321
FXUS66 KSGX 312012
AFDSGX

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

.SYNOPSIS...
Warm and dry today and 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...

With a lackluster marine layer this morning temperatures quickly
rose, reaching around 5 degrees above normal today. The heat will
push many people to the beach both today and tomorrow, where a
strong southerly swell is creating elevated surf and a high rip
current risk - swim with caution and always near a lifeguard. 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 both today and
tomorrow, but monsoonal moisture will start creeping up late
Monday and may dampen highs both Monday and Tuesday, potentially
5-8 degrees below where they will be today. 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.

With the ridge firmly in place by early week, monsoonal moisture
will advect northwestward Tuesday morning, 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
1PM. 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...
312015Z...Mostly clear this afternoon with VFR conditions. BKN low
clouds with bases 400-700 ft MSL will build into the area after 08Z
Mon covering up to 10 miles inland across San Diego County, slightly
less across Orange County with moderate to high confidence. This
will be accompanied by areas of vis 1-3 sm (locally under 1sm on
higher coastal terrain) with clearing 15-17Z Mon.

Otherwise...VFR conditions for mountains and deserts with SCT mid-
level clouds developing over the mountains AOA 10,000 ft MSL during
the afternoons.

&&

.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.

&&

.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...Zuber
AVIATION/MARINE/BEACHES...APR