I used to live up that way and I know it can get below zero and in the single digits for a few weeks. Definitely talk with others that do top bar, and in the cold. And as I'm learning here, local advise only goes so far when the guys are doing it the same way it was done 20 years ago. I had a local guy tell me that walk away splits would not create queen cells, that I would have to wait until spring to split a hive to get a queen. After I explained what I have read about splits, he still told me it would not work. I asked him how long he kept bees and he replied, "ten years". I'm not saying the guy you met don't know, I'm just saying, learn as much as you can from many resources.