To get the entire lake surface temp to 80 would take weeks of air temps in the 80's and 90's and lows in the 70's. Nightime temps have as much to do with lake temps as do daytime as there are almost as many nightime hours as daytime hours.
The lake can and does hit 70 in the far SW (near Ashland and the Apostle Islands) many times. However, mid lake temps typically to out in the upper 50's.
-John