396
FXUS66 KSGX 242042
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
142 PM PDT Sun Aug 24 2025

.SYNOPSIS...
Daily chances for showers and thunderstorms will continue through
the middle of the week as the monsoonal weather pattern continues
across the mountains and deserts. Valleys west of the mountains
may see a storm or two, but otherwise, all areas will remain
humid through the early part of the week. Cooler weather will
occur by Tuesday through Friday. A weak trough offshore will help
to lower chances for monsoonal storm activity across the
mountains and deserts by the end of the week with less humid
conditions. The marine layer will be of similar depth over the
next few days with patchy fog near the coast, pushing further
inland by the end of the week.

&&

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

Clouds continue to build over the mountains this afternoon with
weak thunderstorms forming across southeastern San Diego County.
Thunderstorm activity will continue to increase this afternoon
over the SD/RIV Co mountains, locally up into the eastern SBD mts.
Rain rates will generally be under one half inch across these
areas with desert areas seeing another chance to see light
rainfall. Areas west of the mountains are seeing another warm and
sticky afternoon as dew points reach near 70 degrees for western
San Diego and Orange Counties. The marine layer will slightly
deepen tonight, where most places along the coast will see morning
clouds and fog by Monday.

The monsoonal moisture influx will peak on Monday as PWATs surge
above 1.5 inches across the lower deserts. Atmospheric sounding
profiles show a fairly deep layer of moisture between 300-700mb
and adequate instability to have scattered thunderstorms occur
across the mountains and deserts. Hi-res guidance shows the
greatest chance (70-80%) of storms to occur will be from the
Mexican border north to Big Bear, with chances closer to 40-60%
across the deserts. Confidence in how heavy the rainfall will be
is moderate as models depict areas seeing rates near 0.50-0.75
inches per hour by Monday afternoon. These storms may be
accompanied by heavy downpours and minor flooding, small hail and
gusty winds. The flow aloft is fairly southerly, so storms will
not drift too far west off the mountains, but adjacent valleys
have a smaller chance (15-30%) to see any rainfall. These same
areas will see another chance for storm activity by Tuesday, but
chances are expected to be slightly lower (50-65%) as moisture
decreases with PWATs closer to 1.25" across the lower deserts.
Hourly rain rates therefore, will lower closer to one half an inch
or less; chances also decrease to see rainfall across the far
western valleys.

Areas west of the mountains will remain on the humid side through
Tuesday as monsoonal moisture hangs on. The good news will be a
weak trough off the California coast will begin to form as the
high aloft weakens. This will allow for high temperatures to cool
a few more degrees closer to average. High temperatures will
remain very similar Tuesday through Friday. By Wednesday,
monsoonal moisture and instability will continue to decrease,
leaving around a 15-30% chance of thunderstorms across the
mountains and deserts. The trough`s influence becomes more
pronounced as 500 mb winds, that are now southerly, begin to turn
westerly. This will help to bring in drier air aloft and
stabilize the atmosphere by Thursday into the weekend. Storm
chances across the mountains will be minimal by this time period,
with less humid weather expected for all areas.

Ensemble models show the high across the desert southwest may
restrengthen some by next weekend, which may bring slightly
warmer weather. Looking further out into early next week, a trough
across the Pacific may begin to influence the weather across
California, which would bring near to below average temperatures
and dry weather for our area.

&&

.AVIATION...
242100Z...Coast/Valleys...VFR conditions with some mid and high
clouds at/above 15,000 ft MSL through this evening. Patchy low
clouds based 500-800 ft will slowly develop overnight tonight, after
08z Monday. Fog will redevelop in inland valleys and higher coastal
terrain up to 10 miles inland with VIS locally below 1SM. VIS
improves around 15z and clearing 16-17z.

Mountains/Deserts...SHRA/TSRA have developed over the mountains. 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. 25-35% chance a SHRA/TSRA could drift over or
develop over the low deserts through 02z this evening.

Higher chance and greater coverage of +SHRA/TSRA over the mountains
and deserts Monday afternoon after 19z. 45-55% chance of SHRA/TSRA
over the lower deserts with more widespread impactful conditions -
erratic winds, hail and CG lightning - Monday afternoon.

&&

.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
AVIATION/MARINE...Westerink

NWS Tucson (SGX) Office