Warmer conditions expected through early next week with a shallower marine layer and periods of weak offshore flow. Areas of fog that is locally dense is expected through Sunday morning along the coast. Both temperatures and offshore winds are expected to peak Sunday and Monday. Cooler conditions expected by the middle to end of next week. A windier and potentially wetter pattern is expected to develop for the end of next week.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
At 9 AM, visible satellite was showing low clouds lingering along the immediate coast. Ground observations through the morning have been showing reduced visibility across much of the coast and western valleys. The most impacted visibility has been on the coastal mesas, like at Brown and Montgomery Fields, where dense fog has been reported. With building high pressure aloft, the marine inversion is fairly strong which lowers confidence in complete clearing along the immediate coast this afternoon. The marine layer will become a bit shallower into Sunday as the high pressure strengthens. Areas of fog and dense fog will become more confined to coastal locations for this evening into Sunday morning.
In addition to fog, strengthening high pressure aloft and periods of offshore flow will bring warming conditions across the area, particularly to the coast and valleys this weekend. The peak of the heat is expected for Sunday and Monday when highs will be as much as 12-17 degrees above seasonal averages. Areas of minor to moderate HeatRisk are expected, with the highest HeatRisk expected in the western valleys. Weak to locally moderate northeast to east winds are expected on the coastal slopes of the mountains and through passes and canyons. Strongest winds are expected on Sunday and Monday when gusts of 30-40 mph are expected in the favored locations.
For the middle of next week, guidance is becoming more inline. There is increased confidence in the cooling trend starting for areas west of the mountains on Tuesday, spreading further east into Wednesday. We also have much higher confidence that Wednesday will stay dry. Chances of precipitation enter the forecast as early as Thursday north of San Diego county, with chances increasing and spreading south and east into Friday. Ensemble members of both the ECMWF and GFS are beginning to show the highest chances of precipitation being sometime on Friday. There does remain considerable uncertainty in amounts of precipitation with some members showing much wetter solutions than others. This likely has to do with lingering uncertainty in the timing of the low pressure system and how far south it will dig. This uncertainty continues to impact other factors of the forecast including the temperature, wind, and chances of snow. Current forecast for the end of the week closely follows the NBM with cooler and cloudier conditions and chances of precipitation starting as early and Thursday and continuing through next Saturday.
081630z, Coasts, Low clouds based 300-600 ft MSL currently over coastal areas up to 10 miles inland. Vis reductions 1-4 SM at sea level and dense FG along elevated coastal terrain will slowly improve through 18Z. Clouds scatter to the beaches 18-20Z but likely to linger along the immediate coast through Saturday afternoon. Low clouds 400-800 ft MSL move back ashore after 00Z Sun, spreading up to 10 miles inland again with similar vis reductions initially. Expecting bases to lower slowly overnight with areas of dense FG down to sea level after 06-09Z (including KSAN).
Valleys/Mountains/Deserts, Clear skies and VFR conditions through the period. East to northeast winds picking up after 12Z Sun downwind of passes through the Inland Empire and along coastal slopes of the mountains leading into the foothills. Intermittent gusts 25-35 mph in these areas.
Low clouds over coastal waters today will slowly lower toward sea level starting this evening. Visibility is expected to drop below 1 SM widespread after 06-09Z Sun with areas of dense fog through Sunday morning.
Ca, None. PZ, None.
