Wednesday’s balmy 50s were quickly forgotten as the season’s strongest arctic blast sent temperatures crashing more than 40 degrees in less than 24 hours. Howling northwest winds with gusts as high as 50 mph sent wind chills tumbling to subzero levels, assuring Chicagoans a more seasonable Christmas than last year when the mercury peaked at 57. The cold was accompanied by some snow showers that finally gave the city its first measurable snow of the month. With 0.2 inches Saturday, Chicago’s seasonal snowfall has reached 0.9 inches, far below the typical 7.4 inches expected by Christmas Eve. The cold will be short-lived as temperatures break freezing Saturday and reach the lower 40s Sunday. A new storm system threatens to bring substantial snowfall to portions of the Midwest next week.