Powerful storm leaves trail of destruction in US northwest

Warning signs near a flooded area in the San Francisco Bay. Photo / Getty Images

The US Pacific coast's most ferocious storm in years has moved south, bringing hurricane force winds and heavy downpours to southern California one day after claiming two lives in the Northwest.

News reports on Friday said a homeless man and a young boy were killed by falling trees in the state of Oregon.

Severe flooding, mudslides and high winds - some gusting up to 225km/h - wreaked havoc as the unusually powerful storm bore down the California coastline, buffeting Los Angeles and San Diego.

The Los Angeles Times newspaper said the storm trapped some residents inside their homes and cars, and led officials to issue mass evacuations of some areas, which also were hit by widespread power outages.

The National Weather Service said among southern California's hardest hit areas were foothills denuded of vegetation after being scorched by wildfires over the past few years, and which now are particularly prone to mudslides.

"Flash flooding and debris flows will be a particular threat in and below the recently burned areas," the National Weather Service said in a bulletin on Friday.

The inclement weather sweeping into the region was being carried on a current with the unlikely name "Pineapple Express" - an intense stream of moisture stretching from Hawaii to the US West Coast.

"It's clearly the strongest storm to impact the West Coast in the last three years," Todd Morris, a spokesman for the National Weather Service, told AFP.

Blizzard warnings were issued for mountainous areas, the first such alerts since 2008, with up to a metre of snow expected.

Hundreds of flights were cancelled on Thursday at San Francisco International Airport, but Los Angeles International Airport on Friday reported no increase in major delays.

Flashflood watches and warnings were in place all along the western United States from the Pacific northwest to San Diego at the Golden State's southernmost tip.

- AAP