Stuck in an Unexpected Storm : Weather: Snow in the mountains closes highways and strands thousands of Southland drivers.

A cold, blustery winter storm descended unexpectedly on Southern California on Tuesday, stranding thousands of commuters along routes blocked by snow, snarling traffic and closing portions of two freeways that had been damaged by rain.

A 100-foot section of the San Diego Freeway in Hawthorne sank after heavy rain undermined its foundation Tuesday, closing one northbound lane, the California Highway Patrol said. Two lanes of the southbound Ventura Freeway north of Ventura have been closed since Sunday when the roadbed collapsed in the rain, leaving a hole “as deep as a car and as long as a semi,” a CHP dispatcher said.

The CHP closed Interstate 5 in both directions on the Grapevine near Gorman on Tuesday afternoon, sending traffic on a 100-mile detour through the Antelope Valley. The detour itself was blocked half an hour later when snow clogged the Antelope Valley Freeway near Agua Dulce.

All northbound routes into the Antelope Valley were closed by early evening, cutting off thousands of motorists from their homes.

More than 40,000 residents of the Antelope Valley are estimated to have jobs in the Los Angeles Basin. Those stranded by the snowfall jammed restaurants, gas stations and motel rooms or sat in cars on the shoulder of the Antelope Valley Freeway and other routes linking the valleys.

The freeway was closed to northbound traffic at Escondido Canyon Road near Acton and to southbound traffic at Palmdale Boulevard in Palmdale. The snow left the section in between littered with cars that had spun out and become stuck. Tow-truck operators shuttled some motorists back to roadblocks. Others had to walk.

“It was horribly frightening to be on that freeway,” said Mary Fairley, who was stranded on the Antelope Valley Freeway for two hours with a co-worker, Alberta Jones.

By 10 p.m., the Antelope Valley Freeway remained closed, but the CHP had begun escorting traffic over the Grapevine, a spokesman said.

Scores of skiers were trapped at the mountain resort of Wrightwood in the San Gabriel Mountains, filling hotels and restaurants to capacity. Among those trapped were 36 children from the San Fernando Valley and Westside whose bus skidded off the road and into a snow bank during an outing organized by the Sierra Ski and Pack Club. No one was injured, but trip organizers called parents to tell them not to expect the children home Tuesday.

By 5 p.m., 0.25 of an inch of rain had fallen on the Los Angeles Civic Center, raising the season’s total to 5.53 inches, 0.4 of an inch above the normal seasonal total for the date.

The high temperature in downtown Los Angeles was only 51 degrees, just four degrees above the normal low temperature for the date and only one degree above the coldest maximum ever recorded here on Jan. 7, in 1913.

The snow, which dusted the Tehachapi, San Gabriel and San Bernardino mountains throughout the day, began falling heavily in the late afternoon. Chains were required above 1,500 feet throughout the San Bernardino Mountains.

An Orange County couple was rescued in those mountains Tuesday after being stranded on a road near Lake Arrowhead for more than two days. Searchers found them near their vehicle, which they had just left to seek help.

Stephen Stevens, 24, of Fullerton, and Cheryl McCaslin, 23, of Anaheim, were rescued at 6:45 p.m., 2 1/2 days after they had left on a scenic drive near Big Bear.

“They stayed with their vehicle and that’s what saved them,” said Mark Youngblood, search and rescue coordinator for the Big Bear station of the San Bernardino County Sheriff’s Department.

However, searchers were still looking for a team of rescuers who had not returned.

The storm, which originated in the Gulf of Alaska, had been expected to move eastward across Northern California.

Instead, the weather system rode high-altitude jet-stream winds south along the coast, hitting the seaside community of Half Moon Bay, about 80 miles south of San Francisco, before dawn.

Gusts of more than 40 m.p.h. intensified as they swept through a coastal canyon, damaging at least 30 mobile homes at the Canada Cove Mobile Home Park.

“It took some of the carports and threw them 20 feet away,” said Ed Dudley, manager of the park.

Steve Burback, a meteorologist with WeatherData Inc., said the storm should have moved out to the east by dawn today, leaving clear skies and high temperatures from the mid-50s to the mid-60s.

Another, weaker storm system is expected to bypass Southern California on Friday, with generally clear weather on Saturday. But Burback said a third storm could bring more rain Sunday.

Times staff writers Marla Cone, Greg Braxton, John Chandler, Blaine Halley and Nieson Himmel contributed to this story.