512
FXUS66 KSGX 291916
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
1216 PM PDT Tue Sep 29 2026

.SYNOPSIS...
Strengthening high pressure will bring warming this week. A long-
duration heat wave, with widespread Major to locally Extreme
HeatRisk for the coast and valleys, is likely for this weekend
through much of next week into the following weekend. Record warm
overnight temperatures are likely for the coast and valleys for
Saturday through Monday.

&&

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

Tonight through Wednesday...

An upper level low is continuing to exit the southwestern region,
with northerly flow filtering in behind it. These winds will
translate to weak off shore winds mixing in overnight, which will
help to impede the development of the marine layer overnight. There
may still be some patchy areas of low clouds, with the best
probability for the coastal areas of San Diego County, due to the
development of an eddy in close proximity of San Clemente Island. A
few factors are also contributing to the lack of marine layer
development overnight, given that there are relatively warmer sea
surface temperatures currently, which has made it difficult for a
good inversion to establish itself. This, in combination with weak
offshore flow and drier air mixing into the boundary layer will
likely give way to the marine layer struggling to form throughout
the night.

High pressure is going to continue to build back in tomorrow, with a
positively-tilted ridge axis extending up and over the region from
the southwest. This will result in high temperatures further
increasing. There will be better onshore flow for tomorrow, which
may also help to advect in a little more moisture within the
boundary layer, and aid in the development of better marine layer
coverage for tomorrow night. It will also allow for westerly winds
to increase and become gusty during the afternoon for mostly of the
coastal and inland basin, with enhanced onshore flow as the
compelling factor. Higher wind gusts are also expected for San
Gorgonio Pass, near Banning, where there could be some gusts
approaching 40-45 mph. For overnight, there may also be some patchy
areas of fog developing within some of the higher inland valleys,
also also where low level clouds intersect the west-facing slopes.
As the ridge further expands and strengthens, heights will increase
to values approaching 590 dm over the region, which will allow for
the temperature warming trend to continue into Wednesday, with highs
generally up to around 5 degrees above the seasonal average for this
time of year.

Thursday through next Tuesday...

The ridge will further strengthen from Thursday into Friday, which
will bring temperatures roughly around 5 to 10 degrees above
average, and then 10 to 15 degrees above average for both days,
respectively, for locations west of the mountains. For the deserts,
temperatures will warm through Wednesday, and then plateau on
Thursday. However, for locations west of the mountains, temperatures
will be notably warmer on Thursday as the ridge expands back a
little further to the west. By Saturday, some locations will for the
coastal and inland basins will be around 20 degrees above average,
with triple digit heat for most localities within the Inland
Empire. At the moment, the highest probability for the warmest
afternoon temperatures will be on Monday of next week, as heights
top 590 dm over our deserts. Given the duration of the week, there
will likely need to be some heat products warranted for issuance.
There is one caveat, and that is overnight lows being well too
mild towards the end of the week as a recent bias due to warmer
nights with relatively higher moisture content under a more
tropical environment in recent weeks, that is not reflected well
within the national blend of models, as guidance has been
overperforming. Because of this, the projected temperatures in
the current forecast package might be a bit underdone with
temperatures not being cool enough, as the pattern is showing a
drier airmass mixing in, with better confidence of more efficient
radiational cooling as a result. There will also likely be a
better marine layer establishing itself, towards the end of the
week, with a stronger inversion in place. This may also allow for
there to be a higher probability of patchy fog developing for some
of the coastal areas by this upcoming weekend. Only a slight
weakening in the ridge will translate to slightly cooler
temperatures for next Tuesday.

&&

.AVIATION...
291745Z...Coast/Valleys: SKC this afternoon and evening. Areas of
low clouds based 1500-2000 ft MSL develop over coastal San Diego
County starting around 06z Wed. As a coastal eddy takes shape
overnight, clouds should spread northward into Orange County and
reach 10-15 miles inland by 12z. Clouds scatter back to the coast 15-
17z.

Mountains/Deserts: FEW-SCT mid-level clouds at and above 9 kft
through 01Z.

Otherwise...mostly clear skies today and tonight.

&&

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

&&

.BEACHES...
High tides will peak at 6.8-7.1 feet between now and noon today and
again late Wednesday morning, leading to minor coastal flooding in
low lying areas and coastal erosion. A Coastal Flood Advisory is in
effect through Wednesday afternoon.

Please note - mentioned tide predictions in the Advisory have been
adjusted to reflect actual measured tide observations.

&&

.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.


&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Coastal Flood Advisory until 5 PM PDT Wednesday for Orange
     County Coastal Areas-San Diego County Coastal Areas.

PZ...None.

&&

$$

PUBLIC...Stewey
AVIATION/MARINE/BEACHES...KW