come down in torrents(о дожде, ливне; The wind got up, and then rain started coming down in torrents. – Ветер усилился, и затем дождь начал лить, как из ведра. TarasZ)
pour piss out of a boot(to describe a stupid person; to describe a very large rain storm (Urban Dictionaty): You're so dumb you couldn't pour piss out of a boot; Look at those clouds, its about to pour piss out of a boot!КГА)