Major winter storm to trek 1,500 miles across the United States, dumping snow, sleet and freezing rain

Tens of millions of Americans are bracing for a huge winter storm that could bring the heaviest snowfall and coldest temperatures in over a decade.

The storm, which started in the middle of the US, will move east in the next couple of days, the National Weather Service (NWS) said.

A state of emergency has been declared in the states of Kentucky and Virginia, and parts of the US unaccustomed to severe cold, including Mississippi and Florida, have been warned to expect treacherous conditions.

Forecasters say the extreme weather is being caused by the polar vortex, an area of cold air that circulates around the Arctic. 

The polar vortex of ultra-cold air usually stays penned up around the North Pole, spinning like a top. But sometimes it escapes or stretches down to the U.S., Europe or Asia — and that's when large numbers of people experience intense doses of cold.

Studies show a fast-warming Arctic gets some of the blame for the increase in polar vortex stretching or wandering.

Severe thunderstorms, with the possibility of tornadoes and hail, were also possible ahead of the storm system's cold front as it crosses the Lower Mississippi Valley, the National Weather Service warned.

Parts of upstate New York saw 3 feet or more of snow from a lake effect event expected to last until late Sunday afternoon.