Jackson County lies in the southwestern part of Oregon along the California border. It is wholly within Climate Division 3 (Southwestern Interior) established by the National Climatic Data Center. Below is a description of the climate of Division 3 followed by specific descriptions of Jackson County. Climate tables for various parameters, as observed at long-term climate stations in Jackson County, are included below.
Climate Division 3 — Southwestern Interior
The southwestern interior of Oregon is one of the more rugged parts of the state. Deeply indented river valleys separate mountains and ridges, with most of the rivers flowing westward towards the Pacific Ocean. Although much of the area lies in somewhat of a rain shadow, sheltered from the Pacific by the Coast Range to the west, many of the higher elevation sites receive abundant precipitation with some locations receiving in excess of 120 inches per year.
Due to the ruggedness of the terrain, much of Zone 3 remains sparsely settled. The only major urban areas lie in the broader valley areas, particularly the Rogue and Umpqua valleys. Ashland, Medford, and Grants Pass are the major cities in the Rogue valley, while Roseburg is the primary urban area in the Umpqua valley.
Long an important forestry area, southwestern Oregon is perhaps best known for its fruit crop, particularly the Rogue River pear industry which covers more than 10,000 acres. Additional agricultural and livestock commodities of importance include hay, grain, seed crop, beef cattle, poultry, sheep, and the dairy industry.
As in the case of the rest of western Oregon, most precipitation in Zone 3 falls during the months of November through March. Of Medford's average annual precipitation of 18.8 inches, about 75 percent falls during that five month period. Occasional summer thunderstorms cause precipitation during the warmer months, but average monthly totals during summer are quite low. Table 1 lists monthly and annual normal precipitation at Zone 3 sites.
Total precipitation in a given area is strongly influenced by elevation, and Zone 3 is no exception. In general, the driest areas are those at the lowest valley locations, while precipitation increases steadily at higher elevations. The driest area in Zone 3 extends from Central Point through Medford and nearly to Ashland, all of which receive less than 20 inches per year. Not far south of Ashland, however, is Mount Ashland at 7,500 feet, which receives an excess of 50 inches per year. Perhaps the wettest area in Zone 3 is in the remote, mountainous area east of Roseburg near Quartz Mountain. Although precipitation data in that area is scarce, it has been estimated that some of the higher peaks receive an excess of 120 inches of rain per year. Another very wet area lies in the Klamath Mountains near Cave Junction in the southwestern part of Zone 3. Some of the higher peaks, such as Onion Mountain and Squaw Mountain, probably receive more than 120 inches per year, although lack of precipitation measurement stations makes this somewhat uncertain. Figure 1 shows NOAA climate stations in Zone 3, which were in operation during the 1961-1990 period. Figure 2 shows the Jackson County region from the Oregon annual precipitation map. Table 2a and 2b list the average number of days with precipitation amounts exceeding certain thresholds.
Table 4 lists average monthly and annual snowfall total for the various stations. Snow falls nearly every winter in southwestern Oregon. In the valleys, the annual total is about 20 to 30 inches per year , although snow on the ground seldom lasts more than a few days at a time. At higher elevations, a great deal more snow is reported. At Sexton Summit (3,836 feet), for example, the average annual snowfall is about 100 inches. The frequency of snowstorms also varies widely with elevation differences. Medford, for example, has an average of 3 days per year with at least one inch of snow while Sexton Summit averages 30 inches.
Due to its separation from the coast, Zone 3 has greater temperature extremes than the remainder of western Oregon. During summer, it is generally the warmest part of the state. Medford, for example, averages about 55 days per year with maximum temperatures of 90 deg F or above; in fact, the average daily maximum for July is above 90 deg F. Winter temperatures can be quite cold. The average extreme low temperature in Medford during December and January is about 18 deg F, and an average of twenty days in January have low temperatures of 32 deg F or below. Medford's monthly mean temperature ranges from 72.5 deg F in July to 37.7 deg F in December, a range greater than most other stations west of the Cascades. Table 3 lists the normal monthly temperature at stations in the area.
Table 5 and 6 lists median frost dates and mean growing seasons, respectively, for four different temperature thresholds. Valley locations such as Riddle, Drain, and Grants Pass have longer growing seasons than higher elevation sites such as Howard Prairie Dam and Prospect. Most of the valley locations have at least 140 days in between spring and fall 32 deg F temperatures. Length of time between 28 deg F temperatures is generally more than 200 days.
Cloud cover is greatest during the winter months, averaging more than 80 percent of total potential cloud cover during December and January. Mid-latitude storms generally produce extensive middle and high clouds, while fair weather periods between storms often produce extensive valley fog, sometimes lasting for many days. Summers, however, are mostly cloud-free, averaging only about 20 percent mean sky cover.
Established: Jan. 12, 1852
Named for President Andrew Jackson, Jackson County was formed in 1852 from Lane County and the unorganized area south of Douglas and Umpqua Counties. It included lands which now lie in Coos, Curry, Josephine, Klamath and Lake Counties. The discovery of gold near Jacksonville in 1852 and completion of a wagon road, which joined the county with California to the south and Douglas County to the north, brought many pioneers.
County information obtained from Oregon Blue Book)
Climate Tables (Jackson County, Oregon)