Won't keep your knees completely dry, but if you ride decently fast, it will keep them a lot drier. You'll go faster also. Put Zzipper fairing on your bike. Those clear plastic bubbles that fit on the brake levers from the '70s and '80s. That and full fenders with a deep flap for the front fender go a long ways toward cmfort in the rain. I used to do a 15 mile commute in Seattle riding south against the winter storms. What a difference!

And a fun anecdote - I rode up Oakland's Joaquin Miller to Skyline in a winter rain, stopped at the top to dry my glasses and kept my head low; above the fairing but below the slipsteam "bubble" for the descent and arrived at the bottom with still dry glasses.

Ben