except in summer when the northward expansion of the North Pacific High creates generally sunny and warm weather. Winters are consistently wet and cloudy, and often foggy, but quite mild. Although night frosts are common, the temperature almost always rises above the freezing point in the daytime. Snow occurs on occasion, but accumulations are normally light, and in some winters no snow whatsoever falls. Very cold temperatures are atypical; the temperature very rarely falls below , and readings of or lower occur only about once every 25 years. Summers are characterized by warm, sunny afternoons with little or no humidity, and cool evenings. Sometimes, heat waves can occur, with temperatures rising above and occasionally even reaching , but the nights usually bring relief. Precipitation varies considerably across the valley and is closely correlated with elevation. Annual totals range from at the lowest elevations to more than in the foothills. Eugene, at the southern end of the valley, is above sea level and receives per year. Conversely, at the northern end of the valley, Portland is above sea level and receives only per year. Most rainfall occurs from October to May, and it tends to be heaviest between November and January, when disturbances coming from the Pacific Ocean are at their most intense. Growing seasons are long, averaging 150 to 180 days per year in the lowlands to about 110 to 130 days at elevations above . Severe storms of any kind are rare, although snow and ice storms can sometimes occur when surface low pressure systems move south along the coast, inducing offshore flow which advects cold air from the Columbia Basin westward through the Columbia River Gorge, filling the valley to the north of the surface low track. 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. Tornadoes are rare, but do happen a few times a year with minimal damage. Agriculture The agricultural richness of the valley is partly due to the Missoula Floods that inundated the valley approximately 40 times between 15,000 and 13,000 years ago at the end of the last ice age. The floods were caused by the periodic rupturing of the ice dam of Glacial Lake Missoula, the waters of which swept down the Columbia River and flooded the Willamette Valley as