Chicago finds itself in a situation that occurs frequently in the cold season: In a transition zone between significant snow to the northwest and little or no snow the southeast. The zone can be widely variable in width, ranging from 100 or more to as little as a few miles. In addition, the transition zone can remain nearly stationary or it can shift many miles to the north or south. In the present situation, the transition zone is shifting northward to lie from northern Chicago into extreme southern Wisconsin Saturday morning. For the Chicago area, this means snow will change over to sleet and freezing rain and then to rain early Saturday, then switch back to a period of a little light snow by afternoon. Much colder air arrives in the afternoon and remains with us into Tuesday, but milder air makes a quick return Wednesday.