Jefferson County lies in the central part of Oregon and rests against the Cascades and Ochoco Mountains. It is within both Climate Division 6 (North Central Oregon) and Climate Division 7 (South Central Oregon) established by the National Climatic Data Center. Below is a description of the climate of Division 6 and 7 followed by specific descriptions of Jefferson County. Climate tables for various parameters, as observed at long-term climate stations in Jefferson County, are included below.

Climate Division 6 — North Central Oregon

North Central Oregon, climatic Zone 6, is a relatively dry region lying east of the Cascade Mountains. The Cascades serve as an effective moisture barrier, causing storms to dump much of their moisture west of the peaks and leaving areas to the east in a "rain shadow." As a result, Zone 6 is generally rather dry. The region extends from the Columbia River southward over hill country to the forested mountain areas, which border climate Zone 7. The Columbia is used in irrigation, transportation and hydroelectric power, and therefore dominates the area.

This region is Oregon's major wheat producing area. Grain production on dry land farms is the main source of agricultural income except for the Hood River Valley, which produces mostly tree fruits. Despite relatively small dimensions, the latter is one of the most important production areas in the Northwest. Its annual income of approximately $60 million derives mostly from pears, apples, and cherries. Other important commodities produced in Zone 6 include green peas, irrigated truck crops, beef cattle, sheep, alfalfa, and poultry.

Just as most of Oregon, this region has a definite winter rainfall climate. The months of November through February generally receive the most precipitation due to winter storms, which bring rain to lower elevations and snow to higher ridges and peaks. Annual totals vary greatly and are proportional to elevation; some of the lower elevations receive less than 12 inches per year, while a few of the higher areas receive more than 40 inches. Occasional summer thunderstorms bring localized, occasionally heavy showers.

Figure 1 shows NOAA climate stations in Zone 6, which were in operation during the 1961-1990 period. Figure 2 shows the Jefferson County region from the Oregon annual precipitation map. Table 1 lists normal monthly and annual precipitation for stations in Zone 6. Locations at the lowest elevations (adjacent to the Columbia) such as Arlington and Hermiston receive less than 10 inches per year. Precipitation increases steadily with elevation. Highest annual totals are found in the Blue Mountains along the extreme east border of the region, where totals exceeding 50 inches occur. Table 2 lists the average number of days with precipitation amounts exceeding certain thresholds.

The Columbia Gorge is a major east-west passageway connecting Zone 6 with the Willamette Valley and Oregon coast. Vigorous winds are common in and around the Gorge. During summer, wind direction is predominantly from the west, causing strong, steady winds within the Gorge and along the northern edge of Zone 6. These winds, in fact, make Hood River a world-renowned wind surfing location. Winter winds can blow from the west or the east and can reach speeds sufficient to cause widespread damage.

A major effect of the Gorge is a moderation of air temperatures near the Columbia by allowing maritime air to reach the area from the west; this can occur both in summer and winter. Occasionally, however, large-scale easterly flow brings very cold continental air to the region, resulting in extremely cold conditions. During such periods, the cold air passes westward through the Gorge, creating extreme conditions in the western valleys as well.

Table 3 lists normal monthly and annual temperatures in the region. Highest summer temperatures are observed at the low-lying points near the Columbia (i.e. Arlington, Hermiston, and Milton Freewater), while mean temperatures decrease with increasing elevation. Winter temperatures follow the same pattern with mildest temperatures at the lower elevation sites.

Table 4 lists average monthly and annual snowfall total for the various stations.

Median frost dates and length of the growing season are listed in Tables 5 and 6, respectively. These also follow the same elevation relationship evident in the temperature data: the longest growing seasons are in the mild and low elevation sites, while increasing elevation generally causes a shortening of the season. Arlington and Condon, both at nearly 3,000 feet above sea level, have much shorter growing seasons than lower sites such as The Dalles and Arlington.

Table 7 and 8 list average monthly and annual heating and growing degree days.

Climate Division 7 -- South Central Oregon

South Central Oregon, the largest of the Oregon climatic divisions, is a vast area of high desert prairie punctuated by a number of mountain ranges and isolated peaks. This region is predominantly livestock country; in addition to beef cattle, there are large numbers of sheep, dairy herds, horses, and swine. There are large amounts of land under irrigation as well, particularly in the Deschutes, Crook, Jefferson, and Klamath Counties. Among the major field crops grown are potatoes, alfalfa and other hay crops, mint, wheat, oats, barley, and onions. In the remaining counties comprising this zone (Grant, Harney, and Lake), irrigated acreage is much smaller; grazing lands and dry land farming predominate.

Figure 1 shows NOAA climate stations in Zone 7, which were in operation during the 1961-1990 period. Figure 2 shows the Jefferson County region from the Oregon annual precipitation map. Most of this region receives relatively low amounts of precipitation. As can be seen in Table 1, most of the stations in Zone 7 receive less than 15 inches per year. However, some of the higher mountain sites receive significantly greater precipitation. For example, Steens Mountain in Harney County, whose summit is more than 9,000 feet above sea level, receives more than 40 inches per year at its higher elevations. Other mountainous locations are also known to receive high annual amounts. Most of the stations in Zone 7 receive their highest monthly precipitation in the winter months with a secondary maximum during late spring and early summer. For other locations, the precipitation is greatest during spring and summer. Stations near the Cascades (such as Sisters, Bend, Chiloquin, Klamath Falls, and Madras) tend to have annual distributions very similar to those in western Oregon: winter maximum are followed by a steady decrease, with lowest monthly averages in midsummer. Farther east, however, spring-summer peaks are much more pronounced. At Hart Mountain, for example, the four wettest months are March through June. The months of July through September are generally the driest of the year throughout the region. These months are characterized by isolated local thunderstorms. Some months are very wet and others almost completely dry.

Table 2 lists the average number of days with precipitation amounts exceeding certain thresholds.

Table 3 lists normal monthly temperatures for Zone 7 measurement stations. Summers are generally quite warm, although the relatively high elevations tend to moderate the temperatures somewhat. Pelton Dam and Dayville, with mean maximum temperatures in the 90's during the warmest summer months, are the hottest stations in this region. The coldest sites listed are Brothers, Hart Mountain, Sprague River, and Ochoco Ranger Station. It is certain that some of the higher elevations are colder than the areas listed here, however.

Table 4 lists average monthly and annual snowfall total for the various stations.

Tables 5 and 6 list median frost dates and mean growing seasons, respectively, for four different temperature thresholds.

Table 7 and 8 list average monthly and annual heating and growing degree days, respectively.

County Description

Established: Dec. 12, 1914
Population: 19,400
Area: 1,791 sq. mi.
Economy: Agriculture, forest products and recreation.
County Seat: Madras

Jefferson County was established in 1914 from a portion of Crook County and named for Mount Jefferson on its western boundary. The county owes much of its agricultural prosperity to the railroad, which arrived in 1911, and to the development of irrigation projects in the late 1930s. The railroad, which links Madras with the Columbia River, was completed after constant feuds and battles between two lines working on opposite sides of the Deschutes River. Vegetable, grass and flower seeds, garlic, mint and sugar beets are cultivated on some 60,000 irrigated acres. Jefferson County also has vast acreages of rangelands and a healthy industrial base related to forest products. The Warm Springs Forest Products Industry, a multi-million dollar complex owned by the Confederated Tribes of the Warm Springs Reservation—partially located in the northwestern corner of the county—is the single biggest industry. With 300 days of sunshine and a low yearly rainfall, fishing, hunting, camping, boating, water-skiing and rock hunting are popular recreations.

(County information obtained from Oregon Blue Book)

Climate Tables (Jefferson County, Oregon)