Commuters try to stay warm as they wait for a train on an L platform during the early morning rush in Chicago, Jan. 7, 2015.

One last blast of arctic air will freeze the Midwest and the Northeast before things begin to warm up — way up. Temperatures will swing from 20 degrees below normal Monday night in some areas to more than 15 degrees above normal by the end of the week, forecasters say.

A jet stream pattern pushed frigid air south from Canada last week, sending temperatures plummeting from Texas to Maine. The pattern is expected to mean more snow Tuesday for interior New England, with a mix of sleet, freezing rain and rain across Pennsylvania and southern New England. Farther south, bitterly cold temperatures will ice over the upper Midwest and the Great Lakes, sending lows below zero in many places through Wednesday.