I say it's coming down hard or pouring, but I've heard a lot of native Americans say it's raining cats and dogs, mostly the "older" generation, I guess. Brazilians say "it's raining pocketknives", which makes more sense than cats and dogs, at least to me. It's interesting how an idiom varies from country to country, isn't it? I myself love idioms. They make languages so much more colorful, especially when you hear one that is not so common.