West of the Cascade Range, winters are chilly with frequent rain and occasional snow. Temperatures can get very cold, but only occasionally, as the result of Arcticcold waves. The high desert region of the state is much drier, with less rain, more snow, colder winters, and hotter summers.[1]
Precipitation in the state varies widely: some western coastal slopes approach Script error: No such module "convert". annually, while the driest places, such as the Alvord Desert (in the rain shadow of Steens Mountain) in eastern Oregon, get as little as Script error: No such module "convert"..Script error: No such module "Footnotes".Script error: No such module "Check for unknown parameters".[2]
The Pacific Ocean, the moisture-laden air above it, and the storms moving from it over the Oregon coast, are major factors in the state's precipitation patterns. As humid ocean air flows east from the ocean and encounters the Coast Range, it rises steeply, cools, and loses moisture through condensation, which produces heavy rain. The heaviest precipitation in the state occurs at Script error: No such module "convert". above sea level in these coastal mountains. At lower elevations along the coast, orographic precipitation is less intense but still produces Script error: No such module "convert". a year.Script error: No such module "Footnotes".Script error: No such module "Check for unknown parameters".
In the Willamette Valley east of the Coast Range, storms "blowing" from the Pacific retain enough moisture to drop from Script error: No such module "convert". annually in the most heavily populated part of the state. East of the valley, the storm air rises again as it meets the Cascade Range, cooling once more and forming condensate at elevations often as low as Script error: No such module "convert"..Script error: No such module "Footnotes".Script error: No such module "Check for unknown parameters". Since volcanic peaks in the range are quite high—more than Script error: No such module "convert". in the case of Mount HoodScript error: No such module "Footnotes".Script error: No such module "Check for unknown parameters".—most of the remaining Pacific moisture falls here in the form of rain or snow.Script error: No such module "Footnotes".Script error: No such module "Check for unknown parameters".
The remaining two-thirds of the state is relatively dry, classified as semi-arid, with large areas receiving no more than Script error: No such module "convert". a year. Exceptions occur at higher elevations in the Blue Mountains and the Wallowa Mountains to the northeast, which get Script error: No such module "convert". a year.Script error: No such module "Footnotes".Script error: No such module "Check for unknown parameters".
Across Oregon, the wet season runs from November through March, when the jet stream is strongest in the Northern Hemisphere. Precipitation is less in the months between winter and summer: April through June in the spring and September and October in the fall. Statewide, the dry months are July and August, when moisture arrives during afternoon thunderstorms, mainly in the mountains, and less often from storms that reach the north coast and adjacent counties.Script error: No such module "Footnotes".Script error: No such module "Check for unknown parameters".
Snowfall in Oregon is greatest in the Cascade Range. Based on data from ski resorts and a few official weather stations, average annual snowfall in the Cascades can range from Script error: No such module "convert"..[2] The state's largest annual snowfall on record, Script error: No such module "convert"., occurred at Crater Lake in the Cascades in 1950.Script error: No such module "Footnotes".Script error: No such module "Check for unknown parameters". In the Blue Mountains of eastern Oregon, snowfall totals can also be large, between Script error: No such module "convert".. On the other hand, most winter precipitation in the Coast Range falls as rain, though heavy snow sometimes occurs.[2]
In most mountain areas in Oregon, the ground above Script error: No such module "convert". is covered with snow from December through April. Snow depths, which vary with elevation and time of year, average an estimated Script error: No such module "convert". in the Cascades and Script error: No such module "convert". in the Blue Mountains at the end of January; by the end of April, they diminish to Script error: No such module "convert". in the Cascades and Script error: No such module "convert". in the Blues. Glaciers remain year-round on some Cascade peaks higher than Script error: No such module "convert". above sea level.[2]
Annual snowfall along the coastal plain averages Script error: No such module "convert". a year, including years with none. Further inland, between the Coast Range and the Cascades, snowfall generally averages from Script error: No such module "convert". a year. East of the Cascades, in non-mountain settings, the annual totals range from Script error: No such module "convert"., depending on location; they are smallest in the north-central region and the Snake River basin in the southeast and largest in the northeastern valleys and in the high plateaus of the south-central part of the state.[2]
In addition to seasonal cycles in solar radiation (more in summer, less in winter), major factors affecting temperatures in Oregon include the moderating influence of the Pacific Ocean combined with variations in surface elevation, especially the Cascade Range. In general, temperatures on Earth drop by about 4 °F (2 °C) per each Script error: No such module "convert". increase in elevation. Throughout the year, high elevations across the state tend to be cooler than low elevations. In addition, the Cascades, running north–south from border to border, generally retain relatively warm Pacific air masses on the western side of the state and relatively cool continental air masses on the eastern side. This prevailing pattern breaks down occasionally when dense cold air flows down the Columbia Gorge into the Willamette Valley and lowers temperatures more than usual from Portland to Eugene.Script error: No such module "Footnotes".Script error: No such module "Check for unknown parameters".
Oregon has a wide range of temperatures, though the extremes are rare.[2] The highest was recorded on July 29, 1898, in Hermiston, Oregon, and again on August 10, 1898, in both Pendleton, Oregon and Redmond, Oregon, and once more on June 29, 2021, at Pelton Dam. All are east of the Cascades, when the temperature reached Script error: No such module "convert"..Script error: No such module "Footnotes".Script error: No such module "Check for unknown parameters". The lowest occurred on February 9, 1933, in Ukiah, and again on February 10, 1933, in Seneca, also both east of the Cascades, when the temperature dropped to Script error: No such module "convert"..Script error: No such module "Footnotes".Script error: No such module "Check for unknown parameters". The temperature in the Willamette Valley is mild compared to the desert regions of the state, with high temperatures at or above Script error: No such module "convert". occurring only five to fifteen times per year, and low temperatures below Script error: No such module "convert". similarly infrequent.[3] The hottest area of the state is the southwest; Jackson County is the warmest place in the state during summer.[4]
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