Storm Blair has hit most of the U.S., bringing freezing temperatures to Texas. Precipitation later this week could mean snow for some areas.

Our chilly weather could bring freezing rain and snow to parts of the Hill Country and I-35 corridor, local forecasters say. Why it matters: A polar vortex hauling arctic air from Canada down across the U.

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. Studies show that a fast-warming Arctic is partly to blame for the increasing frequency of the polar vortex extending its grip.