The Climate of Oregon

1. General Trends and Relationships

Oregon's weather and climate are affected by large-scale circulation in the atmosphere; by regional influences involving the Pacific Ocean, the shoreline, and the interior; and by local characteristics, such as topography. Local conditions are influenced by all of these, and more: distance from the coast, elevation, and terrain orientation (for example, north- vs. south-facing slopes) can have profound climatic effects.

The north-south orientation of the coastline is almost directly perpendicular to the prevailing atmospheric flow at upper levels. The steep and rugged topography leaps from the sea nearly everywhere except where major rivers pour their contents back into the Pacific Ocean. The effect of this combination on moisture-laden air is to produce copious precipitation. On both local and regional scales, the interaction between ocean, land and topography produces considerable complexity in the resulting spatial structure of climate elements.

1.1 Precipitation

1.1.1 Seasonal characteristics

Locations in western Oregon receive the bulk of their annual precipitation during winter. Although there is some variation with latitude, the wettest months tend to be the November-March period. Although significant precipitation can occur during the warm season, average totals during those months are generally lower than during winter. East of the Cascades, however, the annual distribution is much more uniform, with some locations receiving more precipitation in summer than in winter. Table 1 shows average monthly and annual precipitation at selected locations in Oregon, listed by climate division. Table 2 lists the average number of days with measurable precipitation at the same locations. Several additional facts become obvious from perusal of the tables:

· Precipitation is highest near the coast and in higher elevations, and increasingly lower as one moves eastward.

· July is consistently the driest month throughout Oregon

· During winter in western Oregon, more than half the days have measurable precipitation; in summer, only 10-15% of the days are wet.

Zone Station County Elev. (ft) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Ann
1 Astoria Clatsop 10 10 7.59 7.07 4.61 3.02 2.4 1.16 1.33 2.91 5.73 10.05 10.55 66.42
1 Brookings Curry 70 10.85 9.03 9.49 5.3 3.64 1.55 0.53 1.31 2.15 5.84 11.52 12.23 73.44
2 Corvallis Benton 225 6.82 5.04 4.55 2.56 1.95 1.23 0.52 0.87 1.51 3.11 6.82 7.72 42.7
2 Eugene Lane 364 7.03 5.38 5.17 3.6 2.2 1.22 0.46 0.8 1.32 3.35 7.51 7.86 45.9
2 Portland Multnomah 20 5.35 3.68 3.54 2.39 2.06 1.48 0.63 1.09 1.75 2.66 5.34 6.13 36.1
3 Ashland Jackson 1750 2.37 1.72 1.95 1.61 1.29 0.91 0.32 0.58 0.95 1.6 2.82 3.06 19.18
3 Medford Jackson 1300 2.69 1.93 1.82 1.16 1 0.58 0.26 0.52 0.86 1.49 3.23 3.32 18.86
3 Roseburg Douglas 465 5.13 3.7 3.56 2.24 1.43 0.83 0.43 0.73 1.24 2.23 5.36 5.47 32.35
4 Detroit Dam Marion 1220 12.79 10.24 9.42 6.54 4.87 3.27 0.9 1.6 3.56 6.42 13.21 13.98 86.8
4 Government Camp Hood River 3980 13.65 10.01 8.92 7.15 4.75 3.42 1.13 1.83 3.9 6.13 11.92 14.01 86.82
4 McKenzie Bridge RS Lane 1478 9.88 7.33 7.03 5.02 3.58 2.59 0.85 1.44 2.93 5.08 10.01 10.76 66.5
5 Crater Lake Klamath 6475 9.66 7.78 8.09 4.6 3.01 1.98 0.68 1.29 2.38 4.75 10.56 10.84 65.62
5 Silver Lake RS Lake 4380 0.89 0.68 0.76 0.63 0.86 0.85 0.5 0.61 0.58 0.63 1.2 1.1 9.29
6 Hood River Hood River 500 4.56 3.59 2.5 2.47 1.1 0.91 0.44 0.27 0.64 2.69 5.95 5.28 30.4
6 Moro Sherman 1870 1.49 0.98 1.04 0.78 0.77 0.59 0.28 0.47 0.54 0.75 1.66 1.74 11.09
6 Pendleton Umatilla 1492 1.51 1.14 1.16 1.04 0.99 0.64 0.35 0.53 0.59 0.86 1.58 1.63 12.02
7 Bend Deschutes 3660 1.83 0.97 0.92 0.6 0.77 0.86 0.49 0.58 0.47 0.65 1.57 1.99 11.7
7 Burns Harney 4140 0.81 1.17 1.6 0.8 1.07 0.67 0.52 0.43 0.62 0.78 1.41 1.41 11.29
7 Klamath Falls Klamath 4098 1.81 1.28 1.35 0.75 0.85 0.69 0.35 0.62 0.55 1.07 1.97 2.23 13.52
8 Enterprise Wallowa 3880 1.26 1.17 1.65 1.55 2.15 1.72 1 1.4 1.19 0.84 1.5 1 16.43
8 John Day Grant 3063 1.15 0.82 1.12 1.21 1.56 1.4 0.53 0.95 0.84 0.92 1.47 1.4 13.37
8 La Grande Union 2755 1.96 1.47 1.48 1.42 1.61 1.43 0.63 0.92 0.97 1.24 1.86 1.86 16.85
9 Ontario Malheur 2145 1.33 0.87 0.82 0.63 0.73 0.72 0.2 0.44 0.53 0.61 1.3 1.5 9.68
9 Rome Malheur 3410 0.58 0.5 0.83 0.69 1.01 1.05 0.36 0.46 0.58 0.55 0.82 0.63 8.06

Table 1. Average monthly and annual precipitation at selected locations in Oregon

Station Number Zone Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year
Astoria WSO AP 328 1 22 19 21 18 15 12 8 8 10 15 21 22 190
Bandon 2 NNE 471 1 20 18 20 16 12 9 4 6 8 12 20 21 167
Brookings 1055 1 17 16 18 13 9 6 4 5 6 11 18 18 141
Newport 6032 1 20 18 21 17 14 11 7 8 10 15 21 22 187
North Bend 6073 1 19 17 20 16 12 9 4 6 7 12 20 20 160
Tillamook 1 W 8494 1 21 19 21 18 15 10 7 7 11 15 21 23 187
Bonneville Dam 897 2 20 18 20 17 14 10 5 6 9 13 20 21 172
Eugene WSO AP 2709 2 17 15 16 12 10 6 3 4 6 10 17 18 134
Forest Grove 2997 2 19 17 17 14 12 8 4 4 7 12 19 20 154
Portland WSO AP 6751 2 18 15 17 14 12 9 4 5 8 12 19 19 151
Salem WSO AP 7500 2 17 15 17 13 11 7 3 4 7 11 18 19 143
Silver Creek Falls 7809 2 19 16 19 16 13 9 4 5 8 13 19 19 163
Grants Pass 3445 3 15 14 16 11 7 4 2 3 5 9 16 17 118
Medford WSO AP 5429 3 13 11 12 10 7 5 2 3 4 7 14 14 101
Riddle 7169 3 16 15 16 13 8 6 2 3 5 9 17 17 126
Roseburg KQEN 7331 3 18 15 17 14 9 6 2 3 6 10 18 18 137
Toketee Falls 8536 3 17 16 19 16 11 8 3 4 7 11 18 18 149
Belknap Springs 652 4 18 16 19 17 13 9 4 5 8 13 19 19 160
Detroit Dam 2292 4 20 18 20 18 15 10 5 5 9 13 21 21 176
Government Camp 3402 4 20 18 20 18 15 10 5 6 9 13 20 20 172
Santiam Pass 7559 4 20 18 18 16 12 10 5 6 9 12 18 21 164
Crater Lake 1946 5 17 16 19 14 10 8 3 5 6 10 17 18 142
Fremont 5 NW 3095 5 8 7 9 6 5 6 2 3 3 5 9 9 74
Wickiup Dam 9316 5 13 11 12 8 7 6 4 4 5 7 13 13 103
Antelope 1 NW 197 6 10 9 11 8 7 6 3 4 4 6 12 11 91
Hermiston 2 S 3847 6 11 9 9 7 6 5 3 4 4 6 12 12 87
Hood River Exp Stn 4003 6 16 14 14 12 8 6 3 4 6 10 18 17 128
Moro 5734 6 11 9 11 8 7 5 3 4 4 7 13 11 91
Pendleton WSO AP 6546 6 12 11 11 9 7 6 3 4 5 6 12 12 96
The Dalles 8407 6 12 12 11 7 5 4 2 4 4 7 14 13 94
Bend 694 7 10 7 8 6 5 5 3 4 4 5 9 10 75
Burns WSO 1175 7 9 9 13 8 9 6 3 4 5 6 12 10 99
Hart Mountain Refuge 3692 7 7 7 9 8 8 7 3 4 5 6 9 8 80
Klamath Falls 4506 7 11 9 11 7 6 5 2 3 3 6 11 12 86
Lakeview 4670 7 11 10 11 9 8 6 3 3 4 6 11 11 93
Madras 5139 7 8 8 7 6 5 5 3 3 4 5 9 9 73
Ochoco Ranger Station 6243 7 9 8 6 6 6 6 3 5 4 6 9 9 75
Sisters Ranger District 7857 7 10 8 9 7 5 4 3 3 4 6 11 10 82
Squaw Butte Exp. Stn. 8029 7 8 6 8 5 7 6 3 4 4 5 8 7 69
Austin 356 8 14 12 13 9 10 8 4 6 6 8 14 15 115
Elgin 2597 8 14 13 14 11 11 9 5 6 6 9 14 15 130
Enterprise 2 S 2675 8 11 11 14 12 13 11 7 7 7 7 14 10 120
John Day 4291 8 11 9 10 10 10 9 3 5 5 7 11 12 101
LaGrande 4622 8 12 9 12 11 10 9 4 5 6 8 11 11 108
Seneca 7675 8 10 9 10 8 8 7 3 4 4 6 10 12 88
Ukiah 8726 8 13 10 11 10 9 8 3 5 5 7 12 12 104
Burns Junction 1174 9 7 8 7 7 6 5 3 4 4 4 8 7 68
Malheur Branch Exp Stn 5160 9 9 8 8 6 6 6 2 3 4 5 10 10 76
Rome 2 NW 7310 9 4 3 4 4 4 4 2 2 3 3 5 4 41

Table 2. Average number of days with measurable precipitation (.01 inch or more), Oregon locations.

b. Spatial distribution

The most important factors influencing annual average precipitation in the region are elevation and distance from the coast. Locally, elevation is the most important factor; on a regional basis, however, distance from the coast becomes increasingly important.

Orographic (terrain-induced) influences on precipitation are very significant in Oregon. The primary effect of orography on a given mountain slope face is to cause precipitation to increase with elevation; in general, the higher the elevation, the greater the precipitation. Orographic effects typically operate at large spatial scales, responding to smoothed topographic features rather than detailed variations in terrain. Thus, a major terrain barrier such as the Cascades results in abundant orographic precipitation, even though small ridges and valleys embedded in the mountain range may not show demonstrable effects.

Figure 1 is a simplified map of average annual precipitation produced using PRISM, a model developed by Christopher Daly of Oregon State University for creating maps and digital coverages of precipitation and other climate parameters. Evident from Figure 1 are the two major influences on precipitation: generally high precipitation west of the Cascades, due to proximity to the Pacific Ocean; and highest local values in mountainous areas, in both western and eastern Oregon.

c. Long-term distribution

Analysis of long-term time series of precipitation in the study area reveals rather significant year-to-year variations; correlations between successive years are generally quite low. However, the records indicate a number of longer-term cyclical patterns, in which relatively dry years and relatively wet years are bunched together. Figure 2 shows annual precipitation for the Oregon Coast division (Zone 1) since 1896 (for the Water Year, October through September, ending on the year shown). Bars represent annual departures from the 100-year average, while the solid line shows 5-year running averages. Notable in the figure are four distinct periods, and a possible fifth which began in the 1990s:

1896-1917 Generally wet
1918-1944 Generally dry (in several cases 4 or more dry years in a row)
1945-1974 Generally wet (even the "dry" years were not particularly dry)
1975-1994 Generally dry (including 10 in a row at the end of the period)
1995-2001 Generally wet

d. Stream flows

Precipitation and stream flows correlate very well, at least on an annual basis. Total flows are highest during the wettest years, and lowest during dry periods. Figure 3 shows annual stream flow in the Willamette River at Albany since 1896. The generally dry period in the 20's and 30's can be seen, as well as a much wetter regime beginning in the late 40's, and very dry periods in the late 70's and late 80's-early 90's.

During this century, dams have been constructed on many of the rivers and streams in Oregon. Originally dams were built primarily for flood control, but many have served as sources of hydroelectric power (some exclusively so). Dams have caused stream flows to be more uniform, by reducing the highest flows (winter and spring) and maintaining higher minimum flows (summer and autumn).

In addition to obvious changes in river conditions, dams can cause changes in ocean conditions. For example, the significant seasonal changes in water flow in the Columbia River serve to reduce the typical late spring high flows due to snowmelt, and to increase the flow of fresh water to the sea from late summer through early winter. The plume of freshwater that travels southward during summer has been reduced, and the northward flowing fresh water in winter) when winds and currents are predominantly from south to north) has increased. Decreases in salinity along the coast have consequently been noted as far as 1000 km north of the mouth of the Columbia River during winter. Consequences to estuarine environments are unknown.

1.2 Temperature

1.2.1 Seasonal and diurnal characteristics

The strong marine influence in the study region causes the seasonal and diurnal temperatures to be mild and relatively uniform compared with more inland areas. In general, the temperature range during the day and throughout the year is quite small. Table 3 shows average maximum temperatures throughout Oregon, while Table 4 lists average minimum temperatures. Other than the coastal stations, Oregon summer temperatures are warm, with average highs mostly in the 80's. The Willamette Valley's temperatures are moderated by the influence of mild Pacific air, while most of eastern Oregon, although not affected significantly by maritime air masses, are tempered by high elevations. Bend, for example, has average highs in the low eighties. However, if Bend were at sea level instead of 3,660 feet, its temperatures would be much warmer: by 10-15 degrees F. Indeed, the warmest places in Oregon are those sheltered from the marine influence and which lie at low elevations: Medford, Pendleton, and Ontario, for example.

For an illustration of the influence of the Pacific Ocean, consider the case of Newport, Corvallis and Madras, three stations at the same latitude (44.6°N). However, Newport is a coastal station while Corvallis is east of the Coast Range and Madras is east of the Cascades, so their temperature regimes are much different. Table 5 below shows temperatures and degree days for different times of the year. Newport's coastal influence is very strong, enough to keep temperatures mild throughout the year, while Corvallis and Madras are successively more extreme.

Station Number Zone Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year
Astoria 328 1 47.8 51 53.2 56 60.1 64.1 67.5 68.8 67.8 61.1 53.5 48.2 58.3
Bandon 471 1 53.4 55.1 55.6 57.1 60.4 63.7 66.1 67 67.1 63.1 57.4 53.5 60
Brookings 1055 1 54.8 56.3 57.3 59.6 63.3 66.5 67.7 67.6 68.6 64.4 58.5 54.8 61.6
Honeyman State Park 3995 1 50.8 53.9 55.8 58.7 62.9 67 70.3 70.9 70.8 64.2 55.4 50.5 60.9
North Bend 6073 1 51.8 54 54.8 56.5 60.2 63.8 66.3 67.1 66.9 63 56.9 52.4 59.5
Seaside 7641 1 51.1 54.1 55.6 58 61.9 65.3 68.2 69.2 69.9 64.4 56.3 51.4 60.5
Tillamook 8494 1 50 53 54.6 57 60.9 64.8 67.3 68.6 68.8 62.9 54.7 49.8 59.4
Corvallis OSU 1862 2 45.5 50.4 54.9 59.5 66.1 73.1 80.2 81.1 75.4 64.3 52.2 45.6 62.4
Eugene 2709 2 46.4 51.4 55.9 60.5 67.1 74.2 81.7 81.8 76.2 64.6 52.4 46.2 63.2
Forest Grove 2997 2 45.2 50.7 55.6 60.8 67.8 74.8 80.9 81.9 75.9 64.9 52.2 45.2 63
Hillsboro 3908 2 45.3 50.6 55.5 60.3 67.1 73.6 79.7 80.2 74.8 64.2 52.3 45.5 62.4
Leaburg 4811 2 46.6 51.8 56 60.8 67.2 74.2 81.5 82.1 75.8 65 52.5 46.2 63.3
North Willamette Exp. Stn. 6151 2 46.1 51 55.3 59.7 66.3 73.3 79.8 80.3 74.5 63.9 52.8 46.2 62.4
Portland 6751 2 45.4 51 56 60.6 67.1 74 79.9 80.3 74.6 64 52.6 45.6 62.6
Salem 7500 2 46.4 51.5 55.7 60.4 67 74.5 81.6 82 76 64.2 52.4 46.4 63.2
Silverton 7823 2 45.7 50.7 54.9 59.1 65.4 72.1 78.6 79.2 73.8 63.3 52.3 45.8 61.7
Ashland 304 3 46 52.1 56.1 61.8 69.7 78.5 86.6 85.6 78.5 66.6 52.2 45.3 64.9
Drain 2406 3 47.8 53.2 57.6 62.4 69.1 75.9 82.8 83.1 77.9 66.8 53.9 47.4 64.8
Grants Pass 3445 3 47.6 54.9 60.5 67 74.6 82.9 90.1 89.8 83.1 70.3 53.8 46.2 68.4
Medford Exp. Stn. 5424 3 44.9 52.6 57.7 63.8 71.6 79.9 87.8 87.1 79.9 67.5 51.3 44.1 65.7
Medford 5429 3 45.7 53.3 58.5 64.6 72.9 82.1 90.5 89.9 82.8 69.4 52.6 44.3 67.2
Roseburg KQEN 7331 3 48.5 53.4 57.8 62.9 69.3 76.5 83.6 84.1 78.1 67 54.3 48 65.3
Toketee Falls 8536 3 42 48.1 53.5 60.8 69.5 78 85.7 85.4 77 62.9 47.8 41.3 62.7
Belknap Springs 652 4 39.1 44.4 49.6 56.4 64.5 72.8 80.7 80.7 73.9 62.7 47.1 39 59.2
Government Camp 3402 4 35.4 38.5 40.6 45.1 52.2 60 67.7 68.1 62 53.4 40.7 36.2 50
Crater Lake N.P. 1946 5 34.5 35.2 36.5 41.7 49.2 58.1 68 68.5 61.7 52 38.5 34.4 48.2
Wickiup Dam 9316 5 37.7 42 46 53.2 62.3 71.2 79.9 79.8 71.8 60.9 44.9 37.9 57.3
Antelope 197 6 39.4 45.7 51.6 58.1 66.5 75.8 84.1 83.7 74.8 63.4 48.1 39.9 60.9
Dufur 2440 6 40.6 47.6 55.1 62 70.3 78 85 84.5 76.6 64.4 48.7 40.5 62.8
Heppner 3827 6 41.3 47.6 53.7 60.4 69 77.9 85.7 84.8 75.4 64.3 50.1 42 62.7
Hood River Exp. Stn. 4003 6 40.7 46.8 53.7 60 67.5 74.2 80.1 80.5 74 63.4 49.3 41.2 61
Moro 5734 6 37.6 43.7 50.8 57.1 65.1 73.8 81.6 81.3 72.8 61.6 46.7 38.3 59.2
Pendleton Br. Exp. Stn. 6540 6 40 46.9 54.1 61.3 69.5 78.8 88.3 87.2 77.3 65.2 49.8 41.1 63.3
Pendleton 6546 6 39.7 46.9 54.2 61.3 70 79.5 87.8 86.2 76.3 63.7 48.9 40.5 62.9
Bend 694 7 41.6 46.3 51.2 57.5 65.1 73.6 81.4 80.8 73.1 63.1 48.4 41.6 60.3
Burns 1175 7 33.6 39.5 47.7 56.5 65.6 74.4 85.1 83.3 73.6 61.8 45.2 35.2 58.5
Hart Mountain Refuge 3692 7 39.2 41.9 45 52.9 62.4 71.5 81.2 80 71.4 60.9 46.1 39.6 57.7
Klamath Falls 4506 7 38.8 45 50.3 58 67.1 76.2 84.6 83.4 75.5 63.8 47.1 39 60.7
Klamath Exp. Stn. 4510 7 39.2 45.2 50.6 58.3 67.6 76.5 84.9 83.8 76.2 64.1 47.4 39.2 61.2
Lakeview 4670 7 37.9 42.3 47.4 55.7 65 74.1 83.5 82 73.8 62.6 46.6 39 59.2
Madras 5142 7 43.2 49.8 56.2 62.8 71.2 79.8 87.4 87 78.4 66.5 51.4 43.2 64.7
Malheur NWR HQ 5162 7 37.3 43.9 50.1 58.5 67.6 76.1 84.9 83.3 75 63.4 47.6 38 60.5
Ochoco Ranger Station 6243 7 35.7 41.6 47.8 55.3 63.6 72.8 81.4 81.9 73.1 61.3 43.7 35.6 57.8
Paisley 6426 7 41.6 47 51.3 58.9 67.5 76 84.6 83.7 76.1 65.4 49.4 41.7 61.9
Squaw Butte Exp. Stn. 8029 7 35.2 40.8 46.4 54.7 63.6 73.7 82.8 82.3 72.6 61.2 45 36.4 57.9
Whitehorse Ranch 9290 7 40.2 46.3 51.5 59.1 67.8 76.7 86.2 84.1 74.9 65.2 49.7 41.1 61.9
Elgin 2597 8 36.7 43.7 51.3 59.9 68.9 77.6 87.4 86.9 77.6 64.6 47.4 38.2 61.7
Enterprise 2675 8 32.8 37.2 47.1 56.1 61.6 70.6 77.6 78.2 68.6 59 42.6 31.3 55.4
John Day 4291 8 40.4 47 52.7 60 68.7 78.3 88.1 87.2 77.6 66 50.1 41.8 63.2
Union Exp St 8746 8 36.4 43 50.4 57.9 65.6 74.1 83.4 83.7 73.8 62.4 47.3 38.1 59.7
Malheur Br. Exp. Stn. 5160 9 33.4 42.4 54 63.4 73 82.3 91.1 89.2 78.3 64.7 47.5 35.5 62.9
Ontario 6294 9 35.2 44.5 56.2 66 76.2 85.9 95.4 93 81.5 67.2 49.1 36.8 65.6
Rome 2 NW 7310 9 39.8 47.9 54.9 63.4 72.9 82.5 92.6 90.7 80.4 68.2 51.5 41 65.5

Table 3. Mean monthly maximum temperatures, Oregon locations

Station Number Zone Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year
Astoria 328 1 35.9 37.3 38.1 40.3 44.8 49.5 52.4 52.6 49 44 40.2 36.6 43.4
Bandon 471 1 38.2 39.4 39.7 40.7 43.9 48.1 50.4 50.4 47.7 44.3 41.8 38.7 43.6
Brookings 1055 1 40.9 41.9 41.9 42.8 46.1 49.5 51.2 52.1 51.1 48.2 44.7 41.2 46
Honeyman State Park 3995 1 37 38.4 38.6 39.8 43.2 47.9 49.7 50.8 48.8 45.2 41.3 37.6 43.2
North Bend 6073 1 38.9 40.5 41.1 42.4 46.6 50.7 52.5 53 50.5 46.7 43.2 39.7 45.5
Seaside 7641 1 36.7 38.2 38.4 40.6 44.7 48.9 51.6 52.1 49.1 44.7 40.7 37.2 43.6
Tillamook 8494 1 35.7 36.9 36.9 38.5 42.4 46.7 49.2 49.6 46.4 42.2 39.1 36.1 41.6
Corvallis OSU 1862 2 33 35.1 37 39.2 43.1 48.6 51 51.2 47.8 41.7 38 33.9 41.6
Eugene 2709 2 35.2 37 38.9 40.6 44.5 49.7 52.8 53.2 49.3 43.5 39.7 35.9 43.4
Forest Grove 2997 2 31.9 34.3 36.6 39.1 43.8 49.4 52.7 52.4 47.9 41.1 37.1 32.9 41.6
Hillsboro 3908 2 32.5 34.5 36.4 38.8 43.1 49.2 51.8 51.4 46.6 40.6 36.9 33.2 41.3
Leaburg 4811 2 32.9 34.8 36.4 38.9 43.1 48.2 50.5 50.5 47.1 42.4 38.1 33.8 41.4
North Willamette Exp. Stn. 6151 2 32.4 34.4 36.5 39.5 44.1 49.6 52.4 52.3 48.4 41.4 37.4 33.1 41.8
Portland 6751 2 33.7 36.1 38.6 41.3 47 52.9 56.5 56.9 52 44.9 39.5 34.8 44.5
Salem 7500 2 32.7 34.1 35.6 37.7 42.2 48.3 50.9 51.4 47.1 41.1 37.5 33.6 41
Silverton 7823 2 32.8 35.2 37.4 40.1 44.9 50.3 53.4 53.9 49.9 43.4 38.2 33.4 42.7
Ashland 304 3 28.7 31.1 33.1 36 41.4 47.8 51.6 51.5 45.3 38.7 33.3 29.5 39
Drain 2406 3 33.5 35.4 36.8 38.7 42.8 48 49.9 50.2 46.1 42.4 39 34.7 41.5
Grants Pass 3445 3 32.7 34.4 36 38.4 43.6 49.7 53.1 52.7 46.7 41.2 37.9 33.7 41.7
Medford Exp. Stn. 5424 3 28.5 30.9 33.8 36.4 41.2 47.6 50.7 50.7 43.9 37.2 33.5 29.7 38.7
Medford 5429 3 30.4 32.2 35.4 38 43.4 50.7 55.2 55.1 48.2 40.4 35.5 31.2 41.3
Roseburg KQEN 7331 3 33.9 35.8 37.7 39.7 44.5 50.3 53.5 54.3 49.3 43.8 39.3 34.8 43.1
Toketee Falls 8536 3 28.8 30.6 32.5 35.4 40.5 46.9 49.9 49.3 43.9 37.9 33.5 29.6 38.2
Belknap Springs 652 4 27.2 29 30.6 33.4 38.4 45 48.3 48.3 43.5 37.6 32.8 28.4 36.9
Government Camp 3402 4 23.6 25.4 26.6 29.6 34.2 40.8 45.5 46.1 41.4 35.7 29.1 24.7 33.6
Crater Lake N.P. 1946 5 17.5 17.9 18.1 21.1 27 33.8 39.8 40.2 35.5 30.1 22.5 18.3 26.8
Wickiup Dam 9316 5 16.9 19.9 23.5 27.8 33.6 40.3 43.6 42.1 35 29 25.3 18.9 29.7
Antelope 197 6 23.3 27.1 29.8 32.8 38.5 45.8 50.5 50.6 43.8 36.5 29.9 24.5 36.1
Dufur 2440 6 24.4 27.8 30.4 33.4 38.4 44.6 48.2 48.3 42.6 35.3 30.3 24.8 35.7
Heppner 3827 6 25.8 29.8 33.2 36.1 42.1 48.6 52.5 52.9 46.1 38.7 32.5 26.4 38.7
Hood River Exp. Stn. 4003 6 28.3 31.2 34.4 38.4 43.8 50 53.4 52.8 45.8 38.1 34.4 29.4 40
Moro 5734 6 24 28.3 31.8 35.7 41.5 48.5 53.7 53.2 45.4 36.7 31 25 37.9
Pendleton Br. Exp. Stn. 6540 6 24.8 29.1 32.4 35.7 41.4 47.5 51.4 51 42.9 34.4 31.4 25.6 37.3
Pendleton 6546 6 27.2 31.6 35.4 39.4 45.8 52.9 58 57.7 49.9 41 34.1 27.9 41.7
Bend 694 7 21.8 24.8 26.3 29.3 34.8 41.2 45.2 44.8 37.8 31.6 27.4 22.4 32.3
Burns 1175 7 13 19.3 24.9 29 35.9 41.6 47.2 45 36.3 28.1 22 15.1 29.8
Hart Mountain Refuge 3692 7 18 21 22.2 26 32.2 39.1 43.9 43.6 37 30.6 24.2 18.4 29.7
Klamath Falls 4506 7 20.3 24.8 27.7 31 38.2 45.6 50.8 49.7 42.7 34.4 27.4 21.6 34.5
Klamath Exp. Stn. 4510 7 20.4 24.9 27.8 31.1 38.2 45.6 50.9 49.7 42.8 34.6 27.6 21.8 34.7
Lakeview 4670 7 19 23.1 26.3 30.6 37.3 44.6 50.2 48.2 41.1 33.2 26.4 20.6 33.4
Madras 5142 7 23.6 26.3 28.2 31 36.9 43.2 46.4 45.7 39.2 32.3 28.8 23.7 33.8
Malheur NWR HQ 5162 7 17.3 22.4 25.8 30 37.4 44.4 49.3 47.6 38.5 29.9 24.5 17.8 32.1
Ochoco Ranger Station 6243 7 15.9 19.5 23 26.2 31.6 37.7 40.2 40 33.7 28.2 23.8 17.5 28.1
Paisley 6426 7 21.6 25.2 27.3 31.4 38.2 45.3 49.1 48.3 40.6 33.4 26.4 21.8 34.1
Squaw Butte Exp. Stn. 8029 7 18 22.5 25.5 29.4 35.8 43.9 50.2 49.7 42.1 34.3 26 19.6 33.1
Whitehorse Ranch 9290 7 17.8 22 25.4 30.1 37.4 44.9 50.8 49.9 41.2 33.4 25 18.4 33
Elgin 2597 8 20.5 24.2 27.9 31.7 37.4 43.8 45.9 44.9 37.8 30.9 27.8 22.2 32.9
Enterprise 2675 8 12.2 14.3 23.3 27.9 33.4 39.6 41.5 41.1 33 26.5 20.8 10.1 27.1
John Day 4291 8 21 25 28.4 32.3 38.6 45.2 48.6 47.8 40.4 33.2 28.2 22.1 34.2
Union Exp St 8746 8 23.5 27.4 29.9 33.8 39.4 45.9 49.5 48.7 41 34 30.6 24.8 35.7
Malheur Br. Exp. Stn. 5160 9 17.9 24.1 30.8 37 45 52.9 58.1 55.8 46 35.6 28.1 20.3 37.6
Ontario 6294 9 19.1 24.8 30.6 36.2 44 51.6 57.4 54.4 44.2 34.3 28.3 20.5 37.1
Rome 2 NW 7310 9 17.3 23.3 25.8 30.1 38.4 46.3 51.3 48.6 39 30.2 24.2 18.5 32.8

Table 4. Mean monthly minimum temperatures, Oregon locations

Parameter Newport Corvallis Madras
Mean maximum in warmest month 65.7 82.4 87.5
Mean minimum in coldest month 38.6 33.6 23.3
Days with maximum 90°F or more 0.6 14.2 38.3
Days with maximum 32°F or less 0.8 2.7 12.1
Record high temperature 100 108 112
Record low temperature 1 -14 -40
Annual heating degree days @ 65°F 5075 4832 5998
Annual cooling degree days @ 65°F 0 203 277

Table 5. Temperature and degree day characteristics for Newport, Corvallis, and Madras

 

1.2.2 Long-term characteristics

As in the case of precipitation, long-term temperature trends are characterized by significant year-to-year variations as well as noticeable longer-term trends. Figure 4 shows annual average temperatures at Corvallis, Oregon since 1889, chosen because of its long record and minimal urban development near the sensor (urban effects can significantly alter temperatures). Corvallis saw its warmest period in the late 1930's and early 1940's; since then, there was a decrease for several decades and an increase in the last 20 years.

1.3 Winds

Local winds in Oregon are dominated by large-scale pressure patterns over the North Pacific and onshore. During winter (and, to a lesser extent, autumn and spring), frequent cyclonic storms reach the area from the west, greatly influencing winds and other weather elements. Summer months see fewer strong storms, and are more typically characterized by sea-land breeze regimes.

During summer, the North Pacific High, a quasi-stationary area of high pressure off the coast, exerts a significant influence on western U.S. weather. The High moves northward in summer as the jet stream weakens and moves poleward. As a result, Pacific storms, which are already less vigorous than their winter counterparts, tend to be diverted to the north.

Several times each year winds exceeding hurricane strength (74 mph) strike Oregon, especially along the coast. Damage is confined primarily to vegetation and structures; loss of life is rare. But occasional high intensity wind storms affect virtually all of Oregon. The Columbus Day storm of 1962 is the most destructive wind storm in Oregon's history. Gusts exceeded 140 mph in some parts of the state, and topped 110 mph in downtown Portland.

Tornadoes are actually rather common in Oregon, with several reported each year. In nearly all cases, however, they are quite weak and cause little damage. Several people have died during some of the more powerful tornadoes, but in terms of tornado danger Oregon is a very safe state.

1.4 Humidity

The presence of the Pacific, combined with generally mild temperatures, causes average relative humidity along the coast to be quite high, especially during the cool season. As one moves inland, humidities decrease. In Table 6 are 4 p.m. average relative humidities (in general, the minimum values for the day) at several locations within the study area. Astoria and North Bend are coastal stations; Eugene, Portland, and Salem are in the inland Willamette Valley; and the other sites are in southwestern Oregon (Medford and Sexton Summit) or east of the Cascades (the remainder).

Site Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Ann
Astoria 78 76 69 70 69 71 69 71 70 74 78 81 73
Baker 78 74 58 46 47 49 36 36 40 51 67 78 55
Bend 64 60 45 39 36 38 32 30 32 47 64 69 46
Burns 68 61 47 35 35 30 21 22 24 36 54 68 42
Eugene 81 73 66 55 53 49 37 39 43 65 78 83 60
Klamath Falls 70 62 51 37 39 37 26 31 33 43 59 74 47
LaGrande 66 64 55 39 42 43 28 31 32 45 59 71 48
Meacham 80 76 71 60 57 54 35 36 45 65 79 84 62
Medford 74 59 49 42 39 35 27 26 31 46 65 79 48
North Bend 80 79 74 73 73 74 73 74 75 79 80 82 76
Ontario 78 61 42 30 30 29 22 23 27 37 58 76 43
Pendleton 77 66 49 41 38 33 23 26 32 50 71 80 49
Portland 77 69 61 55 54 49 46 48 50 66 74 79 61
Redmond 65 56 49 37 38 36 25 28 31 45 57 66 44
Roseburg 78 68 59 52 49 46 38 39 45 61 76 82 58
Salem 79 71 63 54 52 50 39 40 47 64 76 81 60
Sexton Summit 75 65 67 64 52 52 42 40 42 57 77 76 59

Table 6. Average 4 p.m. relative humidity (percent)

 

1.5 Cloud cover and solar radiation

In general, Oregon is quite cloudy near the coast throughout the year, and cloudy statewide during winter. Table 7 shows the average cloud cover by month for locations throughout Oregon.

Site Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Ann
Astoria 85 84 81 81 77 78 67 66 63 74 80 86 77
Baker 69 67 63 59 57 50 28 28 38 45 60 67 53
Burns 76 72 67 63 61 50 25 34 33 49 65 73 56
Eugene 85 81 79 72 67 62 36 44 49 71 83 89 68
Klamath Falls 84 66 64 59 67 43 13 29 25 43 65 75 51
La Grande 81 79 80 64 47 58 20 36 40 58 71 85 61
Meacham 85 84 80 76 62 61 31 39 47 65 80 84 67
Medford 82 76 72 66 69 48 21 23 33 56 75 86 58
North Bend 86 76 75 66 59 61 46 56 55 63 73 82 67
Pendleton 82 80 72 66 61 52 26 33 40 57 77 84 61
Portland 85 84 82 77 60 68 46 53 56 72 81 89 72
Redmond 80 70 70 56 73 53 15 34 30 45 68 72 51
Roseburg 87 84 80 72 50 57 30 38 48 71 85 89 67
Salem 83 82 79 73 68 65 40 47 51 69 80 88 69
Sexton Summit 77 77 77 68 70 48 23 28 35 56 73 77 58
Troutdale 81 84 86 72 62 68 49 52 52 75 81 87 71

Table 7. Average cloud cover (percent) for Oregon stations

 

The percent of possible sunshine is listed in Table 8 for three sites: Portland and Roseburg in western Oregon and Baker in extreme eastern Oregon.

Site Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Ann
Baker 41 49 56 61 63 67 83 81 74 62 46 37 60
Portland 24 32 37 47 51 47 67 61 58 38 29 21 45
Roseburg 26 30 39 49 52 61 79 74 68 42 25 20 50
Table 8 - Monthly percent of possible sunshine

 

Cloudiness often drops significantly within a few miles of the coast, especially during the warm season. The best indicator of changes in cloud cover is probably air temperature: air temperature increases as one moves away from the coastal strip tend to be good indicators for decreases in average summertime cloud cover. During winter, when the area is dominated by large-scale storm systems, cloud cover tends to be much more uniform throughout the area.

1.6 Snow

Snow is relatively rare along the immediate coastline in Oregon. There is, however, a noticeable (and not unexpected) relationship between latitude and snowfall. Table 9 lists monthly average snowfall at various Oregon stations. Notice, in particular, Crater Lake, one of the snowiest measurement stations in the U.S., which once reported nearly 900 inches of snow in one season.

As one moves inland or upward, the amount of snowfall reported per year increases steadily. For example, Laurel Mountain, Oregon, in the Coast Range at 3590 feet above sea level, averages 110 inches of snow per year. Assuming a ratio of snow to water of 10:1, this represents about 10% of Laurel Mountain's average annual precipitation of 116 inches.

Station Zone Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
Astoria WSO AP 1 2.2 0.4 0.4 0.1 0.2 1.4 4.7
Brookings 1 0.2 0.3 0.1 0.1 0.4
Corvallis 2 2.2 1.3 0.2 0.2 1.6 5.7
Eugene WSO AP 2 3.1 0.9 0.2 0.2 1.7 6.1
Portland WSO AP 2 1.8 0.9 0.1 0.5 2 5.4
Ashland 3 2.7 1.2 1 0.5 0.3 2.4 8.4
Medford WSO AP 3 3 0.8 0.7 0.2 0.4 2.3 7.5
Roseburg KQEN 3 2.3 0.6 0.1 0 0.7 4
Belknap Springs 4 24.4 15.9 13.3 2.7 0.2 0.3 6.8 22.3 88
Detroit Dam 4 7.9 3.7 2.2 0.4 1 3.5 18.2
Government Camp 4 61.6 44.6 49.2 29.8 7.5 0.2 0.3 6.4 34.1 52.7 278.2
McKenzie Bridge 4 13.2 6.9 4.3 0.4 0.3 2.6 8.8 40.1
Crater Lake 5 85.3 73.2 87.5 43 19.3 4 0.5 0.2 4.1 21.3 69.1 84 495
Silver Lake 5 5 2.7 3.1 1.4 0.7 0.3 2.2 6.4 18.6
Hood River Exp Stn 6 14 6.4 1.5 3.4 9.9 36.5
Moro 6 6 2.7 1.2 0.2 0.2 2.5 6.2 19.3
Pendleton WSO AP 6 6.1 2.1 1 0.2 0.2 2.2 5.2 17
Bend 7 10 3.9 4.1 2 0.3 0.2 5.6 9.5 34.8
Burns WSO 7 6.6 6.9 4.6 1 0.3 0.1 0.6 6.4 10.4 42.3
Klamath Falls 7 9.3 4.7 3.7 0.9 0.1 0.5 4.5 10 34.9
Enterprise 2 S 8 12 8.2 8.1 5 1.4 0.2 1.3 7.6 9.4 52.6
John Day 8 6.2 3.7 3.2 1.2 0.1 0.3 2.5 7.1 24
LaGrande 8 9 4.7 1.7 0.7 0.2 3 7.4 28.7
Ontario 9 7.2 2.8 0.7 0.1 0.1 2.2 6.7 20.3
Rome 2 NW 9 4.4 0.8 1.4 0.5 0.2 0.2 1.7 3.7 13.5

Table 9. Average Monthly and Annual Snowfall (inches), Oregon locations

1.7 Thunderstorms

In western Oregon thunderstorms occur only 4-5 times per year, mostly in the inland valleys and near the Cascades. Generally they are not severe and produce little damage. Eastern Oregon receives many more thunderstorms (typically 12-18 per year), and they tend to be much more severe, resulting in greater damage to crops and buildings. Mountains are especially susceptible to thunderstorm damage; each year a considerable number of forest fires are started by lightning.

Hail occurs numerous times in Oregon every year, but in general hail is small and damages are low (typically a few thousand dollars each year, mostly to crops). Most of the damage occurs in eastern Oregon.