I live in Florida. I wait five minutes.

Really, though, my Detours panniers aren't inherently rain-proof (though they're fine in a drizzle), but they came with bright yellow rain covers that do a pretty good job. If I get caught in some rain, I just put everything I care about in the pannier and slip the raincover over it.

I have used a giant trash bag as makeshift rain gear in a pinch.