If it's raining extremely hard, then the rain is torrential — it's absolutely pouring.
Use torrential to describe something that's happening in torrents, or turbulent flowing streams. When there's a torrential storm, there's so much rain falling so fast that you'll be soaked in about three seconds. If a basketball team's offense just keeps coming and won't stop, you could call it a torrential attack. And if you take your sister's shoes without asking, she might shower you with a torrential tongue-lashing.
