Well, as I understand Zion has quite a few streams and rivers which one has to cross and get wet in in the process. And from my recent GC experience I'd imagine that water is cold as it is. So to answer your question, "warm" for me would ideally be higher than 75 degrees or so, at least during the day time.

Does Zion fit the bill for that time of year?

Any other warm places come to mind?