Depends on how you treat the change-
If it's a single spell that makes you into a wolf, and you turn into a human by discontinuing the spell, then rain and circles are a problem.
But I'd always assumed that the transformation spells aren't continuous, so to speak... the spell energy only flows during the change... once changed, they don't really have to maintain the current form, and rain's not an issue. It's two spells- one to turn into a wolf, one to turn back. In between, you simply are what you are, regardless of any restraints placed on you- this is why the alphas have to change before entering a threshold, but don't automatically turn back when they enter (Displayed briefly during GS).
EDIT: The only case I see supporting the one-spell view is that they turn back when they're killed, but that could just as easily be a reflexive (or triggered) casting of the 2nd spell to turn back to their original form- to die as themselves. If a wizard can manage a death curse, this would be their equivalent, I guess.