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

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.

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.