860
FXUS66 KSGX 010944
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
244 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...

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... 010930Z...Coast...Patchy low clouds based 400-700 feet
MSL will develop and dissipate in random and intermittent fashion
through 15Z. When/where they form, vis will be reduced below 1SM
over coastal mesas. Confidence is low that cigs will persist over
any coastal airport for long, but most likely at KSAN. Low clouds
will struggle to redevelop tonight into Tuesday. Isolated cigs at
best after 09Z Tuesday.

Otherwise...VFR conditions for mountains and deserts with SCT
cumulus AOA 10000 feet MSL over mountains 19-00Z.

&&

.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 from 10 AM this morning 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...Stewey
AVIATION/MARINE/BEACHES...MM