High pressure will build over Southern California through Thursday bringing hot weather inland, while a shallow marine layer and the sea breeze keep coastal areas mild, with areas of low clouds and fog at times. Late in the week, a low pressure system will approach the coast, increasing onshore flow, and spreading cooling inland.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
No major changes this evening. The stratus over the coastal waters has started to increase in coverage and will continue to do so through the night. The shallow marine layer will bring in some low clouds to many coastal areas as the evening progresses. Patchy dense fog will be possible for our friends on the water into the coastal valleys into the early morning hours.
Early Afternoon Discussion (Issued at 1:46 PM PDT):
A strong ridge will build over CA through midweek, heating up inland areas, while coastal areas are moderated by the sea breeze and a shallow marine layer. A strong inversion will cap the marine layer and may lead to some May Gray along coastal area this week. There will be sharp thermal transition from the coast to the inland valleys where temperatures will be running some 10-15F above average. Maximum temperatures over the high deserts will eclipse the century mark Wed/Thu, while the lower deserts approach or exceed 110F. An Excessive Heat Warning has been issued for the deserts.
By Friday, a cut-off low will approach the CA coast, helping to flatten the western periphery of the ridge aloft. This in turn, increases the sfc pressure gradient onshore, and will help spread cooling inland this coming weekend. The latest operational solutions of the GFS/ECMWF are fairly close approaching the weekend, but diverge considerably after that into next week.
For the latter part of the forecast, the ECMWF lags in advancing Pacific trough over the West, keeping the main longwave trough axis off the coast through the middle of next week. After some cooling Fri/Sat, warming reappears in most of the ensemble members of the 12Z ECMWF, while all of the GFS ensemble members stay much cooler into early next week. This takes confidence in the temperature forecast, especially inland, off a cliff beyond about Saturday. For the forecast temperatures in latter periods, a blend of solutions was ultimately favored to hedge toward the ECMWF solutions.
260300z, Coast/Western Valleys, Low clouds with bases 500-800 ft MSL and tops to 1000 ft MSL will develop along the coast 04-07Z, latest in Orange County, then spread 10-15 mi inland overnight. Areas of 3-5 SM likely over higher coastal terrain, with local vis 1 SM or less in FG on the coastal mesas and far western valleys, possibly at KCRQ 11-15Z. Stratus and vis restrictions clearing to the coast 16-18Z with some beaches remaining BKN-OVC through the afternoon. Stratus will move begin to move back into the coastal areas 27/00-03Z with bases near 500 ft MSL.
Inland Valleys/Mountains/Deserts, A few high clouds and unrestricted vis through Tuesday evening.
Patchy dense fog could impact the coastal waters during the nights and mornings through Tuesday through Thursday, with the highest potential Tuesday night into Wednesday morning. Visibility may be less than 1 nm at times. Otherwise, no hazardous marine weather is expected through Saturday.
A new long-period south-southwest swell will arrive on Tuesday and peak Wed and Thu at 3 ft, 16-18 sec. A second south swell arrives Friday and peaks Saturday, gradually lowering Sunday in to Monday. This will generate elevated surf of 3 to 5 feet at southwest facing beaches and a high risk of rip and longshore currents through the weekend. Occasional sets of 6-7 ft are possible on exposed south- southwest facing beaches.
Ca, Excessive Heat Warning from noon Tuesday to 7 PM PDT Friday for Apple and Lucerne Valleys-Coachella Valley-San Diego County Deserts-San Gorgonio Pass Near Banning.