The biggest impact in Oklahoma was felt in the south, where one county reported 7 inches of snow. Snow is expected to continue falling overnight in some areas, which will create hazardous driving conditions through Friday.

The snow brought on by a winter storm on Thursday has caused slick and hazardous road conditions in the Oklahoma City metro and throughout the state.

Another massive winter storm is forecast to pummel the southern and eastern U.S., with impacts from Texas to the Carolinas.

The Tennessee Valley could see some of the heaviest snowfall, with 6 inches expected on Friday. A wide swath of the country is under winter weather warnings and advisories and will see impacts including "widespread closures, treacherous travel, scattered power outages, and downed branches," the weather service said.

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.