Done it 9 times, the first in 1989, and got rained on at least 3 times. Yes, it hailed pretty hard one time. Sometimes the wind is terrible. The main thing to watch out for is the other riders. Thousands of people riding at all different speeds can be dangerous. The support is o.k. at best, food o.k. at best, and there are long lines for everything. Enjoy! My club used to do it every year until we all got tired of it.
I prefer smaller centuries like the one I am doing today, less than 100 riders, fantastic support, and wonderful home-cooked food.