721
FXUS66 KSGX 011602
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
902 AM PDT Mon Sep 1 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...

.Morning Update...

Low clouds will continue to clear off the beaches this morning
with mostly sunny conditions expected throughout the day. Another
warm day will be expected with highs near 5 to 10 degrees above
normal. We continue to watch the expected weather pattern shift to
cloudier and increasing chances for wetter weather by Tuesday.
Taking a look at latest hi-res guidance this morning seems to
continue the thinking for increasing clouds and shower chances to
occur by Tuesday morning. As stated below, this broad cloud cover
may help inhibit afternoon thunderstorm development across the
mountains and deserts. Monsoonal thunderstorm chances still look
to decrease throughout the week with a slow cooling trend.

.Previous Discussion (244 AM Monday)...

Today through tomorrow...

The subtropical ridge over the desert southwest has expanded
westward over the region, which has allowed for the 595 dm high
centered over northern Arizona to strengthen and will result in
hot temperatures this afternoon, with values as much as 20 degrees
above the seasonal average for this time of year, especially
within the inland valleys and Inland Empire (IE). Due to this, the
Heat Advisories already in place will continue. There will also
be an ample amount of moisture that will begin to surge
northwestward from the state of Sonora, Mexico, which will begin
to make its way over the region by later this afternoon and
possibly provide a stray shower or thunderstorm to develop over
the mountains and adjacent deserts of the east-facing slopes. By
later this evening, this moisture will continue to move in from
the south and may even allow for a few showers to move over, even
some of the coastal and inland areas. Instability drops off quite
substantially as you go west of the mountains, however, by later
this evening and through tomorrow morning, there might be just
enough instability aloft with elevated MUCAPE in the mid-levels,
that a few flashes of lightning cannot be ruled out. Highs today
will be the warmest, and then an increase in low-level moisture
will allow for temperatures to be not quite as hot tomorrow, yet
with the higher RH values, heat indices will still be high enough
to warrant the Heat Advisories to continue into tomorrow. Chances
of afternoon showers and thunderstorms looks to be better tomorrow
as well. Although, given the abundance of mid-level clouds moving
over during the morning tomorrow, this could keep temperatures
slightly cooler and not allow the convective temp to be achieved,
which may hinder the development of showers/storms later in the
day. During the overnight hours, the marine layer is going to be
sparse and mostly confined this morning right over some of the
inner coastal waters of the SoCal bight, and there will also be
some patchy fog this morning for some of the coastal locations and
even within some of the wind-sheltered valleys.

Tomorrow night through the weekend...

By tomorrow night, most of the clouds from the showers and
thunderstorms that have developed throughout the day will have
dissipated by late evening and through the overnight hours. There
will still be a very slight chance of rain and possibly a few
crackles of thunder during the overnight hours, even over the
coastal areas. By Wednesday, as the ridge begins to become more
flattened and troughing upstream allows onshore flow to become more
persistent, it will allow for a gradual cooling trend, which will
go into Friday and then temperatures will remain nearly steady
heading into the weekend. As the trough upstream begins to
influence the region more and the mid-level flow becomes more
predominant, it will advect in relatively drier air aloft and
likely suppress most of the afternoon thunderstorm activity.
There will will be some residual moisture, so an isolated shower
or thunderstorm developing over the mountains will continue to be
possible, however, most of these will only remain confined to the
mountains and adjacent deserts of the east-facing slopes. Chances
of afternoon showers and storms will continue to drop off going
into the weekend, and a more persistent onshore flow will also
continue to cool the coastal areas with a deeper and better marine
layer coverage, and even cooling off for the inland valleys to
temperatures closer to the normal highs for this time of year.

&&

.AVIATION...
011600Z...Coast/Valleys...Patchy low clouds based 400-700ft MSL
managed to develop along parts of the coast this morning, but are
quickly dissipating with all TAF sites at VFR conditions. Low
clouds will struggle to redevelop tonight into Tuesday morning,
but isolated CIGs 400-600ft MSL and VIS to 1SM or less possible,
generally after 09z Tue up to 5 miles inland. -SHRA possible early
Tuesday morning nearly region-wide, with brief VIS reductions
with any shower.

Otherwise...VFR conditions for mountains and deserts with SCT
cumulus AOA 10000 feet MSL over mountains 19-00Z. -SHRA move in
after 06z Tue, with some -TSRA possible over the mountains
and deserts generally after 12z.

&&

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

&&

.BEACHES...A long-period southerly swell will continue to bring
elevated surf and a high rip current risk to Orange County today,
gradually subsiding thereafter. Surf of 3-5 feet with occasional
sets to 6 feet for south-facing beaches. Longshore currents along
west-facing beaches. Check the Beach Hazards Statement, which is in
effect through this evening, for details.

&&

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