Cycling in the Pacific Northwest is a great thing to do all year. The majority of the region, except those at higher elevations, are mostly snow free, save for an odd snowstorm that quickly turns to mush. However, that doesn't mean you won't have to plan for weather, especially if you plan to ride in the coastal areas west of the mountains. These areas, depending on the time of the year, can get quite rainy. It's important to be prepared if you are planning to ride long distances for hours at a time. Here are things you should pack in your bags and put on your bike just to be prepared.
Layers of clothing:
Much of the year, except for in mid-winter, the daytime temperatures are often warm enough to go with only a light sweater or jacket. As the sun sets, temperatures on that same day can get close to freezing. Therefore, it's a good idea to bring a combination of light and heavy clothing. This includes full-fingered gloves, a cap, and leggings or pants. You should also bring at least one heavy-duty, waterproof jacket.
Protection against water:
Make sure you bring something to protect anything you don't want to get wet. This might mean having as many waterproof panniers or bike bags as possible. If those are out of your price range, then aim at covering or sealing your most important items in plastic bags. You can also buy waterproof shoe coverings; otherwise you could be riding around with soggy, cold feet. On rainy days, wearing goggles may help with making things easier to see.
Accessories for your bike:
You might want to think about outfitting your bike with fenders even if you don't normally use them. Yes, they could slow you down a bit, but they will keep mud and debris from flying up from the road and into your face. You can always remove them when your long-distance ride is done and the weather has cleared up. Keep your tires a little softer than usual to help increase traction in wet weather. Always have a headlight, preferably a waterproof one, and reflectors at various locations around the bike, such as on the wheels. You can also put reflective tape on parts of your clothing to increase your visibility on dark days or at night. Carry a pump and some tools as well.
Cycling in the Pacific Northwest means being prepared for all types of weather. You may luck out and have an entire ride in nothing but good, warm weather, or it could rain for most of your trip. Either way, if you're prepared, you won't have to worry about having to stop or postpone your riding until the weather clears.