A coast-to-coast storm that paralysed roads and blacked out nearly 1 million homes and businesses was set to pound California on Friday, sparking the first blizzard warning for Los Angeles since 1989.

The National Weather Service warned of a "cold and dangerous winter storm" through Saturday. 

As much as 1.5 metres of snow could fall in some mountains near Los Angeles, which could create white-out conditions as winds gust to 120kph and an increased risk of avalanches, forecasters said.

The weather service issued rare blizzard warnings for Southern California mountain areas and urged drivers to avoid dangerous roads.

Storms already have battered the central-US plains states and northern regions of the country for days and the National Weather Service predicted continuing problems from ice, snow and freezing rain into the weekend.

In the Pacific North-West, much of Oregon's largest city was shut down after almost 30 centimetres of snow fell unexpectedly.

It took drivers in Portland by surprise, stalling traffic during the Wednesday evening rush hour and trapping motorists on freeways, some of whom spent the night in their cars.

The snow began to melt later in the day but as temperatures dropped and night fell, roads became slick with black ice.

Portions of Interstate 80 in California and Wyoming were closed because of impassable conditions.

The weather also knocked out power to nearly 1 million homes and businesses in multiple states and closed dozens of schools and contributed to delays, grounding more than 2,000 flights. 

Widespread power outages were reported in Michigan, Illinois, California, New York and Wisconsin, according to PowerOutage.us.

"Utilities across the country fear ice storms like we fear nothing else," said Trevor Lauer, president of DTE Electric.

A Michigan firefighter died on Wednesday after coming in contact with a downed power line in the village of Paw Paw, authorities said.

Winter storm warnings remained in effect in northern Michigan and parts of New England could see sleet, snow and freezing rain as a cold front stretched from the eastern Great Lakes to East Texas, the National Weather Service's Weather Prediction Center said.

But the weather front was creating vastly different conditions in different parts of the country.

Usually balmy California unusual winter weather

Karen Krenis was driving to a pottery studio in Santa Cruz in the San Francisco Bay area on Thursday when she stopped in her tracks after seeing snow on the beach.

She got out of her car and went to take photos. By the time she left, about 50 other people were there. Adults were snapping photos, and children were making snowballs.

"I have lived in California for 30 years, and I've never seen anything like it," Ms Krenis said.

In Southern California, the storm began moving Thursday with widespread rain and some snow flurries.

There was even a dusting of snow or graupel — a sort of soft hail — on Thursday morning in the hills near the Hollywood sign, although it quickly vanished.

Flood watches and warnings were in effect through Saturday afternoon for some coastal regions and valleys, and the potential for heavy rainfall causing flooding and debris flow in some areas burned by wildfires in recent years.

Evacuation warnings also were issued in Ventura County for four areas that were considered unstable after being hit hard by storms last month.

The weather service said temperatures could drop far below normal in the region. That posed a special risk to thousands of homeless people, with shelter space limited and freezing temperatures expected in some areas.

AP