When you absolutely, positively don't want to get wet, you need a completely waterproof cycling jacket. How does a jacket qualify as properly 'waterproof', as opposed to water resistant or windproof? Just hold it under a tap and watch how nothing comes through.
You'll find seams are taped to seal them and zips are specially reinforced to keep the wet stuff out. These days, miracle technical materials even allow some to remain breathable, so you don't overheat on those climbs. Here's our pick:
Unit 28, Wheatley Business Park, Old London Road, Wheatley, Oxford OX33 1XW // email@example.com // +44 (0) 1302 249 323