Just a guess, but if you have snow on your roof I'd assume it would be winter. If it's winter I'd guess the leaves would be off your trees.
I'm thinking your better winter reception has more to due with the lack of leaves than snow. AFA the metal roof, my guess is it would only make things worse, but how bad is hard to tell. As someone else mentioned the further away you can get your antenna from your roof(metal or asphalt) would be better.
Ideally you'll find someone else who when from a asphalt to metal roof, was in a weaker signal area and also had his antenna a short distance from the roof. I just don't know if you'll find someone like that who'll read your message, otherwise it's just best guesses.
I personally like metal roofs and would be of the mind to put what you want on the roof and deal with the consequences later. You may need a more directional antenna or higher mast but you should still be able to keep your OTA, it just may require a bit of tinkering.