I believe that the consensus is that you'll eventually get wet and that what matters is to stay warm.
Good shells will reduce water inflow and protect from the wind. OR Helium is one of many alternatives (I'd consider riding with a Patagonia Houdini) on top of whatever base/mid layer that would be appropriate. The problem is that most shells will trap perspiration and you'll get wet "from the inside". A marathon shell (Houdini) breathes much better but lets a fair bit of water getting in.
Rain pants (OR helium again).
Pay attention to your extremities. A merino beanie, and thin neoprene socks and gloves help you stay warm.
The other crucial aspect pertains to camping. I personally find that setting up camp during a downpour is unpleasant at best and reason enough to justify renting a room. If not possible, I try to focus my mind in why touring is usually great. A ready to eat meal (trail mix) is good idea.
I don't remember ever being caught in non stop rain lasting more than several hours (as would be common during the fall season). You may want to look at weatherspark to get an idea of what to expect.
Nonetheless, I do remember getting cold (wind + rain + riding downhill + cold front) and wearing all of my clothes but for a dry set for the evening.