“It’s raining cats and dogs.”
This idiom means it’s raining heavily. It’s very popular and funny because of the ridiculous imagery it brings of cats and dogs falling from the sky. Even with its popularity, I’ve never actually heard anyone use it in casual conversation. Ever. Instead, we say,
“It’s really coming down out there.”
You can use this for rain or snow. I’ll do another post on common expressions about the weather next week.