Jet Stream Brings Winter Weather Back

High-altitude jet stream winds began pouring into Southern California from Alaska on Tuesday, ushering in a dramatic change in the weather that will see Monday’s high temperatures in the 80s replaced with lows that could dip near freezing by this morning.

Forecasters say the Southland’s flirtation with an ephemeral summer will give way to the reality of winter, with a good chance of rain in the valleys and snow in the mountains by the weekend.

Tom Carlson, a meteorologist with Weather Central, which provides forecasts for The Times, said temperatures stayed far above normal through Monday because of a ridge of high pressure that kept the jet stream well north of Southern California.

Air circulating around this ridge swept down mountain passes into the coastal valleys of Los Angeles, Orange and San Diego counties, heating and drying out by compression as it headed offshore.

Thermometer readings Monday included 90 degrees at San Gabriel and El Cajon, the warmest spots in the nation. Other readings included 88 in Irvine, 86 in Santa Ana, 85 in Montebello, 84 in Pasadena, 82 in Burbank and 81 in downtown Los Angeles.

Carlson said all that changed Tuesday as the ridge moved east.

“When that happened, the jet stream took a big dip, heading down the coast from the Gulf of Alaska and pulling tons of cold air with it,” he said.

A dry but powerful cold front whipped through Southern California Tuesday afternoon, generating gusts that were expected to drop the wind chill below 10 degrees early this morning in some parts of the Tehachapi, San Gabriel and San Bernardino mountains.

Carlson said temperatures in the coldest parts of the San Fernando and San Gabriel valleys could dip close to 30 degrees before dawn today. He said today’s daytime temperatures are not expected to rise above the mid-50s, and brisk north winds “probably will make it feel a lot colder than that.”

It should be a little warmer Thursday, with highs in the 60s. “But the winds will remain fairly strong, so you won’t notice much improvement,” Carlson said. “And it’ll go downhill from there.”

He said clouds will begin increasing Friday as the jet stream starts pushing an Alaskan storm system into Southern California. Scattered light showers should begin falling in Los Angeles late Friday night, continuing through Saturday. The snow level is expected to dip to between 3,000 and 4,000 feet.

“The precipitation shouldn’t be very heavy,” Carlson said. “Something like a quarter of an inch of rain in Los Angeles.”

Forecasters said Sunday should be cloudy, dry and cool, with high temperatures at the Civic Center in the 50s. Carlson said there’s a slight chance of more rain from another storm system Monday night.