If the pool is uncovered, the snow and ice shouldn't be a problem. If you're going to keep it uncovered, and are worried about detritus setting off an algae bloom, I'd wait for the WATER temp to get to 60 or just above to close it. Below 60 inhibits algae growth.

Personally, I'd keep it very clean as the temp goes down, keep the FC at or near shock level, then when I'm ready to close, I'd add a 1/2 to a full quart of Polyquat 60%, wait 48 hours (FC will drop precipitously), raise FC back to shock level, then close.