Area Forecast Discussion
Issued by NWS San Diego, CA

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480 FXUS66 KSGX 180410 AFDSGX Area Forecast Discussion National Weather Service San Diego CA 810 PM PST Sun Dec 17 2017 .SYNOPSIS... High pressure will bring fair skies, lighter winds and mild weather Monday and Tuesday with seasonal temperatures. Strong onshore flow will develop Wednesday as low pressure swings into the Great Basin, pulling in the marine layer briefly. As the low passes, high pressure will build inland, forcing flow back offshore across Southern California with another round of gusty northeast winds below passes and canyons. Cooler, with more seasonal temperatures into the weekend. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... The Cajon Pass and areas just below along with the Santa Ana coastal slopes continue to see northeast winds gusting 30-40 mph. However, winds will be relaxing overnight. The Wind Advisory was allowed to expire at 8 PM. Temps have fallen a bit quicker in portions of the area, so adjusted the lows for tonight down a tad, mainly for the mountains. A strong ridge remains parked just off the West Coast. The ridge will expand across the west on Monday, then flatten as a strong shortwave trough slides SE from higher latitudes. The trough will dig SE into NV on Wednesday as the ridge off the coast builds northward. Surface pressure falls over the Great Basin will set the stage for strong onshore flow over our mts/deserts later Wed/Wed night, before winds turn back offshore on Thursday as the wave passes. Current indications are for westerly winds gusts of 50 to 60 MPH over the mtn crests/desert slopes late Wednesday before swinging northerly and then NE on Thursday. The NE winds could be quite strong as well, especially through the passes and canyons and along the coastal slopes of the Mts. This event looks to be short-lived though as models show the trough moving east rather quickly. The weekend looks like a transition period with higher uncertainty as the hemispheric pattern begins to undergo a rather large shift. Both the 12Z GFS/ECMWF runs appear to be hinting at establishing the massive EastPac ridge farther to the north, allowing shortwave energy to undercut. How this energy manifests itself remains to be worked out, but it will at least keep us cooler and tamp down the persistent dry offshore flow we have been experiencing this month. The shift in the ridge, if it does occur as seems to be indicated, will also open the door to Pacific systems that could eventually bring much needed precipitation. && .AVIATION... 180345Z...Mostly clear with unrestricted vis through Monday evening. && .MARINE... A trough of low pressure moving through the region will bring strengthening northwesterly winds gusting 20-25 kt Wednesday night through Thursday morning. && .FIRE WEATHER... Winds will subside overnight, with moderate RH recovery. Drier most areas on Monday as daytime RH falls into the teens over the mtns/valleys, but winds will be light. Onshore flow will be increasing and become strong and gusty over the mtns/desert slopes by Wednesday afternoon and evening as low pressure passes to the northeast. Gusty offshore flow will follow on Thursday and could be quite strong into Thursday night. && .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...Gregoria/JAD AVIATION/MARINE...SS is the U.S. government's official web portal to all federal, state and local government web resources and services.