380 
FXUS66 KSGX 080358 AAA
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
900 PM PDT Thu Oct 7 2021

.SYNOPSIS...
A Pacific trough of low pressure will interact with an atmospheric 
river to bring rain, mostly light to moderate, late tonight and 
Friday. It will be unseasonably cool Friday and Saturday, but then 
after the trough moves east out of the region, warmer weather will 
briefly return Sunday. A cold upper low pressure system will move by 
to the northeast early next week, and that will bring colder weather 
with strong gusty west to northwest winds in the mountains, deserts 
and coastal waters as well as a slight chance of showers, including 
possible snow in the mountains. The winds will likely turn 
northeasterly Tuesday, possibly lingering Wednesday.

&&

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

Evening forecast update:

The forecast remains on track this evening. The rain event is still 
looking on track. The latest high res guidance and forecast and 
synoptic trends still suggest rain developing across our corner of 
SoCal after 2 am tonight, with the rain tapering off by midday 
Friday. This is a classic short duration but efficient rain maker 
storm system, tapping into some decent subtropical moisture, 
interacting with an upper level low pressure trough. More details 
regarding the forecast are below from the midday forecaster. 

Remainder of the earlier Discussion, issued at 2 pm PDT:

The trough of low pressure will move through the West Coast through 
Saturday with a couple main waves. The first one will be the 
significant one for extreme southern California Friday and interacts 
with an atmospheric river from the southwest, which both the 
deterministic GFS, NAM and ECMWF indicate as briefly moderate 
strength between 5 and 11 AM Friday. The best dynamics will be 
farther north, but the moisture from the AR should bring up to 
around 1.5 inches of precipitable water and that should result in 
widespread stratiform precipitation, with orographic enhancement on 
the SW facing mountain slopes. Best amounts will be in San Diego 
County with lighter amounts farther north. Overall, we are 
maintaining general rainfall totals between 1/4" and 1/2" with local 
amounts around 1" in the mountains, with the bulk of that between 5 
and 11 AM Friday. Hourly rainfall could be up to around 0.15" except 
locally higher on coastal mountain slopes. The air mass dries out a 
lot by Saturday, so when the second wave moves through before the 
trough (then oriented more NE-SW) shifts to the east, there should 
not be any further precipitation.

After fair warmer weather Sunday, a cold closed low should move 
through the Great Basin and bring mostly stronger winds to our area, 
though models/ensembles have trended this a bit farther east. There 
could still be enough moisture for a few showers, which could fall 
as snow in the mountains, with the potential snow levels falling to 
around 5000 feet. If the low moves fast enough/far enough east, 
there could also be some Santa Ana winds behind it Tuesday, possibly 
lingering into Wednesday. With the cold air mass there is some 
possibility of high desert frost Tue night as NBM indicates some 30s 
up there, though warmer weather should return Wed/Thu.

&&

.AVIATION... 
080330Z...Coast/Valleys...BKN-OVC low clouds spreading inland west 
of the mtns tonight. Bases will be 1500-2500 ft MSL with layers to 
20000 ft MSL. Cigs likely to reach KONT and KSBD after 09Z. SHRA 
expected to move from west to east after 10Z, becoming isolated SHRA 
after 20Z Fri. Vis restrictions 2-4 SM BR in areas of SHRA. Higher 
terrain obscured through Fri afternoon. 

Mountains/Deserts...SCT-BKN clouds at or abv 10,000 ft MSL with 
unrestricted vis tonight. West winds gusting 25-40 kt at times 
through the passes and along the desert slopes tonight. BKN clouds 
AOA 8000 ft MSL Fri with periods of SHRA 12-22Z Fri. Vis 
restrictions 2-4 miles in rain and fog near SHRA in the mountains. 


&&

.MARINE...
Northwest winds occasionally gusting around 20 kt likely Friday and 
Saturday afternoons in the outer waters. Stronger northwest winds 
likely next Monday-Tuesday, with gusts 25-30 kt. 

&&

.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...Maxwell/JMB
AVIATION/MARINE...PG