 The mountains themselves create a rain shadow over the eastern half by blocking weather fronts holding precipitation from carrying over the mountains and descending onto the Columbia River Basin.This creates the arid desert-like conditions with a more continental climate in Eastern Washington and heightens the roles of rivers—most notably the Walla Walla, Yakima, Snake and Columbia River—in the region's viticulture.In addition to providing vital irrigation sources, the rivers also help to moderate temperatures during the winter which is prone to severe frost and freeze coming from the Arctic.