LOS ANGELES - CALIFORNIA took a pounding from a powerful storm that collapsed the roof of a trade school, made roads treacherous and forced residents from homes out of fear of mudslides from fire-stripped hillsides.

Interstate 15, the main route connecting Southern California and Las Vegas, was closed by heavy snow in a mountain pass east of Los Angeles. Interstate 5 north of Los Angeles was snarled by snow that forced the California Highway Patrol to shepherd motorists over another pass.

About 20,000 customers lost power across the state, utilities reported.

At its height on Monday, the storm triggered hundreds of fender-bender traffic accidents on freeways, highways and surface streets across California, Washington and Nevada.

In Anaheim, students and teachers fled a trade school when the roof of a classroom collapsed during heavy rain on Monday. Twelve students and two staff members were hospitalised with mostly minor injuries, and all but one were released by afternoon, said Lynn Porter, assistant superintendent of educational services.

About 150 students were at the center, which prepares high school students and adults for medical occupations, said spokesman Pat Hansmeyer.

In Yorba Linda, where 100 homes were lost to a wildfire last month, hundreds of residences in the canyon areas were ordered evacuated on Monday morning because of the threat of mudslides. But the threat eased later in the day.

The area was one of several placed under a flash-flood watch because hillsides and canyons had been stripped of vegetation by recent wildfires, increasing the chances of severe flooding and mudslides.

A section of the Pacific Coast Highway in the Sunset Beach area was closed in both directions by flooding.

At least three deaths were linked to the storm. A California Highway Patrol officer directing traffic around an accident in Hacienda Heights, east of Los Angeles, was hit by an out-of-control car, said patrol Officer Terry Liu.

One person was killed in the San Diego area when an armored truck slid off a wet highway and rolled down an embankment.

Authorities said the storm also contributed to the death on Sunday of a 61-year-old man who was washed off a jetty by large waves in Crescent City.

Ski areas east of Los Angeles got a big coating of snow, but travel in the rugged mountain resort region was nearly impossible in many areas.

The storm had moved into California from Oregon, where it delayed flights out of Portland on Sunday and broke the record low temperature for the day on Monday when the airport reported minus 5.5 deg C.

Some highways in north-western Oregon were closed over the weekend because of icy conditions.

It also brought snow and freezing weather to Washington state, where Seattle-Tacoma International Airport hit a record low of minus 7 deg C early on Monday.

Other US regions were hard-hit by winter weather: - In the North-east, utility crews worked to restore electricity to thousands of homes and businesses in six North-eastern states still without power three days after a devastating ice storm.

- In the Midwest and West, meanwhile, temperatures crashed to record lows well below minus 18 deg C as a huge mass of arctic air blustered through the region.

The cold and remnants of the weekend blizzard that accompanied it closed hundreds of schools from the northern Plains to the Great Lakes states on Monday.

Major highways in Minnesota, and North and South Dakota reopened after Sunday's blizzard dropped as much as 36cm of snow.

Monday morning lows in North Dakota included minus 29 deg C in Bismarck.

Monday's cold was an abrupt change for many areas. Illinois had unseasonable warmth on Sunday with temperatures in the 50s, but Monday morning lows were in the single digits across the northern part of the state.

The storm and ensuing cold have been blamed for at least nine deaths, mostly from weather-related car accidents. -- AP