728 FXUS66 KSGX 130409 AFDSGX Area Forecast Discussion National Weather Service San Diego CA 910 PM PDT Sun Sep 12 2021 .SYNOPSIS... High pressure over Southern California will slowly weaken over the next few days, as a trough develops along the coast. Increasing onshore flow will spread cooling inland this week, with a slow return of coastal marine clouds and fog. Stronger onshore flow will build the marine layer further late in the week, continuing the cooling trend. A long-period south swell will impact the beaches with high surf and dangerous swimming conditions through Wednesday. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... Skies were clear over SoCal at 9 PM PDT, but changes were afoot offshore where a solid marine stratus layer was making inroads over the CA Bight. Another patch had formed just off the immediate coast. Given the weak onshore sfc pressure gradient trends, expect we will see better coverage of the stratus by morning. Our 00Z Miramar sounding was on board as well, with the marine inversion base up to 1300 FT MSL from around 1000 FT yesterday evening. The Heat Advisory has expired, so the forecast was updated to remove the headlines. No other forecast updates are planned. High pressure aloft over SoCal will weaken through midweek and be replaced by a weak trough by the end of the week. This will set up a slow cooling trend that will drop daytime temperatures back below average by midweek along and west of the mountains, while desert temperatures drop to about average. The incoming airmass will be quite a bit drier through Wednesday, then humidity will increase as winds aloft turn more southerly and the marine layer builds farther inland for the latter half of the week. Look for a slow increase in marine clouds this week, with a crescendo by next weekend. && .AVIATION... 130340Z...Coast...Patchy stratus developing along the San Diego County Coast this evening. Periods of CIGS 600-900 ft MSL possible at KSAN and KCRQ 08-16Z with vis 3-6 SM in BR/HZ. Clouds locally spreading 5-10 sm inland overnight. Clouds/vis restrictions clearing 15-17Z. Patchy stratus redeveloping along the coast after 14/04Z with bases 700-1100 ft MSL. Mountains/Deserts...Clear skies with unrestricted vis through Monday evening. && .MARINE... No hazardous marine weather is expected through Thursday. && .BEACHES... A series of Southern-Hemisphere swells of 2-4 ft from 190-200 degrees with periods of 16-20 seconds will move through the coastal waters this week, with the most significant swell Monday of 4 ft/18 sec. Surf of 5-8 ft in Orange County/far north San Diego County and 3-6 ft south of Carlsbad will continue through at least Wednesday. Strong rip and longshore currents are likely at all beaches through much of the week as well. A High Surf Advisory remains in effect for Orange County, and a Beach Hazard Statement for San Diego County, both valid 9 AM Monday- 9 PM Wednesday. && .FIRE WEATHER... Stronger onshore flow and an influx of drier air Monday through Wednesday will elevate fire weather conditions, over the mountains and deserts. The main areas of concern will be below the desert passes and along the desert slopes where minimum RH of near 10% and southwest to west winds of 15-30 mph with gusts 35-40 mph are expected late each afternoon and early evening. Relative humidity will increase for the latter half of the week. && .SKYWARN... Skywarn activation is not requested. However weather spotters are encouraged to report significant weather conditions. && .SGX WATCHES/WARNINGS/ADVISORIES... CA...Beach Hazards Statement from Monday morning through Wednesday evening for San Diego County Coastal Areas. High Surf Advisory from 9 AM Monday to 9 PM PDT Wednesday for Orange County Coastal Areas.NONE. PZ...NONE. && $$ PUBLIC/FIRE WEATHER...10 AVIATION/MARINE/BEACHES...SS