You can try walking them out with an umbrella over them. If that it too much of a hassle to do every time it rains, encourage them with food. Try walking them out on a leash holding a favorite treat in front of them, then when you walk them to where they do their business, unhook the lease, feed them the treat and then tell them to go potty. If they book back to the door, let them stay by the door for awhile. They will eventually have to go. Then next time, do the same thing but instead of giving them the treat before the command, do it after. Unhook them, tell them to go, if they run, they do not get rewarded.