I have NEVER actually heard anyone use the phrase "It's raining cats and dogs." It's funny, because it is such a common example of an English expression or idiom that even in the U.S. everyone thinks that it must be used somewhere, but nobody has ever heard it.
I am now going to say something that is LITERALLY true, because it really WAS raining hard yesterday. I'm going to write it just as if I were talking to someone.
"It was raining hard yesterday, and I mean really hard. It was a real downpour. For half an hour it was coming down in buckets. We must have gotten a couple of inches before it stopped."
"It's pouring" is common. "It's a downpour" is common. And, yes, "coming down in buckets" is common.
Elementary school kids chant:
"It's raining, it's pouring,
the old man is snoring
Bumped his head on the side of the bed
and didn't get up in the morning.
Rain, rain, go away
Come again some other day."
The most famous song of the American musical composer Jerome Kern, "Till The Clouds Roll By," was written in 1917. But the phrase "coming down in buckets" is perfectly up-to-date. It's raining, and man is hoping to get invited in.
She: "What bad luck, it’s
Coming down in buckets;
Have you an umbrella handy?
He: "I’ve a warm coat,
Waterproof, a storm coat,
I shall be all right, I know...."