Rain Shadow Regions: Dry Leeward Slopes (Mendoza and Eastern Washington)

The Cascades show how a rain shadow works: the mountains take the rain and leave the far side dry. The Cascade range runs from the Canadian border south into Oregon and divides Washington into a wet western side and a dry eastern side. The Columbia Valley is protected from wet weather systems by two major mountain ranges, the Olympics and the Cascades, and because of this rain shadow effect Washington sees far less rainfall than many other wine regions. Eastern Washington, where almost all of the state’s wine grapes are grown, is an arid and semi-arid desert with hot summer days. In Argentina, Mendoza rests under a rain shadow from the Andes, and its conditions are sunny and desert-like.

• West of the Cascades, Washington has very few vineyards: a few hectares on the islands of Puget Sound and a few more in Woodinville.
• East of the Cascades is the dry side, where the vineyards are; it is mostly desert, quite hot in summer and reliably sunny.

Here is how a mountain range takes the rain, step by step, using the Cascades. Moist air from the Pacific rises over the Cascades, and as it rises the air cools. Cooler air is less able to hold moisture, so the moisture comes out of the clouds as rain or snow. By the time the air arrives east of the mountains, it has little moisture left. That leaves Washington with a wet western side and a dry eastern side. The Columbia Valley lies in this rain shadow and averages 6 to 8 inches of precipitation a year.

• Rising air cools, and cooler air is less able to hold moisture
• The moisture falls as rain or snow before the air reaches the east side of the mountains
• The air that arrives east of the Cascades is depleted of moisture, which is why eastern Washington is dry

Low rainfall east of the Cascades allows growers to control water. In Washington, irrigation is required in most places to grow wine grapes, and the use of irrigation gives growers a high degree of control over quality. The Columbia River is one of the largest rivers in the country, and the river, its tributaries and aquifers provide ample water for grape growing; in the Yakima Valley, the Yakima River and local aquifers supply irrigation water. In Mendoza, irrigation is the lifeblood of the high, semi-desert vineyards. The industry there is converting to drip irrigation, although carefully controlled flooding still has its supporters. Mendoza’s sunny, desert-like conditions also give producers the opportunity to farm organically with little threat of rot or mildew.

• Low rainfall east of the Cascades allows growers to control water
• In the Columbia Valley, irrigation is required to grow vinifera grapes
• In Mendoza, flooding has kept phylloxera at bay, so new drip-irrigated vineyards need to be planted on resistant American rootstocks

In Washington, the Columbia Valley AVA, wholly located east of the Cascades, is by far the state’s largest growing region at 11+ million acres and is home to over 99% of Washington’s vinifera acreage. It averages 6 to 8 inches of precipitation a year, and Decanter puts the rain on the vineyard land east of the Cascades at just 200 mm a year. The Yakima Valley, a sub-appellation of the Columbia Valley, was designated in 1983 as the first federally recognized wine-growing region in the Pacific Northwest. It averages 8 inches of precipitation a year, and the Washington State Wine Commission puts its share of the state’s planted vinifera acreage at nearly one-third. In Argentina, Mendoza rests under a rain shadow from the Andes. Decanter quotes producer Roberto de la Mota describing Mendoza as a desert with around 200mm of rainfall per year. Mendoza has five divisions: Northern, Eastern, Central, Uco Valley and Southern, and Luján de Cuyo is a department within Central Mendoza. In the Wachau, cool and wet air masses flow down into it through little forested stone valleys from the Dunkelsteiner forest south of the Danube and the Waldviertel in the north.

• Columbia Valley AVA: lies in the rain shadow of the Cascades, averages 6 to 8 inches of precipitation a year, over 99% of Washington’s vinifera acreage
• Yakima Valley AVA: designated 1983, averages 8 inches of precipitation a year, with cooler areas toward the Cascade foothills and blazingly hot areas by the center of the Columbia Basin
• Mendoza: under a rain shadow from the Andes, around 200mm of rainfall per year (Decanter, quoting Roberto de la Mota)

Basalt covers much of eastern Washington, northern Oregon, and western Idaho: millions of years ago, fissures in the earth released massive volumes of this dark, fine-grained volcanic rock. Like almost all of eastern Washington’s growing regions, the Yakima Valley has soils strongly influenced by the Missoula Floods. Moderate to deep silt-loam is layered over gravel or directly onto basalt bedrock, which creates well-drained soils that are ideal for irrigated viticulture. Across the wider Columbia Valley, the soils are loess overlying gravel and slackwater sediment, with a basalt bedrock. Mendoza’s high, semi-desert vineyards grow on sand and clay/loam over gravel, limestone and clay. Altitude matters there too: one study has found that grapes grown at higher altitudes receive more ultraviolet light and produce more antioxidants.

• Yakima Valley: moderate to deep silt-loam over gravel or basalt bedrock, shaped by the Missoula Floods, well-drained
• Columbia Valley: loess overlying gravel and slackwater sediment, with a basalt bedrock
• Mendoza: sand and clay/loam over gravel, limestone and clay

Eastern Washington has long, warm summer days that provide ripe fruit flavors and cool nights that help lock in acidity. In the Yakima Valley, stretches of cooler terrain are home to almost half of the Chardonnay and Riesling grown in the state, while its many warmer sites yield significant percentages of Washington’s best Merlot, Cabernet Sauvignon and Syrah. In Argentina, Malbec styles range from full-bodied, richly fruited wines at lower altitudes to more elegant, fresh and floral wines at higher altitudes.

• Eastern Washington: warm days for ripe fruit flavors, cool nights to help lock in acidity
• Yakima Valley: Chardonnay and Riesling on cooler terrain; Merlot, Cabernet Sauvignon and Syrah on warmer sites
• Argentine Malbec: fuller and richly fruited lower down, more elegant, fresh and floral higher up

Primary sources: https://www.washingtonwine.org/grapes-soil-sun/, https://www.washingtonwine.org/resource/columbia-valley-ava/, https://www.washingtonwine.org/resource/yakima-valley-ava/, https://www.washingtonwine.org/fast-facts/, https://extension.oregonstate.edu/sites/extd8/files/documents/manual12.pdf, https://www.vinea-wachau.at/en/the-wine-region/the-wachau. Secondary sources: https://www.decanter.com/premium/top-20-washington-state-wines-to-try-429038/, https://www.decanter.com/wine-travel/north-america/decanter-travel-guide-washington-state-19989/, https://www.decanter.com/wine/wine-regions/mendoza/mendoza-world-wine-capital-30632/, https://www.wsetglobal.com/knowledge-centre/blog/2021/april/12/understanding-grapes-spotlight-on-malbec, https://www.decanter.com/features/argentinas-mendoza-in-focus-247908/, https://wine.wsu.edu/viticulture-enology-extension-news-spring-2014/. Last verified 2026-11-01