Winter Sub-Freezing Periods and Significant Thaws in the Boreal Forest Region of Central North America

Winter daily maximum temperatures were examined for 56 sites in northern portions of Ontario, Michigan, Wisconsin, and Minnesota over the period 1960-88. The longest sub-freezing period of winter averaged 20-30 days in the southern portion of the region, 30-40 days around Lake Superior, and 90-100 days in extreme northwestern Ontario. These are twice as long as sub-freezing periods at similar latitudes in eastern Canada. The sub-freezing period is shortened by about one week along the shores of the Great Lakes. There is annual spatial correlation of the longest sub-freezing period, indicating regional synoptic-scale control. The late 1970s had the longest sub-freezing periods but no significant linear trend was found in lengths of sub-freezing periods. The average date of the first significant thaw (> 10°C) ranged from early March in the south to late April in the north and is delayed 1-2 weeks along the shores of the Great Lakes.