Washington state is located in the Pacific Northwest of the USA, immediately north of Oregon. Although its wine history is relatively short, it now produces more wine than any other US state, other than California. Almost all wine production occurs in Washington's hot, desert-like eastern part, although there is some grape growing and one AVA (Puget Sound) in the cooler, wetter west.
The Cascade mountain range defines the geography of the region by acting as a barrier between the two parts. The mountains create a rain shadow, effectively blocking most precipitation. This means that the Columbia River Basin in the east only receives about eight inches (200mm) of rain per year, resulting in a continental climate.
Without irrigation from local rivers, which include the Columbia, the Walla Walla, the Yakima and the Snake, grape growing would not be possible. The rivers also play an important role in moderating both summer and winter temperatures. In summer, a flow of cool, moist air rises off the rivers and blends with the dry desert air to freshen the surrounding areas and cool down the vines. In winter, when overnight temperatures can drop as low as -15F (-26.1C), the rivers keep the air circulating and help to moderate the cold. Some vineyards employ wind machines and use trellising which is more open in order to prevent frost from settling and freezing the vines.
Ice Age floods created much of the Columbia River Basin, leaving behind layers of gravel, sand and silt which have mixed with volcanic soils. This mixture of predominantly free-draining, sandy soils and cold winters suits viticulture and has repelled any outbreak of phylloxera. As a result, most grapes are grown on their own rootstocks, which is advantageous from a quality point of view.
Grape growing here is also influenced by Washington's latitude of 46°N, which produces, on average, 17 hours of sunlight per day in the growing season – two hours more than California. When combined with the extreme diurnal temperatures, this results in vines which are capable of achieving good ripeness while maintaining vital acidity.