Rainfall values are low, ranging from in lower portions of the range to near annually in higher elevations to the west. This is because the hills lie east of the Cascades. The Cascade Mountains cause a rain shadow that makes nearly the entirety of eastern Washington and Oregon semi-arid, and much drier than areas to the west of the Cascade Crest Most precipitation falls during the winter, with the dry season occurring during the summer. Despite the low average rainfall, summer thunderstorms can stall over the hills causing flash flooding. Flooding was most severe in Kennewick before construction of the Zintel Canyon Dam in 1992.