According to the Köppen climate classification, most of Western Oregon has a warm-summer Mediterranean climate (or Csb type), which features warm, dry summers, and wet winters with frequent overcast and cloudy skies. Eastern Oregon falls into the cold semi-arid climate (or BSk type), which features drier weather.
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 Arctic cold waves. The high desert region of the state is much drier, with less rain, more snow, colder winters, and hotter summers.
Precipitation
Precipitation in the state varies widely: some western coastal slopes approach annually, while the driest places, such as the Alvord Desert (in the rain shadow of Steens Mountain) in eastern Oregon, get as little as .
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 above sea level in these coastal mountains. At lower elevations along the coast, orographic precipitation is less intense but still produces a year.
In the Willamette Valley east of the Coast Range, storms "blowing" from the Pacific retain enough moisture to drop from 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 . Since volcanic peaks in the range are quite high—more than in the case of Mount Hood—most of the remaining Pacific moisture falls here in the form of rain or snow.
The remaining two-thirds of the state is relatively dry, classified as semi-arid, with large areas receiving no more than a year. Exceptions occur at higher elevations in the Blue Mountains and the Wallowa Mountains to the northeast, which get a year.
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.
Snow
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 . The state's largest annual snowfall on record, , occurred at Crater Lake in the Cascades in 1950. In the Blue Mountains of eastern Oregon, snowfall totals can also be large, between . On the other hand, most winter precipitation in the Coast Range falls as rain, though heavy snow sometimes occurs.
In most mountain areas in Oregon, the ground above is covered with snow from December through April. Snow depths, which vary with elevation and time of year, average an estimated in the Cascades and in the Blue Mountains at the end of January; by the end of April, they diminish to in the Cascades and in the Blues. Glaciers remain year-round on some Cascade peaks higher than above sea level.
Annual snowfall along the coastal plain averages a year, including years with none. Further inland, between the Coast Range and the Cascades, snowfall generally averages from a year. East of the Cascades, in non-mountain settings, the annual totals range from , 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.