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