SEATTLE - Sequim is the driest spot in western Washington, getting just 18" of rain a year. And they can thank their proximity to the Olympic Mountains.
Most of our rainy weather around here comes from the southwest. The moist air will rise up the southwest side of the Olympics, then condense and squeeze out its moisture. (Think of the Olympics as a big sponge in this case.) That's why the Olympic rain forests get over 200" of rain a year.
On the flip side, once the air reaches the summit, all its moisture is gone. Plus, as it then goes down the northeast side of the Olympics, it sinks and dries out -- right over Sequim. That's known as the Olympic Rain Shadow.
For More Information:
KOMO Weather FAQ.