SEATTLE - A rain forest generally needs two ingredients -- a moist airflow and some mountainous terrain. In the Olympic Rain Forest's case, our general weather pattern features a constant west or southwest flow off the Pacific Ocean. That of course, carries a lot of moisture along from being over the ocean for thousands of miles.
As that flow hits the west and southwest side of the Olympics, the terrain forces the air upward. As air rises, it cools and condenses, squeezing out its abundant moisture and soon, you've got a rain forest. The Olympic rain forests gets over 200 inches of rain a year.
Hawaii has rain forests from the tropical trade winds running into their mountains, and South America has gobs of rain forests from the moist flow running into the Andes Mountains.