Dress for how you will feel 20 minutes into the ride. Instead of a heavy jacket, choose 2 or 3 (or 4), light, breathable layers under a wind-breaking but breathable shell. That way you can adjust on the fly to meet conditions as they change. If you sweat you will freeze. If you don't ride with a rack or bag (front or rear) a small bungee or strap will keep rolled up layers secured under your saddle or under your top-tube. I also layer my gloves and socks.