Currently (April 2014), the fraction of the Great Lakes covered by ice is more than ten times the median amount for this time of year, the median amount being indicated by the green line in the below graph and representing the 1980-2010 time period. The source of the graph is the Canadian Ice Service.
(in the graph, the x-axis is year, 1980-2014; the y-axis is percent ice cover 0-40%; Link to graph)
While the data in the graph are only back to 1980, looking at data from the NOAA Atlas of Great Lakes Ice Cover it can be seen that the amount of ice for this time of year is unprecedented back to 1973, the earliest year in the atlas.
My question is, how far back would one have to go to find a year with an equal or greater amount of ice coverage on the Great Lakes at this time of the year? Obviously the Great Lakes were glaciated thousands of years ago :), but is there any historical record of a similar ice coverage event?
(x-axis is year, 1980-2014; the y-axis is percent ice cover 0-30% Link to graph)
As we close out April, the amount of ice remaining on the Great Lakes is more than 4 times greater than any other year in the 1980-2014 time period, and about 50 times the median value for this time of year.
Vast areas of Lake Superior which were considered by Canadian Ice Service as having a zero-probablity of ice-cover at this time of year are in fact covered in very thick ice:
A small amount of ice still remains on Lake Superior. This is the first time in the satellite era (1973-) that ice has been present in June.