They will be OK at this time of year so long as they have shelter if they want to choose it. In the winter the rain itself won't hurt them but it's probably a good idea, if you have the facilities, to shut them up under cover for a couple of hours before roosting time. This is so that their feathers get a chance to dry off before they go in the coop, thus avoiding high humidity and condensation inside the coop during the long winter nights, when they may be in there for up to 15 hours of darkness. Damp, stuffy conditions in a coop can cause bronchial problems and possible illness, any germs will be passed around in the humid air just like in a doctor's waiting room in the flu season!