357
FXUS66 KSGX 180351
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
851 PM PDT Sun Aug 17 2025

.SYNOPSIS...
Low pressure along the West Coast will bring cool weather and
extensive coastal clouds early this week. High pressure will
build this week, bringing a warming trend and less marine layer
cloud cover. Thursday through Saturday will be the hottest days.
Monsoon moisture and the chance of showers and thunderstorms will
increase, mainly over the mountains and deserts Thursday through
at least Sunday, with the highest chance Friday and Saturday.

&&

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

Update: Low level clouds continue to form over the inner coastal
waters of the SOCAL bight with some of them advancing onshore over
portions of San Diego County, and will continue to surge eastward
and into the inland areas overnight, although likely not having
cloud coverage being as extensive as in previous nights. This marine
layer will begin to mix out with clearing shortly after sunrise for
most areas and likely even clear out for all the coastal sites
tomorrow, with temperatures being notably warmer. The marine layer
will further become thinner and lower with cloud heights over the
course of the next week as high pressure strengthens over the
region. Some locations within the Inland Empire (IE) could get close
to the 100F degree mark, especially within the vicinity of Hemet,
and surrounding areas. The warming trend is going to continue
throughout the week, with highs being well above average by mid
week, and then possibly needing the issuance of heat products on
Thursday and Friday within the IE, and maybe for the low deserts.
Temperatures will only slightly cool for the following weekend.
Models have undergone little change in recent days, and still show
the subtropical ridge over the desert southwest retrograding back
towards the west and expanding over the region, which will not
only help to increase temperatures, but also allow for there to be
the intrusion of some mid-level monsoonal moisture being advected
in from the east around the high pressure center. This could
allow for there to not only be the possibility of some afternoon
showers and thunderstorms developing over the mountains and
deserts, but may also help to keep temperatures from getting quite
as hot for the lower deserts just enough to hold off the issuance
of an Excessive Heat Watch (or warning). This will come into
better clarity over the upcoming days before a decision needs to
be made.

(Previous discussion submitted at 125 PM):

Skies cleared quite fully even from the beaches today, and with
less smoke and haze, you can see the mountains much better. The
coastal eddy will continue to weaken as low pressure over the West
Coast gives way to high pressure building and expanding over the
Southwest. The higher pressure will push down on the marine layer
and warm SoCal`s air mass over the next few days. As the ridge
axis moves northward, the door will begin to open for monsoonal
moisture to flow in from the southeast. This will be a gradual
process, increasing a little each day Monday through Friday. Each
day will be a little warmer, the marine layer cloud cover a little
more confined to the coast. While temperatures today are very
much on the cooler than average side, by Wednesday they rise to
around average, then above average Thursday through Saturday, when
max temps will be 2 to 10 degrees above average. Look for temps
in the Inland Empire 97-105, high desert 98-107, low desert
109-115, western valleys in the 90s, 80s near the coast and and
mid-upper 70s at the beaches. The monsoon flow could bring gulf
surges into the low desert each morning, which would dampen the
max temp potential there on those hottest days. The monsoonal
moisture and instability eventually become sufficient to produce
some showers and thunderstorms Thursday and beyond. Look for the
first isolated thunderstorms in the mountains Thursday afternoon.
Then on Friday and Saturday the chances increase and expand in
space to all mountains and deserts, and also expand in time into
early evenings. We`ve been watching guidance play this same tune
for several days now, so our confidence is relatively high for
this scenario to play out through Saturday. Six days is pretty
good for a narrow range of outcomes, so we`ll take it. For next
Sunday, ensembles widen the range of outcomes, but most solutions
indicate a tepid cooling trend to begin and a gentle nudge aside
of monsoonal moisture for decreasing chances of thunderstorms.

&&

.AVIATION...
180330Z...Coast/Valleys...Low clouds based 1000-1500 ft MSL have
formed along the immediate coast of San Diego County with isolated
patches along Orange County. Clouds move into western SD valleys and
fill into the remainder of the Orange County coastline by 09Z,
reaching up to 15 miles inland overnight, with only a 25% chance of
a CIG at KONT. VIS down to 2-5SM for coastal terrain. Low clouds
retreat to the coast by 17Z, with full clearing by 19Z. Low clouds
700-1000 ft MSL will be on the patchy side late Mon evening, with a
60% chance for CIGs to reach KSAN and KCRQ by 09Z.

Mountains/Deserts...Mostly clear with VFR conditions prevailing
through Monday.

&&

.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.

&&

$$

UPDATE...Stewey
PUBLIC...MM
AVIATION/MARINE...CSP