This isn't historic or record-breaking cold, but it will be memorable. So, why is it so cold? Well, first you have to have cold air sitting in place for a while near the North Pole or Siberia, allowing the air to continually get colder and colder. Then, that frigid air needs to be displaced by a change in the jetstream. You can see on the map below where our air originated. The Arctic air started off bottled up in Siberia, then crossed over the North Pole, then straight down to Kentuckiana!
The second graphic shows how long ago it was this cold. We had a low this morning of -1, just as cold 5 years ago in 2009. The low forecast tonight is -4, so tonight will be the coldest since 2004 (10 years ago). If we drop to -6, it will be the coldest in 18 years (1996). The record low today is -14, and tomorrow it's -8, it's not likely that we'll get that cold. Take care in this extreme cold, and keep your pets warm.