786
FXUS66 KSGX 030413
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
913 PM PDT Tue Sep 2 2025

.SYNOPSIS...
Widespread showers and thunderstorms will return to the mountains
and deserts Wednesday afternoon as monsoonal moisture persists.
Locations west of the mountains will see warmer and humid weather
through Wednesday as well. A cooling and drying trend will occur later
in the week over the entire region. There will be minimal chances
for monsoon thunderstorms by the weekend into early next week as
temperatures stay near average.

&&

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

An inverted trough moving northward brought widespread showers and
numerous thunderstorms to the region today. The showers and tstms
have moved to the northeast with the system, leaving more stable
conditions over extreme southwestern CA. More convection is
unlikely, but not impossible tonight. The marine layer remains in
place but any low clouds have been pushed offshore, and high-
resolution models show limited low clouds returning to the coastal
areas late tonight.

From previous discussion...
There will be clearing overnight as this system exits the area.
This will lead to a more typical monsoonal weather pattern as
daytime heating and instability aid in thunderstorms forming over
the mountains and deserts Wednesday afternoon. This will produce
another round of locally heavier rain, gusty winds and cloud to
ground lightning. The best chance will be seen over the mountains,
but some storms may form in the valleys of San Diego County and
the eastern half of the Inland Empire. HREF hourly rain rates show
rates surpassing one half inch per hour, with rates near one
quarter inch possible the deserts and western valleys. Warmer and
more humid weather will continue across the region, with highs
well into the 90s in the western valleys and widespread 70s/80s
near the coast and in the mountains.

Monsoonal moisture will begin to decrease for Thursday into
Friday as precipitable water values across the deserts lower
below 1.5 inches. High pressure across the western U.S. will also
start to weaken. This will lead for a slow cooling trend into the
end of the week. Highs near 100 today in the Inland Empire will be
closer to 90 degrees by Friday with highs only near 100 across
the lower deserts. These temperatures will be near to 5 degrees
below normal and will continue through the weekend.

Ensemble models hint at a trough of low pressure moving toward the
California coast and an area of high pressure building over the
ocean southwest of San Diego by the weekend into the first half of
next week. The positioning and strength of these features is still
uncertain, but their presence will indicate drier weather for our
region overall as greater onshore flow moves in aloft. The
confidence in the drier forecast is moderate to high, but just how
warm it gets is where we have less confidence. For now, NBM
paints a picture well with keeping highs near average as we move
into the early half of next week.

&&

.AVIATION...
030305Z...Coastal Areas...Patchy and intermittent coastal low clouds
with isolated CIGs 600-900 feet MSL and vis 0-5SM possible (40-55%
chance KSAN/KCRQ, 20% chance KSNA) within 10 miles of the coast 11-
15Z, clearing after 15Z.

Mountains/Deserts...-SHRA and isold TSRA expected 19-03Z, mostly
over the mountains and high desert but possible over the low deserts
(20-40% chance). Chances for TSRA over KPSP/KTRM is about 15% 18-
00Z. CB based 9000-12000 ft MSL. Gusty and erratic winds, low
visibility, and lightning will accompany any TSRA, along with lower
bases 6000-9000 feet MSL.

&&

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

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.

&&

$$

PUBLIC...PG
AVIATION/MARINE...CSP

NWS Tucson (SGX) Office