Cloudy skies have dominated the opening days of December, keeping nighttime weather mild. Daily low temperatures have averaged 6.6 degrees above normal. The extensive cloudiness has limited the diurnal range in temperature. Through Dec. 5, the average change from morning low to afternoon high has been only 4.8 degrees, about a third of normal. Our relatively mild nights are about to end. Late Monday evening, two cold fronts were rapidly advancing across the northern Plains, sending temperatures into single digits. Sustained winds of 35 mph prompted blizzard warnings across much of North Dakota. The leading edge of this mass is expected to reach Chicago midday Tuesday. A second push of polar air is to follow Tuesday night. Thursday and Friday will have the lowest readings since February.