This came up earlier in the comments to my wintring post a few days ago. Tom Fiorillo wrote from New York to ask if Lake Mendota had frozen yet. "To me this is really like another canary in the coal mine," he said.
So far, Mendota looks like one wet canary. But the latest freeze date was Jan. 30 in 1932, so I suppose it can still freeze, but it's going to have to cool off some to do it. It's amazing how variable the ice cover has been. Check out this chart, which I'm reposting here to make it more accessible. The freeze and melt days are all over the map, and ice cover has ranged from nearly half a year (161 days) to less than a month (21 days). Guess which is more recent.
1/20 UPDATE: According to Isthmus, the manager of the state climatology office determined that Lake Monona (not Mendota) froze over Thursday, Jan. 18 -- its second-latest freeze date. Extrapolating from its smaller cousin, Mendota probably has about a week to go.
1/22 UPDATE: Not that long. Again according to Isthmus, Lake Mendota froze over on Jan 20, two days after Monona -- and like the Monona freeze date, the second-latest ever.