The Columbia Plateau lies between the Rockies and the Cascade Mountains in Washington state and is the state's largest ecoregion, taking up about one-third of Washington. While you may think of Seattle's legendary wet weather when you think of Washington, the Columbia Plateau has a drastically different climate.
The Columbia Plateau, also called the Columbia Basin, has a semi-arid climate, with a near-desert climate in the lowest sections of the basin. This is largely due to the rain shadow cast by the Cascade Mountains, which causes air to become warm and dry as it descends down the mountain slopes. Summers in the Columbia Plateau are generally hot and dry, while winters are cold.
Annual temperatures in the Columbia Plateau average about 40 to 57 degrees Fahrenheit, with seasonal extremes ranging from below zero to over 100. Summer temperatures stay fairly hot, with maximum temperatures in the 90s and minimum temps in the high 50s. Occasionally, summer highs reach over 100 degrees in this area. In the winter, temperatures generally range from a minimum of 15 to 25 degrees to a maximum of 30 to 40 degrees Fahrenheit. Like the occasional sweltering days in the summer, winter also experiences occasional extreme temperature drops, with recorded temperatures as low as -30.
The amount of rainfall in the Columbia Plateau varies by region, though overall this area sees little precipitation. In the driest parts of the plateau along the Columbia River and the Saddle Mountains, annual rainfall only amounts to 6 or 7 inches. Other areas may see slightly more rainfall, such as the Saddle Mountain area, which averages about 15 inches of rain a year. Rainfall generally falls in early spring and late fall, with occasional summer thunderstorms or hailstorms. In the hottest parts of the summer, it's not uncommon for the area to go four to six weeks without rain.
Snow generally doesn't fall all winter in the plateau in the lower elevations, and yearly snowfall averages about 10 to 35 inches. Generally, snowfall commences around early to mid December and falls on and off until the end of February. Snow depths are usually around 8 to 15 inches, though the Ellensburg Valley may see larger snow depths.
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