776
FXUS66 KSGX 020353
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
853 PM PDT Mon Sep 1 2025

.SYNOPSIS...
Increasing monsoonal moisture on Tuesday with a chance of showers
and thunderstorms area wide. Chance for showers and thunderstorms
will be confined to the mountains and deserts with lowering
chances each day starting on Wednesday. Other areas will start to
see a cooling trend by Thursday into the weekend as an area of low
pressure offshore moves closer to the region.

&&

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

Evening update... High clouds are developing over the coastal
waters and into the western portions of our forecast area this
evening ahead of an upper level low of the coast. There are some
hints of marine layer low clouds along the San Diego County coast,
which should fill in and spread a few miles inland by Tuesday
morning.

The big story is the upper level easterly wave/inverted trough
which will rotate around the upper high and lift northward through
Southern California early Tuesday morning through Tuesday evening.
This is forecast to bring widespread showers and a 5-15% chance of
thunderstorms by Tuesday morning with High-Level Total Totals of
around 30 and elevated MUCAPE of around 200-600 J/kg. NBM
thunderstorm probabilities increase to around 30% over the
mountains and deserts in the afternoon, though this would probably
be dependent on enough thinning of the mid/high cloud cover to get
enough heating for surface-based convection. Despite the storms
being elevated (bases 10-15 kft MSL) and weak instability, hi-res
models are producing peak hourly rainfall amounts of 0.50"-0.75"
from the deep near-saturated layer and ensemble mean PW of around
1.5 inches. If strong enough convection manages to develop to
produce such rates, expect it to be fairly isolated with rates of
around 0.30"/hr or less in the majority of the showers. This wave
lifts off to the north with showers/thunderstorms ending late
Tuesday night or early Wednesday morning. These storms will also
be capable of producing gusty winds, especially Tuesday morning
when a deeper dry sub-cloud layer exists. Otherwise highs a few
degrees below normal in the mountains and deserts, near normal in
the valleys, and 3-5 degrees above normal near the coast.
Decreasing moisture on Wednesday with a trend towards the more
typical afternoon thunderstorms over the mountains and deserts.

Previous discussion...
The area of high pressure over the western states will begin to
weaken by Thursday, allowing for subtle cooling trend to occur.
Triple digit heat west of the mountains will move closer to
widespread 90s the valleys and 70/80s closer to the coast. By
Friday and Saturday, parts of the Anza Borrego desert and Coachella
Valley may stay below 100 degrees (a 50-50 shot at Palm Springs).
Monsoonal moisture will begin to slowly lower each day as the high
weakens with chances sinking below 30% across the mountains and
deserts by Saturday afternoon. By later in the week, model
ensemble show fair agreement in a trough developing off the West
Coast. This will bring drier air into the region and keep
temperatures closer to average as we head into the weekend and
early next week.

&&

.AVIATION...
020315Z...Coast/Valleys...Patchy low clouds will likely redevelop
tonight into Tuesday morning. Isolated CIGs 400-600ft MSL and VIS
1SM or less will be possible, generally after 09z Tue up to 5 miles
inland. -SHRA/-TSRA possible early Tuesday morning beginning around
sunrise nearly region-wide, with brief VIS reductions during any
shower/storm.

Mountains/Deserts...FEW-SCT clouds above 10000ft MSL through this
evening. -SHRA/-TSRA may move in after 12Z Tue, continuing through
much of the afternoon. Gusty and erratic winds, low visibility, and
lightning will accompany any storms that develop. SCT-BKN CIGs
generally above 15000ft MSL for much of Tuesday, although locally
lower with storms.

&&

.MARINE...
Showers are likely over the marine waters late tonight into early
Tuesday afternoon, with an isolated thunderstorm or two possible.
Gusty winds and low visibility near any storms that may develop.
Otherwise, no hazardous marine conditions are expected through
Friday.

&&

.BEACHES...A long-period southerly swell will gradually subside
tonight. A Beach Hazards Statement is in effect until 11 pm.

&&

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


&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement until 11 PM PDT this evening for Orange
     County Coastal Areas-San Diego County Coastal Areas.

     Heat Advisory until 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...SS
AVIATION/MARINE/BEACHES...PG