Spring Valley Lake, California Weather Forecast Discussion

316
FXUS66 KSGX 200926
AFDSGX
Area Forecast Discussion
National Weather Service San Diego CA
226 AM PDT Sat Sep 20 2025
.SYNOPSIS...
Warm and dry today. Tropical moisture returns tomorrow morning
with periods of light to moderate rain showers by later in the
afternoon and into the evening, being mostly widespread across
San Diego County and the mountains. Storm chances become confined
to the mountains on Monday, followed by the potential for more
widespread precipitation Tuesday, depending on the track of the
upper low. Drier on Wednesday, and then remaining dry and
slightly warmer Thursday and Friday.
&&
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...
Today through tomorrow...
At 1:45, satellite imagery is showing a the patchy development of
the marine layer, which should continue to gradually fill in and
extend inland into portions of the inland valleys. There is still
moisture in place from the remnants of Mario, which continue to exit
the region as a trough to the west begins to deepen as the
subgradient jet max exiting this trough will help to move out any of
the remaining moisture with most of the mid to upper levels drying
out by later in the day. Because of this, it will likely remain dry
across the CWA this afternoon, with perhaps only a few towering
cumulus clouds developing over the mountains, with a very slight
chance that one materializes into a shower. Otherwise, conditions
will be much drier for today, and notably warmer as well, especially
for the deserts. By tomorrow, the trough to the west of SoCal will
slightly deepen. Meanwhile, there will be some more subtropical
moisture streaming up from the south. The trough will help to draw
up the moisture, with some showers (mainly light) beginning by
later in the day tomorrow. High res models show that the majority
of this moisture will be passing over San Diego County, with the
highest probability of developing storms over the San Jacinto
Mountains and northern mountains in San Diego County by later in
the afternoon on Sunday. Rainfall amounts on Sunday will generally
be very sparse and minimal (generally less than a tenth of a inch
for most areas), unless there are some heavier cells embedded
within the band of more stratiform type rainfall. High
temperatures on Sunday will also be cooler, and below average for
the inland areas and deserts.
Monday through Friday...
On Monday, the low will continue to deepen and eventually begin to
cut off. Deterministic models and CAMs alike which go out far enough
display more of an abundance of moisture being advected up into the
trough going into Tuesday, which will help to increase the chance of
showers and thunderstorms. Depending on where the position of the
associated U/L low is on Tuesday, will determine how much precip the
area receives. By Wednesday, deterministic models do have some
agreement that shows the U/L low beginning to slowly propagate to
the north slightly and may allow for more of the energy to be
displace further north as well, which may allow for a bit of a lull
in terms on precipitation if this comes to fruition. The U/L low
will continue to meander and remain nearly quasi-stationary going
into the later part of next week, which will keep a slight chance of
showers/thunderstorms around (mainly over the mountains), as well as
temperatures continuing to be on the relatively cooler side
throughout the rest of the week. The marine layer will also become
more persistent during this period in the forecast as the onshore
flow becomes more predominant, and winds will also become more
enhanced with the influence of the trough as well for each afternoon
towards the end of next week, especially for the areas prone to gap
flow winds.
&&
.AVIATION...
200900Z....Coast/Valleys...Low clouds based 700-1100 ft MSL are
slowly forming along the coastline, eventually developing up to 15
miles inland. VIS locally 0-5SM in some inland valleys where terrain
and clouds intersect, though reduced VIS and BR is also possible
closer to the coast as well. Clouds scatter to beaches 16-18Z.
Patchy clouds with lower bases 600-900 ft MSL will form in areas
along the San Diego coast after 21/03Z, then gradually spread into
coastal sites through about 09Z.
.Mountains/Deserts...VFR conditions and mostly clear skies through
early Sunday. Mid and high level clouds AOA 10,000 ft MSL enter the
region late Saturday.
&&
.MARINE...Slight chance for a few thunderstorms over the waters
overnight Monday into Tuesday afternoon. Any storm could briefly
produce lightning, gusty erratic winds, and choppy seas. Otherwise,
no additional hazardous marine conditions are expected through
Wednesday.
&&
.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...Stewey
AVIATION/MARINE...Westerink

NWS SGX Office Area Forecast Discussion