US10 is waterproof it seems for meFor bigger I use a Guy Cotten Uno, that's gone through a lot and doesn't let anything in. Fisherman tested!
My R35 is not very waterproof, it will survive light rain but if its a heavy downpour anything touching the ouside bit of the bag will be wet, not soaking but quite clearly doesn't keep the water out, and that happens after about 20 min ride, nevermind longer...
Ive got one of those roll up top backpacks that are waterproof if I am going to be out in rain for a while.