Rather than having four well delineated seasons, the Pacific Northwest is prone to two major seasons; wet and dry. Winter is dominated by a powerful low pressure channel coming from the Gulf of Alaska. Air generally circulates counterclockwise in a low pressure system meaning that the warm ocean wave producing Westerlies emanating from Hawaii will be picked up and driven in from the southwest, which also contributes to the mild temperatures along the coast known as the Pineapple Express.
Warm air holds more moisture than cold air, and we've already pointed out the extremely moist result. When this warm air rises and is cooled by the lower temperatures higher in the atmosphere, the Pacific Northwest is anointed with more precipitation than anywhere else on the North American continent.
Eighty percent of the precipitation falls between October and April. Between September and July, a high pressure system from the California and Oregon Coast takes root and pushes off those moisture-bearing clouds. The clockwise pattern of high pressure systems doesn't hold the water, and they pick up and deliver air masses from the arctic resulting in cool and dry summer winds.
If you'd like to see trends over the last century, NOAA, particularly on their Northwest Fisheries Science Center website, has all the data you could want about it. What's known as the Pacific Decadal Oscillation (PDO) is a climate index incorporating factors such as sea surface temperature data, sea level pressure, winter land–surface temperature, precipitation, and stream flow. Much of this data goes all the way back to 1900. Check it out on the nwfsc.noaa.gov website because the scientists there explain it far better than this does. So there you have it, your typical and normal weather patterns of the Northwest region. If you've never been there, don't let the weather hold you back, it has areas of gorgeous and lush forests and high desert scenery all split up nicely by awe inspiring mountains.
Image courtesy of the author
National Oceanic and Atmospheric Administration: http://www.noaanews.noaa.gov/stories2010/20101021_winteroutlook.html