Temperatures during the opening days of December have generally been mild, averaging 2.4 degrees above normal. Over the month’s first 13 days, all but three posted a temperature surplus. Friday’s high of 45 and low of 35 represented a 12 degree departure from normal. The city’s mild spell is about to end. On Saturday, increasing northwest winds will usher in a first installment of polar air, drawing on an upstream reservoir of 20-degree temps. Spotty sprinkles or flurries may accompany this transition. Sunday promises to be a cold, but calm day. Focus then turns to a more significant system forecast to pass south of Chicago Sunday night and Monday. Though heavier snow is expected to pass across central Illinois and Indiana, periods of light snow may extend into the metro area. A more robust push of cold air is expected to arrive as the snow system exits Monday night.