An upper level low from the north will bring occasional showers late tonight through Monday morning, along with strong gusty winds in the mountains and deserts and cooler weather. Some snow could fall in the higher mountains. Monday afternoon through Wednesday will be drier with a warming trend as the upper low moves east, though upper level low pressure will likely strengthen again over the West Coast the second half of next week for renewed cooling and possibly a few showers.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Mostly cloudy conditions prevailed this evening, with greatest cloud cover in the mountains and inland valleys. The upper low will continue to approach from the northwest with spotty showers developing overnight, then a front from the west around sunrise Sunday will spread more organized, albeit mostly light, precip onto land. There will be orographics at times over the south and southwest facing slopes due to winds around 850 MB of 25-45 knots and saturation up to 8000 to locally 14000 feet above sea level. After the initial front Sunday morning, the precip should weaken at lower elevations but not weaken much in the mountains due to the orographics, but then the main upper low will track over the Mojave Desert Sunday evening, and a wave associated with the low will enhance the precipitation. Instability will be insufficient for thunderstorms, but some brief moderate to heavy rain could occur in the mountains with maximum rates up to 0.30 or 0.40 inches in an hour. Overall precipitation totals appear to average around 1/4 inch at lower elevations and 1/2 to 1 inch in the mountains, with the potential of local 2 inch amounts on the south slopes in San Bernardino County. Snow levels should mostly be 6500-7000 feet Sunday but should fall to 5500-6000 feet Sunday night, with the potential of a few inches of snow, mainly close to the low in San Bernardino County. With the pressure height gradients aloft producing winds up to 45 knots at 850 MB and 60 knots at 700 MB, there will be strong winds in the mountains and deserts, especially in the usual locations from the mountain crests through the desert slopes and through San Gorgonio Pass. Local gusts could reach 65 MPH Sunday afternoon and evening. Of course, with an unseasonably strong upper low for late May, maximum temperatures will be 15-25 degrees below normal Sunday.
Models indicate some lingering showers Monday, mostly early in the day, with Monday still being unseasonably cool but generally 5-10 degrees warmer than Sunday. Almost all GFS ensemble runs show deep troughing continuing just inland from the West Coast Monday through Wednesday, but at least with increasing heights, temperatures should increase though not quite to normal levels. Some sort of upper low could dive south along the West Coast the second half of next week and bring more cooling, a deep but possibly disorganized marine layer and perhaps a few showers, though moisture will be limited.
260300z, Coast/Valleys, Low clouds off the coast will be moving ashore sometime around 05Z Sunday with BKN-OVC cigs 1500-2500 ft MSL. These low clouds will push into most inland valley locations and obscure coastal slopes and higher terrain. Isolated -SHRA possible after 26/08Z spreading in coverage from west to east. Bases 1500-2000 ft MSL and local vis 3-5 miles possible during showers. These cigs and showers will prevail through much of the day on Sunday.
Mountains/Deserts, SCT-BKN cigs this evening, becoming BKN-OVC. Coastal mountain slopes becoming obscured in clouds/fog/-SHRA after 26/08Z. SW to W winds beginning to pick up this evening with gusts 25-35 kts. Winds peaking afternoon Sunday with sfc gusts 35-45 kt along the mountain crests/desert mountain slopes and below the passes. MOD-STG UDDFS and LLWS expected over and NE/E of the mountains.
No hazardous marine weather is expected through Thursday.
Ca, Winter Weather Advisory from noon Sunday to midnight PDT Sunday night for Riverside County Mountains-San Bernardino County Mountains.
Wind Advisory from 6 AM Sunday to 2 AM PDT Monday for Apple and Lucerne Valleys-Coachella Valley-Riverside County Mountains- San Bernardino County Mountains-San Diego County Deserts- San Diego County Mountains-San Gorgonio Pass Near Banning.