Practice. Learn to ride in rain and wind by getting out and doing it. You quickly learn how to cope with the conditions and learn how to decide when it's time to go home or find cover or grind it out. You also quickly discover what kind of clothing or gear works and what was a waste of money. Do all of that experimentation close to home.
Also a good idea to practice setting up camp in wind and wind-driven rain. You quickly learn how not to let your tent get destroyed, how to keep your gear under the fly till the thing's up, what's important to keep dry and what can get soaked.
Yepper. Although I will say that there are certain places at certain times of year where you can pretty much count on needing wet weather gear that will keep you warm, such as the western section of Northern Tier route starting in late May. In addition to several days of cold rain, it snowed at Rainy and Washington Passes and a few days in Republic and on the east side of Sherman Pass during my first trip that way.
Part of what you will need to feel comfortable depends on your tolerances.