Because of its size, it is necessarily divided into several distinctive sub-AVAs, including?Walla Walla Valley?and?Yakima Valley?which are both further split into smaller, noteworthy appellations. A region this size will of course have varied microclimates, but on the whole it experiences extreme winters and long, hot, dry summers. Frost is a common risk during winter and spring. The towering Cascade mountain range creates a rain shadow, keeping the valley relatively rain-free throughout the entire year, necessitating irrigation from the Columbia River. The lack of humidity combined with sandy soils allows for vines to be grown on their own rootstock, as phylloxera is not a serious concern.