Snow will turn into sleet and freezing rain along parts of the Southeast, as well as southern Arkansas, the weather service reported. Forecasters warn that icy conditions, between a tenth and a quarter inch of ice, will make travel dangerous for many. Power outages and downed branches are also a concern.

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.