textproduct: San Diego

SYNOPSIS

Weakening offshore flow will transition to weak onshore flow with slow deepening of the marine layer through the weekend with a greater return of low clouds and fog to coastal areas for the weekend. Gradual cooling will spread inland through the weekend, but with 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.

DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE

SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES...

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.

AVIATION

112100Z...Clear skies and VFR conditions still expected at all sites through 02Z this evening. Chances for Patchy fog and low cloud development (30-60%) from the coast up to 15 SM inland will increase after 03Z for Orange County and after 05Z for San Diego County. Ceilings around 100-500 ft and visibilities less than 1 SM will be possible through 16-18Z. Conditions will begin to clear after 16Z in San Diego County and will linger in Orange County through 18-19Z.

MARINE

There is a 30-60% chance for fog development this evening through Friday morning. Should fog develop, visibilities below 1 nautical mile will be possible. Otherwise, no hazardous marine conditions are expected today through Wednesday.

SGX WATCHES/WARNINGS/ADVISORIES

CA...None. PZ...None.