LOS ANGELES — A cold and blustery winter storm unleashed downpours and extensive snowfall on Southern California, triggering a tornado and snarling post-Christmas travel on major routes Thursday.

Snow shut down vital Interstate 5 in Tejon Pass through the mountains north of Los Angeles and stopped traffic on Interstate 15 over Cajon Pass in the inland region to the east. I-15 finally reopened in both directions in the afternoon but then authorities later shut down about 45 miles of the freeway from Baker, California, to Primm, Nevada, on the way to Las Vegas because of snow and ice.

A truck driver was found unresponsive in a rig stopped along I-5 in Tejon Pass. Kern County firefighters pronounced him dead, according to the California Highway Patrol’s online incident log.

It was not immediately known if the death was weather-related.

After being stuck for five hours in Cajon Pass, motorist Johnny Lim wasn’t sure he could reach Las Vegas, where he hoped to spend time after Christmas. He worried about driving his car through the pass when it reopens.

“The freeway is full of snow and ice,” he said, adding that his car “is not built for conditions like this.”

I-5 rises to more than 4,100 feet in Tejon Pass between Los Angeles and the San Joaquin Valley. Cajon Pass rises to more than 3,700 feet between the San Gabriel and San Bernardino Mountains on I-15, the major connector between Southern California and Las Vegas.

Snow also stopped or slowed traffic on other California roads, including heavily traveled Interstate 8 in eastern San Diego County, and numerous trees were toppled.

In San Diego County, a 60-year-old woman died Thursday when a car veered off a snowy road near Warner Springs and crashed into an embankment. The woman passenger wasn’t wearing a seatbelt and the driver was going too fast to control the car, California Highway Patrol Officer Jeff Christy told KSWB-TV.

On Wednesday night, a small tornado briefly hopped across Ventura Harbor, the National Weather Service reported. Gusts downed five trees, according to the Ventura County Star. Winds up to 65 to 85 mph (105 kph to 137 kph) also caused minor damage to nearby buildings, forecasters said.

Localized flooding inundated roads and freeway lanes while snow fell to low elevations, leading to numerous traffic accidents.

The town of Wrightwood at the eastern end of the San Gabriel Mountains had received 25 inches of snow by midday.

Famed Palomar Observatory in the San Diego County mountains said it would be closed at least through the weekend because of heavy snowfall.