401
FXUS66 KSGX 210940
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
240 AM PDT Thu Aug 21 2025

.SYNOPSIS...
The heat is expected to peak for most areas today and Friday with
slight cooling for the weekend and continued cooling early next
week to around average for the middle of next week. Monsoonal
moisture will increase into the weekend with increasing chances
for showers and thunderstorms over the weekend, most likely
during the afternoon and early evening, and mostly for the
mountains and deserts. There will be gradual drying of the
monsoonal moisture for the middle and latter part of next week
with decreasing chances for showers and thunderstorms. Night and
morning coastal low clouds and fog will be patchier and extend
locally inland into coastal areas.

&&

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

.SHORT TERM (Today through Saturday)...
The heat is expected to peak for most areas today and Friday with
high temperatures 5 to 10 degrees above average for inland areas.
High temperatures through Friday will be in the mid 70s to lower
80s near the coast to the upper 90s to 108 for the Inland Empire
and for the Apple and Lucerne Valleys with 112 to 116 for the
lower deserts. High temperatures for inland areas on Saturday will
cool a few degrees.

The marine layer will remain around 1200 feet deep. Night and
morning coastal low clouds and fog will be patchier with the
greatest coverage over the coastal waters into adjacent coastal
areas.

Monsoonal moisture will increase into the weekend with increasing
chances for showers and thunderstorms over the weekend, most
likely during the afternoon and early evening, and mostly for the
mountains and deserts.

For Friday into Saturday, model guidance generally shows a mix of
elevated convection moving along in south to southeast flow aloft
as well as some diurnally-driven convection tied to the mountains.
The HREF only has isolated rainfall of one-tenth inch or less over
the mountains on Friday. Chances for measurable rainfall from
showers and thunderstorms will increase for the mountains and
deserts for Saturday afternoon and early evening, but with chances
for one-half inch or more of rainfall at 25 percent or less.

There could be a few daily high temperature records tied or broken
today and Friday, mainly for mountain locations. Nights will be
warmer for the weekend into early next week with greater potential
to tie or break daily records for record warm low temperatures.
That would not be limited mostly to the mountains but could also
occur for the deserts, valleys, and inland Orange County.

&&

.LONG TERM (Sunday through Wednesday)...
Greater chances for afternoon and early evening showers and
thunderstorms will continue through Tuesday though with daily
chances for one-half inch of rainfall mostly 25 percent or less.
High temperatures on Sunday will be a few degrees cooler than
Saturday followed by gradual cooling through the middle of next
week with Wednesday high temperatures around average.

&&

.AVIATION...
210800Z...Coast/Valleys...Patchy low clouds with bases 200 to 700 ft
MSL will develop along the coast 10Z-16Z Thu. Where low clouds
develop expect VIS reductions 1-4 SM, especially on coastal mesas.
Low clouds and fog likely to clear to the immediate coast by 17Z
Thu. Low confidence in cigs developing at coastal TAF site before
12Z Fri.

Mountains/Deserts...SCT-BKN clouds 15-20kft MSL through Thu. CU or
possible TCU over the mountains Thu afternoon, with a 10 percent
chance of iso -SHRA or -TSRA 21Z Thu-01Z Fri.

&&

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

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Heat Advisory until 8 PM PDT Saturday for San Bernardino and
     Riverside County Valleys-The Inland Empire.

     Heat Advisory until 8 PM PDT Friday for Riverside County
     Mountains-San Bernardino County Mountains-San Diego County
     Mountains-San Diego County Valleys.

     Heat Advisory from 10 AM this morning to 8 PM PDT Friday for
     Orange County Inland Areas-Santa Ana Mountains and
     Foothills.

     Extreme Heat Warning until 8 PM PDT Saturday for Apple and
     Lucerne Valleys-Coachella Valley-San Diego County Deserts-
     San Gorgonio Pass near Banning.

PZ...None.

&&

$$

PUBLIC...17
AVIATION/MARINE...CO