Another blast of winter storms is closing schools, snarling flights and putting millions of residents on alert across parts of the Deep South and south-central U.S. The National Weather Service says heavy snowfall and icy conditions are likely Friday in Mississippi,

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.