Well, well...
Today, with us, one of the classics...
As always, do not translate this idiom literally: It wouldn't make any sense!
Today, to guess its meaning, you only have to look out of your window.
It's the perfect Sunday morning to spend some time studying English, isn't it? (a teacher can wish...)
And here's the correct meaning for my international readers (I'm not familiar with the weather forecast in your home country).
And here's the idiom in a real conversation:
It's raining cats and dogs out there!
It's a wonder any of the men can see what they're doing!
See you tomorrow in class!