914
FXUS66 KSGX 181655
AFDSGX

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

.SYNOPSIS...
A warming trend is expected this week with less marine layer cloud
cover. Thursday through Saturday will be 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...

Visible satellite at 9 AM was showing low clouds had cleared most
coast and valley locations, with low clouds lingering over the
coastal waters. Mostly clear skies are expected to continue for
inland areas today, with the potential for some low clouds to linger
along the immediate coast. As the week goes on and an upper level
area of high pressure moves further west the marine layer will
become shallower.

An upper level high currently just east of the Four Corners region
will slowly migrate west through the week. Highs today are expected
to be warmer than yesterday for most locations, with the most
noticeable warming in the Inland Empire, mountains, and High Desert.
Highs will continue to warm a few degrees each day through Thursday.
NBM chances of the Inland Empire reaching or exceeding 105 degrees
on Thursday is 70 to 80 percent and 80 to 90 percent chance of
reaching or exceeding 95 degrees in the mountains below 5000 ft. The
low deserts have over 90 percent chance of exceeding 110 degrees
Thursday, but almost no chance of exceeding 120 degrees. The chances
of exceeding those higher temperature thresholds in the deserts will
likely be tapered by an increase in monsoonal moisture. By Friday,
there may be a few degrees of cooling as the high weakens and is
in an ideal position to allow a more significant influx of
monsoonal moisture to make it into the area.

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 to 25 percent chance of showers and thunderstorms over the
mountains and High Desert. Chances increase to around 20 to 45
percent 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 to 15 knots 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 percent 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...
181530Z...Coast/Valleys...Satellite shows low clouds are burning off
quickly with most coastal sites likely to be clear by 16-17z and
full clearing by 18z. Bases are largely 1100-1500 MSL with tops to
1700-2000ft MSL. 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 06z Tuesday.

Mountains/Deserts...Mostly clear with VFR conditions prevailing
through 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-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...CO/SS
AVIATION/MARINE...Zuber