My Brooks B17 Imperial has been outside in all kinds of weather, often overnight for the past 10 years. Even in -20C (-4F)

As long as you give it a good layer of proofide or Grangers waterproofing wax every 3-4 years you will be fine.
Untreated leather might get damp which might cause issues when it freezes.

So treat it and keep it dry under a porch or something.