In article ,
"Capt. Geoffrey Thorpe" The Sea Hawk at wow way d0t com wrote:

The temperature in the shaded areas (like where you see the 39 F) is not
reliable due to clouds. 70ish is prety good for most of the lake. The upper
lakes are a lot colder, but Erie runs on the warm side...

Only at the western half, which is shallow. The central and Eastern half
are quite deep. When the wind is blowing strong out of the east, it is
not unusual for the depth at the west side to increase by several feet.