I haven't actually tried it in rain yet, but I purchased a Mavic jacket (demo model) from a dealer at our semi-annual bike swap meet in Tucson - I think it's the Cascade or Cascadia style. It wouldn't quite fit into a jersey pocket, but it's supposed be waterproof, windproof but breathable. I can say it's windproof for sure. Not cheap- even at 50% off it was $100 but it's light and comfortable, has arm and underarm vents. I Maybe you could find one discounted on line if it sounds like what you want.
I try to avoid rain (easier to do here in the desert) but it has been good on cold and/or windy rides so far. Hope that is helpful
2016 Specialized Ruby Comp disc - Ruby Expert ti 155
2010 Surly Long Haul Trucker - Jett 143