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Seasonal Summary for the Great Lakes - Winter 2011- 2012

Lake Michigan

Above normal temperatures were reported over Lake Michigan in November and December. A brief period of colder than normal temperatures in the second week of December resulted in new lake ice formation at the southern end of Green Bay and in the Bays de Noc. Fast ice started forming normally on the Bays de Noc in the third week of December. By the end of December, the fast ice extent was a little lower than normal in the Bays de Noc and ice growth was about a week slower than normal in the southern portion of Green Bay. 

January was also warmer than normal, with a few brief colder than normal periods near mid-January. Around mid-January, ice formation was one to two weeks late in the Straits of Mackinac and in Green Bay. Green Bay became ice covered in the third week of January, about two weeks later than normal. At the end of the month, the southern third of Green Bay was consolidated, and the Straits of Mackinac was covered with thin and medium lake ice. Little fast ice was present in the Straits of Mackinac, compared to what is normally observed.

Average air temperatures were warmer than normal over Lake Michigan in February. Most of the ice in the southern third of Green Bay became mobile again in the second week of the month. Colder air than normal spread over the south-western portion of the lake around mid-February causing some new ice to develop around the western shore of the lake. This new ice was destroyed within a few days.

The month of March was far warmer than normal over Lake Michigan. A brief period of colder than normal temperatures early in the month resulted in ice formation in Green Bay and the northeast portion of the lake and the peak ice cover of the season was observed in the first week of March. Temperatures then turned warmer and some average daily temperatures of over 20°C above normal were observed before the end of the month.  Ice break-up in the Bays de Noc occurred in the third week of March, two to three weeks earlier than normal. Green Bay, the Bays de Noc and the Straits of Mackinac became open water to ice free in the last week of the month, also two to three weeks earlier than normal.

Again, what is remarkable is what did not happen; normally the ice in most of Green Bay will become completely consolidated except for a portion of the northern third of the bay. The ice then usually remains fast for more than two months. Last winter, only the southern third of the bay became consolidated and this lasted for only about two weeks.  Also, the normal pattern is for fast ice to develop in the north-eastern portion of the lake and in the Straits of Makinac; this ice usually remains fast for about two months. No such fast ice formed last winter.

The total accumulated coverage for Lake Michigan (TAC, Figure 6, which is representative of the seasonal-average) was the lowest in CIS records, dating back to the winter of 1972-1973.

Weekly ice coverage for Lake Michigan

Figure 5: Weekly ice coverage for Lake Michigan [data]

Total accumulated ice coverage for Lake Michigan

Figure 6: Total accumulated ice coverage for Lake Michigan [data]