The proximity of the Pacific Ocean, orientation of numerous canyons and valleys, and varying elevations produce many different distinct microclimates in the Paso Robles AVA.
The area benefits from the largest swing between high daytime and low nighttime temperatures of any region in California as a result of the cool marine air that flows east through the Templeton Gap and south along the Salinas River Valley from the Monterey Bay. The region’s summer is characterized by warm, clear days, generally unencumbered by clouds, fog or severe winds. Daytime high temperatures in the summer typically fall between 85 and 105 degrees Fahrenheit, but nighttime low temperatures usually can drop by 40 to 50 degrees, cooled by a marine layer that moves over the region in the mid to late afternoon. This diurnal fluctuation is considered a key by winemakers and wine grape growers to attain the intense varietal character displayed in wine grapes from the area.
September, October and the first half of November are typically rain-free and warm, giving Paso Robles vines the advantage of time to produce fully mature fruit, while the overnight cooling keeps the grapes’ acid chemistry in balance. The first rainfall of the season is typically about two weeks later than Napa or Sonoma, and a month later than Mendocino, giving winemakers the luxury of waiting for optimal ripeness. Winter temperatures tend to dip into the low twenties in the cooler regions, with most vineyards becoming fully dormant by mid-December. Frost is a potential threat through mid-May, especially following a northern weather system.
The rainfall of the region, like its climate and soils, varies greatly depending on the vineyard’s proximity to the Pacific Ocean and the Templeton Gap. Average annual rainfall for the City of Paso Robles is 15.5 inches, but rainfall ranges from eight inches in the eastern portions of the AVA to as much as 45 inches on the far western ridges. The first rains typically arrive in early-to-mid November, with the heaviest amounts usually occurring January through March. These rain totals are typically dominated by relatively few, but substantial, Pacific storms that can contribute several inches of rain in just a few days.
The City of Paso Robles rests at 740 feet above sea level. Paso Robles vineyards east of the Salinas River range from 700 to 1,200 feet in elevation while those to the west range from 850 to 2,000 feet.
Due to cool nights, warm days, and typically late rains, Paso Robles vines tend to have a longer growing season and grapes have more hang time compared to other wine regions, resulting in fully mature fruit whose acid chemistry is kept in balance through the area’s overnight cooling.