Reported by a Matrix forum user, the severe winter of 1708-1709 gets prolonged 6 months instead of 2, following a data error in the last version of 1709 scenario.
Here is the fix, just copy-paste it over the original scenario in the Scens folder.
The tooltip is still showing blizzard conditions in May 1709 after the fix. Is this WAD, as I don't want to continue if it is not fixed. This is not Cleveland after all