134
FXUS66 KSGX 241555
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
855 AM PDT Sun Aug 24 2025

.SYNOPSIS...
High temperatures will remain near or a little above average
through Monday. High temperatures will cool to a few degrees
below average for Tuesday through Friday. Significant monsoonal
moisture will remain in place through Tuesday with chances for
afternoon and early evening showers and thunderstorms each day,
mainly for the mountains and deserts. Drying will begin on
Wednesday and continue into next weekend with decreasing
thunderstorms chances each day. Night and morning low clouds will
mostly be limited to coastal areas through the weekend, but could
return to the western valleys for the latter half of next week.

&&

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

.Morning Update...

Mostly clear skies were seen over the region as the marine layer
erodes across the San Diego County coast. Lingering low clouds
will cover the SD Co beaches through the morning with gradual
clearing by early afternoon. This morning`s sounding shows
sufficient moisture aloft with a less pronounced dry layer near
the surface. This will lead to chances for showers and
thunderstorms across the mountains and deserts with locally heavy
downpours at times. Hi-res models show rain rates mainly staying
below one half inch per hour along the spine of the mountains from
the Mexican border to Big Bear. Monday will likely be the most
active day over the next few days as model soundings show a very
moist profile from the surface to the mid levels with PWAT values
over 1.50" across the deserts and a favorable wind profile from SE
to S. This may lead to showers or storms drifting westward into
adjacent valleys by late afternoon Monday into the evening.

I concur with my colleague`s thoughts below on elevated chances
remaining in the forecast through Tuesday. As stated below, a weak
trough offshore will begin to bring winds out of the SW and W,
bringing in drier air and diminishing storm chances by the latter
half of the upcoming week.

.Previous Discussion (228 AM Sunday)...

Today through tonight...

A subtropical ridge has expanded over the region, and allowed for
high pressure to build in quite strong. The center of the high
pressure is located over northern Arizona, and this has allowed the
clockwise flow to advect in some mid-level monsoonal moisture over
SoCal. Due to this, humidity has increased, keeping temperatures
somewhat modified and slightly cooler over the deserts. There will
be lack of mid to upper level cloud coverage this morning, unlike
yesterday, which will result in temperatures being slightly
warmer for the deserts. Showers and thunderstorms will develop
again over the mountains, and possibly over the deserts,
especially adjacent to the east- facing slopes of the mountains.
Water vapor does reveal that there will be less moisture to work
with in the mid levels this afternoon, so coverage should be quite
sparse and mainly just over the higher terrain (with orographic
lifting as the forcing). Given the steering flow being primarily
out of the south, any showers/storms that do develop are likely
not going to get into the inland valleys. The marine layer is
going to be similar tonight, although there should be better
coverage as onshore flow is enhanced throughout the day tomorrow.
There could also be some areas of patchy fog along some of the
coast for Orange and San Diego counties, as well as over the inner
coastal waters and within some of the inland valleys.

Monday through next weekend...

Beginning Monday, the ridge is finally going to begin to break down
slightly with better influence from the trough upstream. This
will begin a gradual cooldown which will continue through the
middle of the week, as the trough begins to propagate ever so
slowly towards the region with lower heights aloft advancing in
from the northwest. There will still be residual monsoonal
moisture moving in some the southeast around the flow of the high
pressure to the northeast, which will keep the threat of afternoon
showers and thunderstorms still alive over the mountains, and
possibly the deserts, at least through Wednesday...with even a
lesser chance of the inland areas (less than a 20 percent chance).
As the troughing continues to slowly advance towards the region,
the tightening of the pressure gradient will allow for afternoon
gusty winds to increase over the higher elevations and within the
areas prone to strong and gusty/gap flow winds. By Thursday, there
should be enough drier mid-level flow between 700-500mb due to
the flow becoming more southwesterly that this should help inhibit
anymore afternoon convection, and this should remain the same
through next weekend, with similar temperatures. The marine layer
will also become slightly thicker and elevated throughout the
course of the week.

&&

.AVIATION...
241500Z...Coast/Valleys...Patchy low clouds and fog up to 10 miles
inland in San Diego Co based 600-800 ft MSL are clearing. VIS
improving where patchy fog has developed. Full scatter out by 17z.
Mid and high cloud cover at/above 15,000 ft MSL through this
evening. Low clouds with similar bases will slowly develop overnight
tonight, after 09z Monday, and fog will redevelop up to 10 miles
inland with VIS locally below 1 SM. Similar clearing Monday morning,
15-17z.

Mountains/Deserts...25-45% chance of SHRA/TSRA over the mountains
and deserts 19-02Z this afternoon. CB bases near 12 kft with tops to
35 kft. SHRA/TSRA may produce local VIS of 1-3 SM, gusty and erratic
winds, cloud-to-ground lightning, and small hail. Otherwise variable
mid and high clouds at/above 15,000 ft MSL through this evening.

&&

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

&&

.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...APR/Stewey
AVIATION/MARINE...Westerink