This is a great question that gets asked more than you would think. First, you have to realize Presque Isle Bay is not where the official temperature is read. The bay freezes first, but if you look out into the open waters, you will see the lake is not frozen. The official lake temp is taken pretty far out in the lake. Also, lake Erie (despite what it looks like on the map) is a large body of water, so there are warm spots and cool spots. When they mix, it can cause water temperature swings as well.

Ray Petelin,
WSEE Storm Team Meteorologist