977 FXUS66 KSGX 100407 AFDSGX Area Forecast Discussion National Weather Service San Diego CA 907 PM PDT Tue Oct 9 2018 .SYNOPSIS... Onshore flow will increase with a deepening marine layer as a weather disturbance arrives late Wednesday. Temperatures slightly below seasonal averages will continue through Thursday. Light precipitation is possible Wednesday night into Thursday morning west of the mountain peaks, including the coastal waters. Tropical moisture from Sergio will arrive on Friday and Saturday but the remnants will remain to the east. Another deeper Pacific trough is expected to form along the West Coast on Friday and could tap into the remaining moisture for the weekend. This will then be followed by offshore flow increasing Sunday and Monday. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... After complete clearing over the water and land, the stratus is filling back in along the coast and locally inland. This trend will continue but appears that it will be broken up over land and coast most of the night with clear areas shifting around. An upper level trough will arrive late Wednesday and a rapid expansion of clouds in the late afternoon and evening after initial daytime clearing. The marine layer will deepen as well Wednesday evening and areas of drizzle should form and even isolated showers. This wave is out of the region on Thursday but we remain in cyclonic flow aloft. The deep low cloud deck should be disrupted on Thursday for partial clearing and then a weak offshore flow develops keeping much of the low cloudiness offshore. TC Sergio mid level moisture (precipitable water plume) is forecast to reach San Diego County by Friday night while the low level remnants stays well to the south and makes landfall across Baja and Mexico. An amplified pattern allows for a large deepening closed upper low along the West Coast to form on Friday and then settle southward to just southwest of San Diego for Saturday. This scenario could bring showers and elevated thunderstorms where it taps the remnant moisture of precipitable water values 1.2 inches along the San Diego and Orange County coast...and less threat for mountains. This pattern is tricky because the upper low center track is critical to showers reaching land or staying just offshore and thus various NWP runs have varied. Latest ECMWF, GFS, NAM and ensemble mean vary slightly in the strength and position of the close low but all bring deep moisture into the northeast quadrant and a real threat for showers training from southeast to northwest along the coast for the weekend. The track record since late last week has been inconsistent as the systems phase differently. This is then followed by a deeper short wave trough across Utah that would combine with the closed low drifting east across northern Baja to bring enhanced Santa Ana wind flow (offshore winds) late Sunday and into Monday and Tuesday. The pattern can often yield upper level support and locally strong offshore winds. && .AVIATION... 100355Z...BKN-OVC015-020 at the coast spreading 25 miles inland overnight and reaching KONT and KSBD between 11-14Z. Vis of 4 miles in HZ at KONT and KSBD from 13-17Z. Clouds clearing back to the coast through 17Z Wednesday. After 17Z...mostly clear. && .MARINE... No hazardous marine weather through Saturday. && .BEACHES... A 200 degree long-period south swell from Sergio will arrive Thursday bringing high surf to area beaches Thursday and Friday. The swell and surf will decrease late Friday and Saturday. Minor coastal flooding is possible during the 6 ft morning high tides. There will be strong rip and longshore currents. For details, see the Beach Hazards Statement and the Surf Zone Forecast. && .FIRE WEATHER...Offshore winds developing and strongest for late Sunday into Tuesday. Initially some precipitation is possible. && .SKYWARN... Skywarn activation is not requested. However weather spotters are encouraged to report significant weather conditions. && .SGX WATCHES/WARNINGS/ADVISORIES... CA...Beach Hazards Statement through Friday afternoon for Orange County Coastal Areas-San Diego County Coastal Areas. PZ...NONE. && $$ PUBLIC/FIRE WEATHER...Tardy AVIATION/MARINE...Moede