Weakening offshore flow will transition to weak onshore flow with a greater return of low clouds and fog to coastal areas for the weekend. Gradual cooling will spread inland, but weekend high temperatures still as much as 15 to 20 degrees above average for portions of the inland valleys and lower coastal slopes of the mountains. There will be slight warming on Monday with slightly stronger offshore flow possible, followed by slight cooling for Tuesday and Wednesday.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
It was another warm day today and San Jacinto (86 deg), Escondido (86 deg), Lake Cuyamaca (71 deg), and Borrego (85 deg) all broke their daily high temperature records today. Lake Elsinore (87 deg) and Ramona (82 deg) both tied their daily high temperature records. More information on today's records can be found on our weather.gov/sandiego homepage. Highs tomorrow are expected to be a few degrees cooler than today for the coast and valleys, with similar temperatures expected for the mountains and deserts. Big Bear, Lake Cuyamaca, and Borrego are all in contention for breaking or tying high temperature records tomorrow.
From Previous Discussion Issued at 2 PM December 11,
The upper level ridge overhead continues to dominate the weather pattern across southern California, providing a stout subsidence inversion and limiting the strength of onshore winds. The weak offshore winds that have been in place for the inland areas will continue to gradually wane over the next few days with onshore flow slowly becoming more established in the afternoon as the ridge axis pushes eastward into the weekend. The effect of this pattern is resulting in abnormally warm afternoons with high temperatures today/Friday 15-20 degrees above climatological norms and cool nights for the inland valleys. The subsidence inversion (peaking at about 3000ft) is keeping the foothills quite mild overnight, with some spots seeing record warm overnight temperatures (15-25+ degrees above normal). As the ridge slowly slides eastward into the weekend, this inversion will weaken some and moderate the abnormally warm nights for the mid slopes. The marine layer has been largely squashed from the inversion and lack of onshore flow, but will slowly restrengthen tonight and moreso on Saturday morning. Any cloud cover tonight from the marine layer will be primarily over the waters and nearshore coasts but some clouds may leak over the immediate coast tonight into Friday morning, likely serving as patches of fairly dense fog with visibilities 1 mile or less. Cloud cover should be a bit more uniform and push a bit more inland, with areas of fog expected again late Friday night into Saturday morning. Afternoon highs cool about 5 degrees for Saturday/Sunday afternoon compared to today, but still about 10 degrees above normal.
A weak shortwave rounding the displaced ridge moves through central California on Sunday, and through the Great Basin on Monday. While the influence of the ridge has limited onshore flow in the afternoon and allowed weak offshore flow in the overnights, this shortwave early next week will likely serve to provide a setup for slightly stronger offshore winds Monday afternoon, albeit still weak. The marine layer will likely be more shallow come Tuesday morning as a result, with afternoon high temperatures staying similar to this weekends, about 10-15 degrees above normal. Upper level flow turns quasi-zonal Tuesday into Wednesday, reestablishing onshore flow and a more present marine layer. Temperatures respond with some cooling for midweek, with high temperatures running about 5 degrees above normal.
120430z, Coast, Low clouds and FG over the nearshore waters will spread into portions coastal Orange/San Diego Counties up to 10 miles inland starting 06-09Z. Coverage should remain patchy with intermittent clearing. Ceilings around 100-500 ft and visibilities less than 1 SM where FG develops. Any clouds and FG that develop will scatter out 15-17Z in San Diego County, but may linger in Orange County through 18-19Z. Low clouds and FG return to coastal areas 03-06Z Sat.
Valleys/Mountains/Deserts, SKC and VFR conditions expected through Friday evening.
A patch of low clouds and FG over coastal waters will likely expand through the overnight hours. Areas of visibilities below 1 nautical mile are likely (50-70% chance). Low clouds may intermittently scatter out Friday afternoon, but low clouds and visibility restrictions should become widespread again by 00Z Sat. Otherwise, no hazardous marine conditions are expected through Wednesday.
Ca, None. PZ, None.
