390
FXUS66 KSGX 051010
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
310 AM PDT Sun Oct 5 2025

.SYNOPSIS...
Below normal temperatures through Monday, followed by a gradual
warming trend into the latter part of the week. Areas of night and
morning low clouds and fog will continue along the coasts and
parts of the valleys through midweek. Mid and high level tropical
moisture moving into the region will bring a slight chance of
showers late Thursday through Saturday.

&&

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

Low clouds are patchy and slow to develop again this morning as
only a weak marine inversion is in place. Hi-res guidance still
suggests a coastal eddy spinning up later this morning, which
would help with the development of low clouds across the coastal
areas and far western valleys. Elsewhere skies will remain clear.

Weak troughing remains across the Western US today with a weak
short wave dropping south through the Pac NW. Highs will be a
couple degrees higher today but overall a few degrees below
normal. A closed low develops from this wave off the Central CA
coast Monday and remains in place through about Tuesday, meanwhile
high pressure builds across the southern US. Gradual height rises
across So Cal from this building high pressure will bring minor
warming through Tuesday with highs returning to around normal.
This high will also help strengthen the marine inversion, allowing
for more widespread low cloud coverage across the coastal areas
and western valleys each night and morning. Little change for
Wednesday as the low opens up and then progresses eastward to our
north.

Main story this week remains the potential for tropical moisture
from what is now Tropical Storm Priscilla being drawn into the
region late Thursday through Saturday. Ensemble mean Precipitable
Water from both the EC and GEFS are around 1.5", which isn`t bad
for early October. However, it looks like most of this moisture
will be above the 700 mb level. The real question is how much
forcing we`ll have to work with. Instability remains weak with
MUCAPE less than 100 J/kg, so thunderstorm probabilities are less
than 10%. While there is less spread amongst the ensembles with
respect to the position of the large upper low off the coast
during this period, enough spread remains that uncertainty in
forcing and therefore rainfall amounts remain. With the low
further west, placing us more under the influence of the ridge to
the east, accumulating precipitation will be harder to come by. A
further west track of the low would provide a little more lift and
precipitation. Given the lack of strong dynamics either way, this
generally looks like a case where rainfall rates will remain on
the low end absent any thunderstorms developing, really limiting
the flooding threat. NBM 10th/90th percentile 48 hr rainfall
accumulations range from zero to around an inch for late Thu-late
Sat, though the 90th percentile is definitely skewed by a couple
of very wet ensemble members. NBM mean is closer to 0.2-0.3".
Ensemble PW falls off a cliff on Sunday as the upper trough
passes, pushing tropical moisture out of the area. Highs will
remain near normal through Friday, then a cooling trend follows
for Saturday and Sunday as the trough moves through.

&&

.AVIATION...
050930Z....Coasts/Valleys...Currently SKC except a few patches of
low clouds over nearshore waters. Clouds based 1800-2400 ft MSL
should develop and fill into most of coastal San Diego County/Orange
Counties after 12Z. 10 to 15 miles inland extent at most. Clouds
mostly scatter out 17-19z, but may linger to 21Z at the immediate
coastline. Clouds with similar bases will start to develop after 00z
Mon at the coast, broadening in coverage, and eventually filling
into SD/Orange County valleys overnight.

Mountains/Deserts...VFR conditions will prevail into Monday morning.

&&

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

&&

.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...SS
AVIATION/MARINE...KW