I'm confused. Reading your post, it sounds like the bike hasn't been out in th cold yet. If that's correct, why are you blaming the cold for the problem?

While it's true that aluminum expands/contracts at almost twice the rate that steel does, I don't believe the difference (even in a Minnesota winter) would be enough to stress crack it. Plus, it looks like an aluminum headset, in which case, differential contraction wouldn't be an issue at all.

If you bought it used, odd are that something else happened, possibly beginning with galvanic corrosion which stressed the frame causing cracks.