As storms come in off the Pacific, they run into the western slopes of the Olympics, which squeeze out most of the moisture from the storm clouds. So much so, that they get between 150-200 inches of rain a year -- by far the wettest place around the mainland U.S. (Hawaii has rain forests too.)
Just think -- had the Olympics not been there, it'd rain even more in Seattle. We would probably get double or triple the amount of rain we get now if it were flat to our west. The Olympic "rain shadow" is also how Sequim only gets 18 inches of rain from being on the lee side of the Olympics.
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