760
FXUS66 KSGX 130405
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
905 PM PDT Fri Jul 12 2024

.SYNOPSIS...
There will be a slight cooling trend this weekend. Monsoonal
moisture will increase through Saturday with scattered showers and
thunderstorms. Moisture lingers on Sunday with isolated
thunderstorms mainly in the mountains. The air mass dries on
Monday with slight warming trend over inland areas. Night and
morning coastal low clouds will continue to extend inland into the
western valleys but less extensive this weekend.

&&

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

Marine clouds slow to move onshore with lots of mid level
altocumulus, leading monsoon surge in mid levels 600-400 mb layer,
now exiting to the north. An upper low along the coast will clip
region during Saturday morning providing best synoptic lift over
LA, Orange and northward. However, significant moisture surge
with 1.5 inch precipitable water on the backside of the upper
high that is situated over the 4 corners region now. Within the
moderate 25 knots mean southeast flow there are weak disturbances
as well for Saturday. This bring potential for elevated
convection. Surface-based thunderstorms are likely with heating
upslope flow and sea breeze during Saturday afternoon, and a few
will get carried into the foothills and deserts, especially High
Desert. There is already considerable downdraft wind potential
with the warm boundary layer in place. Fire weather watch issued
for these reasons along with recent 2 week heat wave that has
critically dried vegetation. A few cells that can anchor and
become strong will have local heavy rain rates up to 0.5 per hour.
Temperatures on Saturday only lower slightly and we still have
potential for 105F in High Deserts and 110 to 115F in the lower
desert so heat risk continues to be moderate to major in those
region despite increasing moisture. Mid level clouds and moisture
may keep overnight lows slightly warmer.

The pattern changes slightly on Sunday with the upper low along
the California coast well north of Socal, but tropical monsoon
moisture remains in place. Steering flow is less and veers to
south. Low level dewpoints should make it quite muggy with
possible 70s into Coachella Valley. This will lower high
temperatures all inland areas. The scattered favored convection
potential remains but along the mountain spine and into High
Desert due to persistent south steering flow aloft.

Drying is still forecast on Monday and lasting into mid week.
With only slight air mass changes the inland areas will warm
slightly again due to the drier conditions with daytime highs
again above averages and near 115F for lower deserts and getting
back to around 100F for Inland Empire. So the cooling day will be
Sunday it appears with a temporary break in extreme heat.

&&

.AVIATION...
130400Z...Coast/Valleys... Marine layer is holding offshore for now,
with bases near 700-1000 feet MSL and tops around 1400-1700 feet
MSL. CIGs expected at the coastal airports beginning around 13/06z
as the marine layer pushes inland. Bases will be slightly higher
than previous nights, and visibility reductions will be minimal with
localized 4-6SM for the coast. Patchy fog with localized <3SM VIS
possible in the valleys and higher coastal terrain. CIGs scatter out
around 13/16z for the coasts. TSRA possible off the mountains late
morning and early afternoon which may push into the valleys,
although confidence for impacts to TAF sites is too low to mention
at this time. Low clouds will push back inland around 14/06z for
most coastal sites.

Mountains/Deserts... TSRA expected to develop late Saturday morning
through Saturday evening in the vicinity of the mountains with SCT-
BKN120 for much of the mountains. Brief, heavy rainfall and gusty
and erratic winds likely with any storm. Mod up/downdrafts possible
near the mountains. Confidence for impacts from TSRA to TAF sites is
too low to mention at this time. Localized VIS reductions down to
3SM due to smoke in the vicinity of the San Bernardino Mountains
from the Vista fire. Gusty west winds possible up to 30kts around
San Gorgonio Pass through Saturday evening.

&&

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

&&


.FIRE WEATHER...
Considerable concern with incoming moisture surge and still very
warm boundary layer with potential for elevated convection
(anywhere) with upper low riding north along the coast within the
upper high to our east, in addition to the terrain driven
convergence and sea breeze thunderstorms in response to the
increased mid level moisture and instability. Have matched with
offices to the north for fire weather watch for scattered
lightning and erratic outflow winds associated. We had isolated
activity today in the San Bernardino mountains produced 40 mph
wind after initial lightning and some rainfall. Cells in the
mountains will have rainfall but fuel is very dry due to recent
long heat wave. The threat on Sunday becomes confined to the
mountains.


&&

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


&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Fire Weather Watch from Saturday morning through Sunday evening
     for Riverside County Mountains-Including The San Jacinto
     Ranger District Of The San Bernardino National Forest-San
     Bernardino County Mountains-Including The Mountain Top And
     Front Country Ranger Districts Of The San Bernardino
     National Forest-San Gorgonio Pass Near Banning-Victor
     Valley-Apple Valley-Lucerne Valley-Johnson Valley.

     Excessive Heat Warning until 9 PM PDT Saturday for Apple and
     Lucerne Valleys-Coachella Valley-San Diego County Deserts.

     Excessive Heat Warning until 9 PM PDT this evening for San
     Gorgonio Pass Near Banning.

PZ...None.

&&

$$

PUBLIC/FIRE WEATHER...Tardy
AVIATION/MARINE...Zuber/MKM