First of all, it may strike you that I am entirely unqualified to dispense advice about anything more severe than putting on a long-sleeve shirt. Don't let the fact that I live in the acme of fantastic weather, San Diego, California, fool you. I've paid my dues. I have spent several years in Alaska. Much, much worse weather-wise, I have lived in Ohio. My crowning achievement which gives me confidence to give advice to Juneau Jill took place in France.
I had camped on the lower slopes of the HC climb to Chamrousse during the 2001 Tour De France. When I woke up, it was cranking rain, serious full-blast lawn sprinkler stuff. First, to simply camp in this and pack up with all but one component, a tarp, getting wet is a job well done, but the real magic act was yet to come. That morning on my loaded trekking bike, I climbed every meter of that mountain in full driving rain.
At the top was a scary scene. Le Tour attracts many nut cases such as myself and scores of them had also ascended, unfortunately, not as well-prepared as me. They arrived on their light racing bikes in their tropical bird-colored lycra. Getting soaked on the way up didn't seem like a problem to them. I knew better. Le Tour is in July, but at these high Alpine elevations, winter can occur at anytime. On this morning, the temperature of the summit was much colder than the base, just around freezing. These guys got to the top only to find the course closed and their return route blocked. They had to stay at the top in the freezing rain dressed in scant, soaking lycra. I, however, had not even gotten especially wet on the way up. Being close to freezing was actually kind of refreshing for such a climb. Of course I had warm clothes to put on, clothes that were fastidiously kept dry. I stood up there comfortable and happy after having made the climb in the most comfortable way I think it could have been done.
How did I do it? When I was a lad in Ohio, I once asked my wise father, "Say Dad, you grew up on the island where I was born, you know, the one with the wretched weather, and you rode a bicycle. How did you Britons cope?" (Before ruining the entire place with cars.) His answer, "A rain cape." A rain cape? I still remember the first thought I had, "Well that's the gayest thing I've ever heard." Obviously, I was too cool to be caught dead in such a dorky looking garment.
Fast forward many years to early springtime in Bavaria. Sounds nice, doesn't it? Well there's a reason people there invented Mercedes: the weather sucks ass. I was bike touring through this area spending day after day in freezing rain. I was really at the limit of my weather resisting abilities. I stopped for a rest day in a town called Regensburg, literally, "Raintown", aptly named. I had planned to buy some latex dish washing gloves and some waterproof spray, which I did. But I also happened to pop into a bike shop and I asked, "What the heck do cyclists do in this kind of weather?" Answer: a rain cape. Hmm. Ok, I'm getting it by now.
Of course if you say "rain cape" in a bike shop in the USA, you'll be asked if you are speaking Klingon, but in this bike shop, they had a large selection. And a large selection of Ortlieb stuff. You see, when riding in European weather, look to the Europeans for guidance. I picked out a magnificent rain cape and pretty much instantly my fortunes changed. The rain cape is good from about -7C to 25C when it's cranking with rain. Below -7C, it's not raining and some fancy Goretex suit is going to work great. Above 25C, just get wet, it'll feel nice. Most importantly the rain cape is great for that range right around freezing. You can wear the right layers and modulate them as necessary, but they shouldn't need to get soaked from within.
The only thing the cape is not good for is very high winds (which are not direct tailwinds). In this case, I claim that a Gortex suit solution is not going to be too hot because of the breeze.
My other tips are wearing OR Crocs Goretex gaitors. These do not overheat you, yet provide tons of protection right where you most need it. When I'm really serious, I wear them over latex overboots sold in industrial supply places (by the way, this is why I use toe clips when I'm planning on lots of rain, to avoid a big hole in the bottom of my shoe). Also fenders are great. Don't get the stupid American-looking ones that are trying to copy the aesthetic of an internal combustion engine driven two-wheeler. Get the proper European kind (I like SKS/ESGE) that fit close to the wheel and provide long coverage.
Riding in the rain can be done and done comfortably. For me in San Diego, riding in the rain is a special treat which I very much enjoy.