At least 46 million people were under a winter storm warning as of Thursday as the storm spreads along the Gulf Coast Friday, according to the weather service. The storm should weaken by Saturday, bringing lighter snow between 1 and 3 inches to parts of the Northeast, the Mid-Atlantic and the Ohio Valley before exiting off the Carolinas.

A powerful storm that dumped heavy snow and glazed roads with ice across Texas and Oklahoma lumbered eastward into southern states, prompting governors to declare states of emergency.

The polar vortex of ultra-cold air usually spins around the North Pole, but it sometimes ventures south into the U.S., Europe and Asia. Some experts say such cold air outbreaks are happening more frequently, paradoxically, because of a warming world.