692 FXUS66 KSGX 041709 AFDSGX Area Forecast Discussion National Weather Service San Diego CA 909 AM PST Wed Mar 4 2020 .SYNOPSIS... Weak high pressure aloft will bring dry, mild weather through Friday, with some coastal clouds nights and mornings. Light showers are possible this weekend as a Pacific trough swings inland, then heavier showers could develop around Tuesday as another system with tropical moisture takes aim at Southern California. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... Coastal low clouds have mostly cleared in San Diego County and should clear from Orange County later this morning. Elsewhere skies are clear. Surface pressure gradients are very weakly onshore to Barstow and Imperial with light winds across So Cal this morning. Relatively benign weather is expected through Friday as a weak upper level ridge moves east over the region. Thursday will be the warmest day for most areas with highs in the valleys reaching the low 80s, and Friday warmest in the low desert with highs nearing 85 degrees. A touch of weak offshore flow is possible Thursday morning as a surface high slides further down the Rockies, but east winds will be confined to the coastal mountain slopes of Riverside/San Diego Counties and below the San Gorgonio Pass in the Inland Empire. Over the weekend we will switch to cooler weather as a weak trough quickly moves through. This will also bring a much deeper marine layer, gusty southwest to west winds in the mountains and deserts, and a slight chance of precipitation over and west of the mountains. Drizzle or light showers could begin in the coastal/valley locations Saturday morning, but the frontal precipitation should occur late Saturday afternoon into Sunday. Most areas will see 0.10 inch or less of rain, up to 0.25 inch in the mountains. Snow levels will generally be around 5000-6000 ft. A brief break in the weather will occur on Monday. A much more potent storm will impact the region Tuesday through Thursday. There are still many uncertainties with this system, including exactly where the atmospheric river ahead of the low will track (northern Baja versus San Diego County), as well as the trajectory of a short wave trough/heavier precipitation lifting northward around the east side of the low (Orange/Riverside/San Bernardino County vs LA County/areas west). Both of these will play a large role in the rainfall totals and distribution. Naturally, the ensemble spread for storm total rainfall is on the order of 1-2 inches, with considerable differences between the ensemble mean and the deterministic runs as well. The one thing that looks certain is that snow levels will be high with this storm, over 8000 ft during the heaviest precipitation. && .AVIATION... 041630Z...Coast/Valleys...FEW-SCT low clouds at the immediate coast through this afternoon but no impacts at TAF sites. Low clouds are expected to spread inland after 06Z Thursday, reaching portions of the western valleys overnight. Bases will similar to last night but could be slightly lower, possibly between 400-800 ft MSL. Mountains/Deserts...Clear with unrestricted vis through tonight. && .MARINE... No hazardous marine weather is expected through Monday. && .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...PG