A major winter storm swept south through California on Thursday, bringing heavy snow and strong winds to mountains and steady rain elsewhere, while prompting mandatory evacuations for coastal areas to the south that were devastated by deadly mudslides in January.

As many as 30,000 people were ordered to leave an area of Santa Barbara County before the storm arrived early Friday.

Sheriff Bill Brown said forecasters weren't certain how intense the storm would be when it arrives in Southern California. However, modeling indicates "there is a risk for dangerous flash flooding, mud and debris flows," he warned.

"I'm not going anywhere," Montecito resident Harriet Mosson said.

The 76-year-old said the three-story condo building where she lives was not damaged in January because it's on the ocean side of U.S. 101, which helped divert the mudslides that came down the mountains.

"Can it happen again? Yeah, I guess it can. Will it, now? I doubt it," she said. "And if something terrible happens I'll be able to get out of here."

People cannot be forced to leave their homes under a mandatory evacuation order, but authorities said they should not be expected to be rescued while the storm event is occurring.

To the north, a blizzard warning was issued for parts of the Sierra Nevada, where winds could gust up to 125 mph (200 kph) on ridges and 60 mph (95 kph) in some valleys, the National Weather Service said.

The cold front is expected to bring snow to foothill areas as low as 3,500 feet (1,066 meters), and officials warned people to stay off mountain roads.

The California Department of Transportation said 90-miles stretch of Interstate 80 was closed between Colfax, California, and the Nevada state line due to whiteout conditions.

Chains or snow tires were needed on stretches of Interstate 80, U.S. 50 and U.S. 395. Dozens of collisions were reported on San Francisco Bay Area highways.

Northern mountains were expected to receive as much as 5 feet (1.5 meters) of snow and reach 7 feet (2.1 meters) in some areas.

The snow will help the Sierra snowpack, which is vital to the state's water supply and is only about a quarter of its normal depth for this time of winter.

Stephanie Myers, a spokeswoman for Heavenly Mountain Resort and Kirkwood Mountain Resort in Lake Tahoe, estimated snow was falling at about 2 inches an hour.

"We're thrilled about this storm," Myers said. "Once the storm moves out of the area, we'll have a beautiful bluebird powder day."

Predictions of widespread showers raised concern about flash flooding when the storm reaches Southern California.

Santa Barbara County ordered the immediate evacuations for residents of areas burned by three major wildfires over the past 18 months.

The order encompasses Montecito, where a Jan. 9 storm triggered flash floods that destroyed or damaged hundreds of homes. Twenty-one people were killed and two remain missing. Other areas impacted by the order are Goleta, Santa Barbara, Montecito, Summerland and Carpinteria.

The county is following a rigorous new system of alerts that emphasizes evacuations well in advance of storms rather than suggesting residents can use their discretion.

"We want everyone to be out of the area, and out of harm's way, by nightfall," Brown said, adding that evacuations in areas stripped bare by wildfires will be part of "the new normal" following the devastation in Montecito.

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Associated Press writers Amanda Lee Myers and John Antczak in Los Angeles contributed to this report.