It's Raining Cats and Dogs!
I ought to know, I just stepped in a poodle (puddle). One of my all time favourite puns and now I get to share it with you.
There's a lot of conjecture as to the origin of this phrase, but it is most likely only a picturesque phrase relating to heavy rainfall. There is some speculation that cats are associated with rain (via witches' familiars) and dogs with storms & wind (Odin, the God of wind, etc. had dogs or wolves with him).
But it is really pissing down hard today. And there isn't a typhoon and the rainy season hasn't started yet. The closest Japanese equivalent to raining cats and dogs is: 土砂降りである (doshaburi de aru) Earth and sand are falling!