Above average temperatures and weak offshore flow will continue through Wednesday. Areas of low clouds and dense fog, in the coastal areas and western portions of the inland valleys, will continue into Wednesday morning. A cooling trend begins Wednesday and continues through Friday as a low pressure system moves in from the Pacific, helping to build the marine layer and extend low clouds and fog inland. This low will also bring chances of precipitation for locations mainly over and west of the mountains Thursday into Saturday morning. Dry conditions are expected to return Sunday with a warming trend and prevailing offshore flow.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
This evening, weakening offshore flow has enabled low clouds and fog from a shallow marine layer to surge several miles inland. This has resulted in areas of dense fog, mainly on elevated coastal terrain and in western portions of the inland valleys. The reduced visibility will likely have impacts for travel, whether by road or air. The fog should lift and clouds begin to cleat by late Wed morning. Onshore flow will strengthen for Wed and Thu ahead of a low pressure system which will likely spread the low clouds and fog farther inland causing the low visibility to migrate upward to higher terrain, reducing the chances for dense fog in the coastal areas Wednesday night.
From previous discussion, Highs through Wednesday are expected to remain above average as a ridge of high pressure lingers over the US West Coast. By Thursday, an approaching low pressure system will become the main influence on our weather. In addition to spreading low clouds and fog further inland, temperatures will drop 5 to 8 degrees for inland locations between Wednesday and Thursday. Cooling will continue into Friday. There remains some uncertainty on when the low will be far enough east for the ridge to re-build over the West Coast, with 50% of ensemble members keeping Southern California under the influence of the trough into Saturday. Current forecast for Saturday through next Tuesday follows the NBM which follows a warmer solution with a few degrees of warming expected Saturday and continued warming into next week. For early next week, all ensemble solutions have a ridge solidly in place over the US West Coast, although there is some lingering uncertainty in the strength of the ridge. There is high confidence in another period of warmer conditions, with less confidence in just how warm it will get.
The aforementioned upper level low will also bring slight chances of showers to the area. Scattered to isolated showers could begin as early as Thursday with slower solutions continuing chances of precipitation into Saturday for some areas. Current forecast has highest chances of rain in and west of the mountains Thursday and Friday, with lingering chances of precipitation into Saturday morning for San Diego County and coastal slopes of the mountains. Latest high resolution guidance is indicating hourly rainfall rates will range from 0.05-0.25" per hour. Rates at or near 0.25" per hour will be fairly isolated and most likely occur Friday. Chances of rainfall totals exceeding 0.50" from Thursday through Saturday, are 10-15 percent for the coast and valleys and 20-30 percent in the mountains. Snow levels will fall to 6500 to 7000 ft by Friday morning. 1-3 inches of snow are expected with totals at the upper end of the range more likely above 7000 ft.
210400z, Coast/Western Valleys, Areas of low clouds based 400-700 ft MSL over coastal areas are currently producing minor vis reductions (2-5SM) at the coast and pockets of dense FG for elevated coastal terrain (including KCRQ). Low clouds may intermittently move in and out of coastal areas before becoming more widespread after 06Z Wed. Low clouds reaching 10-15 miles inland overnight with areas of dense FG becoming more likely for elevated coastal terrain and western valleys. Vis reductions may be somewhat alleviated after 09Z if bases manage to lift a few hundred ft. Regardless, low clouds scatter to the coast 16-19Z, but likely linger at the coast through the afternoon. Clouds move inland once again Wednesday evening with higher bases and greater inland extent.
Eastern Valleys/Mountains/Deserts, VFR conditions expected to prevail throughout the period.
Most of the low clouds have lifted above sea level this evening, allowing for visibility to improve in most areas. However, patches of dense fog leading to visibilities 1 nautical mile or less remain possible over the waters through Wednesday morning. Otherwise, no hazardous marine conditions are expected through Sunday.
Ca, Dense Fog Advisory until 9 AM PST Wednesday for Orange County Coastal Areas-Orange County Inland Areas-San Diego County Coastal Areas-San Diego County Valleys.
PZ, None.
