I notice my wife and I often come out with "the heavens opened" (usually after sudden, unexpected rain). A bit old fashioned, perhaps.
I notice Gweiadur gives "teeming down" as a translation for "bwrw hen wragedd a ffyn", with an alternative Welsh phrase of "arllwys y glaw" - to teem down. (I don't think I say that very often in English).
My wife and her family lived in N.Wales from the mid-1950s to early 1960s, and she said they used to say "raining stair-rods", but I'm not sure if that's a translation of a Welsh expression, or what the English speakers said. (Google suggests it's northern English, so maybe it was an import).
(Google has also reminded me of "raining buckets", which I'm sure I've heard, and isn't all that fanciful really).