537
FXUS66 KSGX 190318
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
818 PM PDT Mon Aug 18 2025

.SYNOPSIS...
A warming trend is expected this week with less marine layer cloud
coverage. It will be hot inland through the weekend, with Thursday
and Friday as the hottest days. There is a chance of showers and
thunderstorms, mainly over the mountains and deserts Thursday
through at least Monday.

&&

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

Low clouds and fog are tentatively trying to gather along the
coast, but so far this evening they are only patchy. The building
high pressure aloft will continue to make things tougher on the
marine layer and its cloud deck in coming days. Tonight might not
even have a uniform cloud coverage along the coast. Look for even
patchier coastal low clouds and fog for the rest of the week. The
big story with this building high is the heat. Each day will warm
a few degrees, and Thursday and Friday appear locked in to be the
hottest days. Temps on those days will be 2-10 degrees above
average regionwide. Look for max temps reaching 100-106 degrees in
the Inland Empire and high desert, 110-114 in the low desert, and
90s to 102 in western valleys and eastern coastal areas. Hence
the Extreme Heat Watch we have in effect for all areas except the
coastal strip. Increased surface humidity from occasional gulf
surges into the low desert will literally dampen the potential for
temps to rise. A very subtle weakening of the upper high takes
place over the weekend, allowing for a tepid, barely noticeable
decrease in temps. The gulf surges, combined with increasing
monsoonal moisture aloft will gradually deepen. That will
destabilize the atmosphere and allow for deep convection and
thunderstorms to form. Through Wednesday these factors appear
insufficient to produce thunderstorms, but on Thursday we`ll
probably get our first isolated ones in the mountains during the
afternoon. Deeper moisture Friday through the weekend will
increase our chances of showers and thunderstorms, and chances
will extend to deserts and into the evenings. We also can`t ignore
the small potential for storms to move west of the mountains or
occur overnight. For now those chances are so small that we`re
not confident enough to forecast that extracurricular tstorm
activity yet. While a couple degrees get shaved off the heat this
weekend, it appears the monsoon moisture remains pretty deep and
the instability sufficient for continued chances of showers and
thunderstorms at least in the mountains and deserts at least
during the afternoons and early evenings. It might not be until
next Monday or Tuesday, when most guidance is showing a subtle
drying trend, that we`ll lose the chance of thunderstorms.

&&

.AVIATION...
190300Z...Coast/Valleys...Patchy low clouds have started to develop
this evening, mostly offshore. Clouds based 700-1000 ft MSL will
continue to become more widespread and start to move inland to up to
10 miles after about 04Z, but remain patchy and somewhat
intermittent in coverage until late tonight. 60% chance for CIGs at
KSAN/KCRQ by 06Z, increasing to 80% by 12Z (30% and 60% chance for
KSNA by 06Z and 12Z respectively). VIS 1-5SM, locally lower possible
for coastal highlands, 06-16Z. Clouds clear to beaches 15-17Z.

Mountains/Deserts...VFR conditions prevailing through Tuesday,
increasing high level clouds ~15k ft MSL through the day Tuesday.

&&

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

&&

.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Extreme Heat Watch from Thursday morning through Friday evening
     for Apple and Lucerne Valleys-Coachella Valley-Orange
     County Inland Areas-Riverside County Mountains-San
     Bernardino County Mountains-San Bernardino and Riverside
     County Valleys-The Inland Empire-San Diego County Deserts-
     San Diego County Mountains-San Diego County Valleys-San
     Gorgonio Pass near Banning-Santa Ana Mountains and
     Foothills.

PZ...None.

&&

$$

PUBLIC...MM
AVIATION/MARINE...CSP