I don't get it.

It does rain in deserts, of course. But most of California is not a desert anyway.

And also droughts are defined by lower than normal precipitation. So if it didn't rain in a desert, and it's still not raining in a desert, that wouldn't even be a drought anyway.

It’s a joke relying on several layers of not understanding basic concepts for its humor. Kind of worried about the environment in which it “hit well”.