Just how cold is this week’s midwestern polar vortex get?  On Wednesday, it will be colder in Chicago than in Antarctica, Siberia, or the top of Mt. Everest. Local forecasters are predicting temps to reach a record low of 20 below zero on Wednesday. And it won’t just be the Windy City – more than 90 million people in the U.S. will experience below-zero temps this week.  John Gagan with the National Weather Service is calling it ‘once in a generation’ cold.