WASHINGTON

Two powerful storms are set to unleash torrents of rain and snow on the western half of the U.S. as millions of Americans travel ahead of the Thanksgiving holiday Wednesday.

A "bomb cyclone" is wreaking havoc on the West Coast, bringing with it 100 mph (160 kph) winds and blizzard conditions. It slammed into the Pacific Northwest late Tuesday night, dumping several feet of snow in some locations.

The National Weather Service (NWS) predicted snow showers and gusty winds would continue throughout the day near Seattle, Washington Wednesday, cautioning residents to "plan for a few travel impacts."

The exceptionally rare weather pattern is set to dump feet of snow on parts of the western seaboard, particularly mountainous areas, and move eastwards towards Nevada, Colorado and Utah.

The San Bernardino Mountains in southern California are expected to receive up to 13 inches (330 mm) of snow, as the nearby city of Los Angeles has been put on a flood advisory as the NWS forecasted up to half an inch of rainfall per hour in the normally arid city.

That could cause problems for the recently fire-hit city and its environs.

The service warned there "could be minor mud//debris flows in and around recent burn areas."

In the midwest, meanwhile, a "kitchen sink" storm was tearing its way across the Plains on its way to the Upper Midwest.

The NWS's storm prediction center issued a tornado watch for parts of the central and northern Arkansas, as well as southeastern Missouri.

"Primary threats include...A couple tornadoes possible and isolated damaging wind gusts to 65 mph possible," the NWS tweeted.

In the twin cities' Minneapolis/St. Paul International Airport 16 flights have been cancelled and 116 others have been delayed, according to the FlightAware website, which tracks flight data.

Further east at Chicago's O'Hare International, 113 flights have been delayed and 13 others cancelled.