Bucket down
Definition 1 of 1
Pro Tip 1/3Intensity
Use 'bucket down' to describe very intense, heavy rainfall, similar to a bucket of water being dumped. 
I was soaked in seconds; it was bucketing down so hard!
Pro Tip 2/3Informal
'Bucket down' is informal, best used in casual conversations or personal writing, not formal situations. 
I heard it's going to bucket down this weekend, so no beach for us.
Pro Tip 3/3Visual Imagery
The phrase evokes the image of water being poured from a bucket, emphasizing the rain's volume. 
Look at the rain bucketing down outside; it's like a waterfall!