024
FXUS66 KSGX 200418
AFDSGX

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

.SYNOPSIS...
Gradual warming this week with the peak of the heat Thursday and
Friday. Monsoon moisture will move into the region with chances of
showers and thunderstorms each afternoon and early evening Friday
into early next week, mainly over the mountains and deserts.
Very patchy low clouds and fog will continue along the coast each
late night into the morning.

&&

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

Evening update...
A small patch of low clouds is moving south along the southern San
Diego County coast and dissipating this evening. With forecast
continued northerly flow tonight, expect any low cloud development
in the coastal areas to be very late and with patchy coverage.
Otherwise just a few high clouds overnight ahead of the initial
push of monsoon moisture. No changes to the forecast this evening,
though based on latest high-res guidance, thunderstorm chances
west of the mountains look pretty minimal, especially beyond
Friday when mid-level winds gradually veer from easterly to
southerly over the weekend.

Previous discussion...
A 598mb area of high pressure near the Four Corners region will
continue to expand across the Desert Southwest closer to
California. Temperatures 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
on Wednesday, or 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...
200330Z...Coast/Valleys...Isolated patches of low clouds are
intermittently forming offshore this evening, and there is moderate
confidence in these clouds becoming more widespread after 07Z this
evening, albeit likely still patchy, with a 60-70% chance of CIGs
reaching coastal airports 09-12Z. Bases expected to be 500-800 ft
MSL, with isolated areas down to 200-300 ft MSL possible, and vis
restrictions 1-3 SM are expected where clouds are present for areas
just inland of the immediate coast, including KCRQ. 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...SS
AVIATION/MARINE...CSP