374
FXUS66 KSGX 192026
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
126 PM PDT Tue Aug 19 2025

.SYNOPSIS...
Warmer weather is expected by the middle and latter part of the
week. As high pressure moves closer to the region, the area will
see an increase in monsoonal moisture by Friday into early next
week. The marine layer will continue to be thin, leaving morning
fog closer to the coast.

&&

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

A 598mb area of high pressure near the Four Corners region will
continue to expand across the Desert Southwest closer to
California. Temperatures currently in the 80s/90s over many
inland areas will rise further into this week with 850 mb
temperatures expected to move closer to 30C by Wednesday through
Friday. Temperatures will increase another 5 to 8 degrees today
vs. Wednesday, where temperatures will go around 5 degrees above
normal. The marine layer will continue to thin, with morning fog
on Wednesday being confined to areas within 10 miles of the
coast, especially in San Diego County. The pressure system will
bring warmest conditions by Thursday and Friday with areas away
from the coast 5 to 10 degrees above normal. With increased
moisture from the monsoonal weather pattern, humidity will be on
the rise, especially for desert areas, which will see temperatures
near 115 degrees. Other areas in the Inland Empire and western
valleys will be in the 90s to lower 100s. Temperatures also will
not cool off as much at night with upper 60s to mid 70s for areas
below 5,000 feet along and west of the mountains. Please remember
to stay cool by drinking fluids and taking breaks if you work
outside. Temperatures will cool a few degrees by the weekend, but
still remain warm.

As the high further expands westward, monsoonal moisture will
slowly start to increase. Taking a look at forecast soundings and
other ensemble model data, the increase in this moisture will
begin on Thursday. With limited instability, chances for any
thunderstorm activity remain below 10 percent on Thursday
afternoon. Low level moisture will increase by Friday into the
weekend, as the high moves closer to our region. With MUCAPE
values 300-600 J/kg, veering winds aloft, and enough moisture in
place, Friday afternoon will see the first chance of afternoon
thunderstorms. The area of high pressure will become stagnant into
the weekend with increased PWAT and low level moisture, which
will contribute to the highest confidence for thunderstorm
activity. The storm activity will mainly be confined to the
mountains and deserts, with the best chance of drifting storms off
the mountains to western valleys and even coastal areas during
the weekend afternoons with near a 10-20% chance. We continue to
monitor rain rates with these storms, but it is still too early to
pin down exact rates, but mountain areas may see rates one
quarter to one half of an inch. Any thunderstorm activity that
does occur will include cloud to ground lightning, brief heavy
downpours and gusty winds.

Ensemble models show the area of high pressure starts to break
down further by Monday and Tuesday of early next week. This will
help cool off temperatures near to slightly above normal levels.
Though the heat will lessen, the monsoon pattern with higher
humidity looks to linger into early next week. This will continue
the chance for afternoon and evening thunderstorms each day across
the mountains and deserts, even into the adjacent foothills and
valleys west of the mountains.

&&

.AVIATION...
192000Z...Coast/Valleys...Patchy low clouds with bases 700-1300 ft
MSL are lingering along the immediate coast this afternoon. Moderate
confidence in low clouds becoming more widespread around 00z Wed
with a 60-70% chance of CIGs reaching coastal airports between 08-
10z Wed. Bases will be 500-800 ft MSL, locally down to 200 ft MSL,
with vis restrictions 1-3 SM in BR. Low clouds expected to scatter
out 15-17Z Wed.

Mountains/Deserts...VFR conditions prevailing through Wed with high
clouds ~15-20k ft MSL.

&&

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

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Heat Advisory from 10 AM Wednesday to 8 PM PDT Saturday for San
     Bernardino and Riverside County Valleys-The Inland Empire.

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

     Extreme Heat Warning from 10 AM Wednesday to 8 PM PDT Saturday
     for Apple and Lucerne Valleys-Coachella Valley-San Diego
     County Deserts-San Gorgonio Pass near Banning.

PZ...None.

&&

$$

PUBLIC...APR
AVIATION/MARINE...CO