In the fall, winter, and spring, rain gear is essential; if you plan to do much hiking, biking, or skiing, breathable waterproof gear is worth the extra cost, and rain pants are a vital part of the outfit.
If you are traveling into the mountains in the winter, make sure your car has tire chains in the trunk. During snowstorms, many mountain passes are closed to vehicles without chains, and the state patrol takes the task of enforcing this requirement very seriously.