I get a lot of rain too. I used to bring the chicks into the garage when rain was in the forecast but with the last batch I decided... that wasn't necessary. I lean greenhouse panels over the open mesh sides to ward off direct rain, and build up the litter under the brooder several inches, so...
Security would be a bigger concern than temperatures then. As long as you can provide a safe, predator proof spot for the chicks, then outside is an option (or you can stick with the garage if that's the more secure option).