Can a tornado/hurricane occur over Lake Superior?
It is not very likely that a hurricane could ever form over Lake Superior because the water is deep and too cold to support such a large storm. However, a waterspout (tornado over water) could form over any body of water if the conditions are favorable for such a storm to occur. Typically, a waterspout forms over southern bodies of water and is much weaker than a tornado that forms over land.
Tornadoes form when a cold front overtakes a slow moving warm/moist body of air. The warm air is naturally seeking to be higher than the cold air. This instability in the atmosphere creates a mesocyclone (a violent rotating supercell) that has the ability to spawn a tornadic rotation, severe thunderstorm, downbursts, hail storm, and cloud to ground lightning strikes.
The Pennsylvania-Ohio tornado outbreak, May 31, 1985, did include some waterspouts that traveled across Lakes Erie and Ontario. So, it is possible, but not likely that a tornado would form over Lake Superior.
"If you're driving in a midnight rain in October near Lake Michigan, remember that a tornado is not outside the realm of possibility," says Robert Trapp of Purdue University (http://www.livescience.com/environment/050405_tornado_midwest.html)