The polar vortex of ultra-cold air usually spins around the North Pole, but it sometimes plunges 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.

It will be a struggle early next week to get above freezing and nights will fall into the teens to near 20 degrees. DALLAS - Most of the nation is in good shape on Wednesday and will be warmer on Thursday.