Be raining cats and dogs
Definition 1 of 1
Pro Tip 1/3Extreme Weather
It's an expression for exceptionally heavy rainfall, not just a regular shower. 
We had to cancel the picnic because it was raining cats and dogs.
Pro Tip 2/3Use with Caution
It's informal and often used in personal conversations or creative writing. 
I couldn't walk the dog because it was raining cats and dogs.
Pro Tip 3/3Idiomatic Imagery
Imagine animals falling like rain to grasp the intensity; it's not literal. 
The sudden storm made it seem like it was raining cats and dogs outside!