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 2/3
Informal
'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 3/3
Visual 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!