The western coast of the United States has been receiving an atmospheric river of moisture since the end of last week that has caused torrential rain, heavy snow and damaging winds.

Known as the "Pineapple Express", this winter weather phenomenon has triggered the worst flooding in 11 years for parts of northern California and Nevada.

This stream of abundant moisture runs from the central Pacific, near the Hawaiian Islands, to the west coast. Just off the western seaboard a stationary area of low pressure has been the driving force for pulling in extreme amounts of precipitation since Friday.

The rain has been so heavy in California and Nevada that more than 5,000 people were forced to leave their homes owing to rising waters and risk of mudslides.

Nevada's Governor Brian Sandoval declared a state of emergency over the weekend. Just south of Reno, Nevada rain was falling at a rate of more than 25mm an hour at times.

In Oregon and Washington it was the cold and icy roads that proved deadly. Numerous accidents with several deaths were reported on Interstates 84 and 90, where freezing rain caused extremely dangerous driving conditions.

High winds were another element of this storm. Widespread structural damage and downed trees have been reported across much of the western states along with power outages.

On Sunday a wind gust of 278 kilometres per hour was recorded in the Sierra, equivalent to the strength of a Category 5 hurricane.

In California's Calaveras Big Trees State Park, the iconic Sequoia "tunnel tree" was downed by strong winds. The ancient Sequoia was reported to be more than 1,000 years old.

More flooding rain and mountain snow is expected to continue through Thursday for California and Nevada.