558
FXUS66 KSGX 010344
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
844 PM PDT Sun Aug 31 2025

.SYNOPSIS...
Warm and dry through tomorrow, with monsoonal moisture increasing
by Monday evening. Showers and thunderstorms possible most of the
day Tuesday and each afternoon in the mountains through Thursday.
Slightly cooler temperatures and a lesser chance for storms in
the mountains to end the week, with this pattern prevailing into
the weekend. After Wednesday, the marine layer will begin to
deepen and make it further inland each morning.

&&

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

Fair weather cumulus developed over the mountains again but no deep
convection could develop due to the stable conditions even though
the sounding showed nearly an inch of precipitable water. The low
clouds and fog cleared out of the coastal areas this afternoon
and remain well offshore at this hour. High resolution models show
good chances for low clouds and fog to return to the coastal areas
late tonight but not spreading more than a few miles inland.

From previous discussion...
With an amplifying ridge over the Four Corners and the thermal
ridge axis stretching along the Colorado River Valley, heat will
peak tomorrow for most areas including the valleys, Orange County,
and Inland Empire. Highs will reach 6-10 degrees above normal on
Monday, the upper 90s to 106, and a Heat Advisory is in effect for
both Monday and Tuesday for the aforementioned areas. The deserts
will also be abnormally warm, sitting at or above 110 tomorrow,
but monsoonal moisture will start creeping up late Monday and may
dampen highs on Tuesday, potentially 5-8 degrees below where they
will be on Monday. Similar to the past few mornings, the patchy
marine layer will struggle to push more than a few miles inland,
but locations directly along the coast and coastal terrain may
experience brief, dense fog Monday morning with visibility at or
below 1/2 mile. If you will be on the roads Monday morning, drive
with caution and leave extra time to reach your destination
safely.

Monsoonal moisture will begin to advect northwestward Tuesday
morning with an inverted trough, potentially allowing some
showers to develop across much of the region west of the
mountains. Rain rates at times may exceed 0.10"/hr, especially if
any weaker storms manage to develop. For now, localized rain
showers are the primary concern for Tuesday morning, with total
accumulations across the valleys and inland areas possibly
reaching 0.05-0.25" before 1 PM. If the rain does pan out, ample
cloud cover on Tuesday may limit the thunderstorm threat across
the mountains for Tuesday afternoon. PW values of around 1.5" in
the mountains and increased instability indicate a solid chance of
heavy rainfall with any thunderstorm, but should clouds prevail,
storms are unlikely to materialize as currently expected. For now,
the thunderstorm threat looks to extend from the San Diego County
Mountains up to the San Bernardino Mountains where the main
moisture corridor will reside. Lighting, very gusty winds, and
heavy rainfall with rates around 0.5"/hr will all be possible with
any strong storm. Confidence is only moderate on any storms that
develop migrating from the mountains and into the deserts and/or
far eastern urban areas. Any storms that develop should subside by
sunset on Tuesday.

While Tuesday continues to look like the best chances for storms,
monsoonal moisture may stick around into the end of the week. The
ridge will become compressed and stretched up into Canada as a
massive low moves over the Great Lakes region and a separate low
moves down into the PacNW. The thermal ridge axis shifts east, and
temperatures will begin a gradual cooling trend Wednesday through
the weekend. Have maintained at least slight chance for thunderstorms
across the mountains each day through the end of the week,
however, there is high uncertainty in the monsoonal moisture as
flow becomes more southwesterly after Wednesday. If any storms do
manage to develop, there is a very low risk of them getting off
the mountains and into the urban areas. Models continue to
struggle significantly with the upper level pattern into the end
of the week, with some keeping the monsoonal moisture around while
others favor the more dry, zonal flow pattern. The interesting
trend to keep an eye on may be a decaying tropical system moving
up along the Baja Peninsula which, similar to how Juliette did
last week, may bring some limited tropical moisture to at least
San Diego County. Until models become better aligned for the end
of the week and into the weekend, the probability of this
occurring is low as latest forecasts trends maintain the system
too far west of the coastline for us to benefit as the low over
the PacNW moves down. At this point, the most probable solution
will be a cooling and drying trend for the end of the week and
into the weekend.

&&

.AVIATION...
010230Z...Patchy low clouds based 400-700 ft MSL will build over
coastal waters this evening. Clouds push into coastal San Diego
County after 08Z Mon covering up to 10 miles inland. Later onset of
clouds for Orange County with slightly less inland extent. 40-50%
chance of BKN cigs at KSNA. Areas of vis 1-3 sm (locally under 1sm
on higher coastal terrain) with clearing 15-17Z Mon. Low clouds may
struggle to redevelop at the coastline Monday evening as high cloud
cover increases.

Otherwise...VFR conditions for mountains and deserts with SCT
cumulus developing over the mountains AOA 10,000 ft MSL Monday
afternoon.

&&

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

&&

.BEACHES...
A long-period (15-17 sec) southerly swell (180-200 degrees) will
bring elevated surf through Tuesday, gradually subsiding
thereafter. Surf of 3-6 feet with occasional sets to 7 feet for
south-facing beaches. Longshore currents along west-facing beaches
with a high rip current risk for all beaches. Beach Hazard
Statement in effect through late Monday.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement through Monday evening for Orange County
     Coastal Areas-San Diego County Coastal Areas.

     Heat Advisory from 10 AM Monday to 6 PM PDT Tuesday for Orange
     County Inland Areas-San Bernardino and Riverside County
     Valleys-The Inland Empire-San Diego County Valleys-Santa
     Ana Mountains and Foothills.

PZ...None.

&&

$$

PUBLIC...PG
AVIATION/MARINE/BEACHES...KW

NWS Tucson (SGX) Office