According to the U.S. Geological Survey, the average precipitation in eastern Washington is 20 inches and the average on the west side is 70 inches. Local averages range from 7 inches in the driest corner to 150 inches in the Olympic Mountains.
The Olympic Mountains in the far west of the state create a rain shadow effect, which means that while the town of Quinault, in the western Olympic Mountains, experiences an average of 137 inches of precipitation, nearby Sequim, only 56 miles away on the eastern side of the Olympics, has an annual average of less than 16 inches. Other towns in the rain shadow include Port Angeles, Port Townsend, Whidbey Island and the San Juan Islands. These towns experience lower annual precipitation than the rest of western Washington.
Figuring the statewide average precipitation is difficult because of the varying climates. According to the state climatologist, however, the average precipitation for Washington State is approximately 45.7 inches.
Seattle is located between the two mountain ranges and east of the rain shadow in the central part of western Washington. The "Go Northwest Travel Guide," points out that Seattle's average 36 inches of rain is actually less rain than what falls in New York and other locations. The rain in Seattle, however, falls over a longer period of time and a larger number of days. This is where the reputation for rain is derived.
