A low pressure system will be moving onshore this weekend, bringing cooler weather, more clouds and periods of showers. Isolated thunderstorms could develop over portions of San Bernardino County as well. A few showers could linger into Monday morning west of the mountains. Sunday will be the coolest day when temperatures will be more than 20 degrees below average inland. This will be followed by dry and gradually warmer weather through next weekend.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Marine layer low clouds have filled in over the coastal areas and valleys of San Diego County, but are struggling to move into the Inland Empire this morning. Clouds should eventually fill in, with a slight (around 10%) chance of drizzle later this morning, mainly near the coast. Elsewhere there are plenty of high clouds around ahead of an upper level low currently centered off the Central CA coast. This low deepens slightly as it drops south off the coast today. This will maintain cool weather with highs 5 to 15 degrees below normal, with the greatest departure from normal in the valleys and foothills.
The low then begins to progress eastward and into So Cal late this evening into Sunday, with a second weak wave rotating around the back side of the low Sunday night into Monday. Both GEFS and EPS show measurable precipitation for much of the forecast area, especially tonight, which is no surprise as a 100 kt jet streak takes aim at northern Baja, providing plenty of lift. The bulk of the precipitation will be light showers from an extremely deep marine layer along and west of the mountains, with rainfall amounts generally around 0.10" or less. There is some weak elevated instability forecast with this low, mainly north of San Diego County, tonight into Sunday. Most of this instability looks to exist from Riverside/San Bernardino County northward, where a slight chance of thunderstorms has been maintained in the forecast. Some weak and shallow surface based instability follows over the higher peaks of the San Bernardino Mountains on Sunday afternoon, which would allow for locally stronger convection with rainfall rates potentially 0.25"/hr or more. Instability diminishes by Sunday evening, with just some lingering light showers from the deep marine layer Sunday night into Monday morning as the final wave moves through.
In addition to the precipitation, gusty southwest to west winds will develop across the mountains and deserts each afternoon and evening. Peak gusts will mostly be around 35-45 mph, with isolated gusts to around 50-55 mph below the San Gorgonio Pass. Sunday and Monday will remain cool with highs 15-25 degrees below normal.
After the low lifts out Monday, we'll be wedged between a trough moving inland across the Pac NW and a closed low to our SW that sits beneath the Pacific high. This will allow for gradual warming with a marine layer that gradual becomes more shallow. Given that we never get underneath some solid ridging, highs never manage to climb all the way back to normal, with the exception of the beaches and possibly the low deserts. At least there will be more sunshine, especially late in the week, when the marine layer is much shallower.
100935z, Coast/Valleys, BKN-OVC cigs 1500-3000 ft MSL will remain for most areas west of the mountains through mid morning. Moderate to high confidence cigs clearing the Inland Empire 17-19Z with BKN cigs near 2000-3000 ft MSL remaining across most coastal/valley areas of OC/SD counties. -DZ/-RA and cigs pushing into most areas west of the mountains after 03Z Sun.
Mountains/Deserts, BKN250 tonight with no vis restrictions. SCT- BKN150-200 Saturday. West winds increasing after 22Z Sat. 15-25 kt gusting 35-45 kt leading to MOD UDDFS. Winds will continue Saturday night through Sunday.
No hazardous marine conditions are expected through Wednesday.
Ca, None. PZ, None.