Thank you, Olympic Mountains!!
A rain shadow is a dry area on the lee side of a mountainous area (away from the wind). The mountains block the passage of rain-producing weather systems and cast a "shadow" of dryness behind them.
As shown by the diagram to the right, the incoming warm and moist air is drawn by the prevailing winds towards the top of the mountains, where it condenses and precipitates before it crosses the top. The air, without much moisture left, advances behind the mountains creating a drier side called the "rain shadow". http://en.wikipedia.org/wiki/Rain_shadow