Willamette Valley - Climate

Climate

Characterized by cool, wet winters and warm, dry summers, the Willamette Valley's climate is relatively mild throughout the year. It has been described as Mediterranean, though somewhat cooler and wetter. Most valley precipitation arrives as rain, about half of which falls between December and February. Growing seasons are long, averaging 150 to 180 days per year in the lowlands to about 110 to 130 days at elevations above 800 feet (240 m).

Precipitation varies considerably from place to place within the valley. Average annual totals range from less than 40 inches (1,000 mm) at low elevations to more than 80 inches (2,000 mm) in the foothills of the Cascade and Coast ranges. Eugene, at 359 feet (109 m) above sea level, receives 46 inches (1,200 mm) a year, while Portland, just 338 feet (103 m) lower, receives only 37 inches (940 mm). Annual snow accumulations at low elevations average 5 to 10 inches (13 to 25 cm) per year, mostly from December through February.

Severe storms are rare, although ice storms sometimes occur when cold air from the state's interior flows west through the Columbia Gorge and into the northern part of the valley. Weather systems sometimes bring high winds to the northern region of the valley. Average cloud cover can exceed 70 percent in winter but drops to less than 15 percent in summer.

High temperatures range on average from about 45 °F (7 °C) in winter to about 80 °F (27 °C) or slightly higher in summer. Average lows are generally around 30 °F (−1 °C) or slightly higher in winter and around 55 °F (13 °C) or slightly higher in summer. Temperatures exceed 90 °F (32 °C) about 10 to 25 times a year on average, and temperatures lower than 5 °F (−15 °C) occur only about once every 25 years.

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