A major winter storm sweeping the US this week may bring the coldest Christmas in four decades to parts of the country, say forecasters.

The "once-in-a-generation" cold snap - which began in the Pacific Northwest on Tuesday, before churning east - will become a "bomb cyclone" by Friday.

More than 90 million people are under winter weather alerts across 37 states. About 80% of the nation is set to experience sub-zero temperatures, including places as far south as Texas.

Named Winter Storm Elliot by the Weather Channel, the arctic blast is expected to deliver the coldest Christmas to the Midwest since the late 1980s, according to forecasts cited by US media.

The National Weather Service (NWS) said bone-chilling temperatures of -50F (-45C) and -70F were possible by the end of this week in some parts of the country.

The NWS has called it a once-in-a-generation winter weather event, especially as the storm reaches the Great Lakes region, where its pressure is to reach the equivalent of a Category 3 hurricane.

The governors of Kentucky and North Carolina have declared states of emergency. Snowy conditions and freezing roadways are expected to wreak travel chaos this weekend.