eastern portions of the IE
through Monday.

.Mountains/Deserts...FU/HZ possible through the Banning Pass and
into the northern Coachella Valley due to active fires in the Inland
Empire. Once again, SW/W wind gusts 25-35 kts are expected for the
deserts and mountain passes Monday afternoon, leading to periods of
MOD UDDFS just east of the mountains. Otherwise, VFR conditions
prevail through Monday night.

&&

.MARINE...
Afternoon winds 10-15 knots Mon/Tue over the outer coastal waters
with gusts up to 20 knots near San Clemente Island. Otherwise, no
hazardous marine conditions are expected through Friday.

&&

.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...Stewey
AVIATION/MARINE...KW