1.1.1 Eastern Washington Climate Regions

Region 1 – EAST SLOPES OF CASCADE MOUNTAINS: This region is comprised of mountain areas on the east slopes of the Cascade Mountains. It is bounded to the west by the Cascade crest and generally bounded to the east by the contour line of 16 inches average annual precipitation.

Precipitation in this region diminishes as the distance from the summit increases and the elevation decreases. For example, within a distance of 20 miles, the average annual precipitation decreases from 92 inches at Stampede pass (elevation 3,958 ft.) to 22 inches at Cle Elum (elevation 1,920 ft.). The average winter season snowfall decreases from approximately 400 inches near the summit of the mountains to approximately 75 inches at 2,000 feet above sea level.

Region 2 – CENTRAL BASIN: This region is comprised of the Columbia Basin and adjacent low elevation areas in central Washington. It is generally bounded to the west by the contour line of 16 inches average annual precipitation at the base of the east slopes of the Cascade Mountains. The region is bounded to the north and east by the  contour line of 14 inches average annual precipitation.The majority of the area in this region receives about eight inches of average annual precipitation. Many of the larger cities in eastern Washington are in this region including: Ellensburg, Kennewick, Moses Lake, Pasco, Richland, Wenatchee, and Yakima.

Region 3 – OKANOGAN, SPOKANE, PALOUSE: This region is comprised of inter-mountain areas and includes areas near Okanogan, Spokane, and the Palouse. It is bounded to the northwest by the contour line of 16 inches average annual precipitation at the base of the east slopes of the Cascade Mountains. It is bounded to the south and west by the contour line of 12 inches average annual precipitation at the eastern edge of the Central Basin. It is bounded to the northeast by the Kettle River Range and Selkirk Mountains at approximately the contour line of 22 inches average annual precipitation. It is bounded to the southeast by the Blue Mountains also at the contour line of 22 inches average annual precipitation.

Region 4 – NORTHEASTERN MOUNTAINS AND BLUE MOUNTAINS:
This region is comprised of mountain areas in the easternmost part of Washington State. It includes portions of the Kettle River Range and Selkirk Mountains in the northeast, and includes the Blue Mountains in the southeast corner of eastern Washington. Average annual precipitation ranges from a minimum of 22 inches to over 60 inches. The western boundary of this region is the contour line of 22 inches average annual precipitation.


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