591
FXUS66 KSGX 180959
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
259 AM PDT Mon Aug 18 2025

.SYNOPSIS...
High pressure will build this week, bringing a warming trend and
less marine layer cloud cover. Thursday through Saturday will be
the hottest days. Monsoon moisture and the chance of showers and
thunderstorms will increase, 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...

Marine layer low clouds continue to fill in over the coastal areas
and western valleys this morning, and should clear back to the
coast by late morning. Elsewhere clear skies will prevail.

An upper level high centered near West TX this morning will begin
to shift westward today, displacing the long wave trough along the
west coast to the northwest. This will bring a few degrees of
warming with high temperatures around 3-5 degrees below normal for
the mountains westward and near normal elsewhere. The warming
trend continues through the week for most areas as the high
settles near the Four Corners region and strengthens. Thursday and
Friday still look to be the hottest days of the week with highs
around 3 to 8 degrees above normal. While the marine layer will be
fairly shallow with patchy cloud coverage during this period,
enough onshore flow will persist to moderate temperatures near the
coast. High temperatures remain questionable in the lower deserts
with the return of monsoon moisture (more on that later), which
may limit daytime heating. That said, any moisture will add to the
Heat Index during the day and keep temperatures elevated
overnight. Given this, an Extreme Heat Watch was issued for the
Inland Empire, mountains excluding the Santa Anas, and deserts for
Thursday and Friday, though the mountains and IE will likely be
more in the "advisory" category with widespread moderate HeatRisk.
The high weakens slightly next weekend into early next week for
minor cooling.

As far as monsoon moisture is concerned, moisture will begin to
increase on Wednesday. However, most of the moisture is above 700
mb with little in the way of surface based instability, so
forecast remains dry. Conditions become a little more favorable on
Thursday as the moist layer begins to lower and weak instability
develops for a 15-25% chance of showers and thunderstorms over the
mountains and high desert. Chances increase to around 20-45% and
expands into the lower deserts Friday through Monday, though the
best chances remain over the mountains and along the sea breeze
front in the high desert. With mid-level easterly flow of 10-15 kt
it`s not out of the question that a couple storms could drift west
of the mountains into the far inland valleys on Friday and
Saturday, but for now have kept PoPs below 15% in these areas.
Will continue to monitor the potential for convection along the
Elsinore Convergence Zone as well, though considering not much has
changed since yesterday the chances remain low.

&&

.AVIATION...
180900Z...Coast/Valleys...After a period of patchiness across San
Diego County, clouds have filled in nicely through 15-20 miles
inland and are spreading through Orange County as well. Bases are
largely 1100-1500 MSL but are sloped lower into Orange County where
bases are closer to 800-1000ft MSL. These clouds may briefly reach
KONT for 12-16z but there is only a 30% chance for long duration
CIGs during this time. Clouds scatter back to the coast by 17-18z
Monday morning with full clearing by 19z. Low clouds begin to fill
back in after 02z Tuesday, but will likely be more patchy. Bases
will be lower than this morning, closer to 700-1000ft MSL, with a
60% chance for CIGs at KSAN/KCRQ by 09z Tuesday.

Mountains/Deserts...Mostly clear with VFR conditions prevailing
through early Tuesday morning.

&&

.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-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
     Gorgonio Pass near Banning.

PZ...None.

&&

$$

PUBLIC...SS
AVIATION/MARINE...Munyan