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.

In these and all subsequuent tables, the values shown are the official "normals," representing an average from 1961 through 1990.
Zone
Station
County
Elev (ft)
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Year
10
10.00
7.59
7.07
4.61
3.02
2.40
1.16
1.33
2.91
5.73
10.05
10.55
66.42
70
10.85
9.03
9.49
5.30
3.64
1.55
0.53
1.31
2.15
5.84
11.52
12.23
73.44
Corvallis
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
42.70
364
7.03
5.38
5.17
3.60
2.20
1.22
0.46
0.80
1.32
3.35
7.51
7.86
45.90
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
36.10
Ashland
1750
2.37
1.72
1.95
1.61
1.29
0.91
0.32
0.58
0.95
1.60
2.82
3.06
19.18
Medford
1300
2.69
1.93
1.82
1.16
1.00
0.58
0.26
0.52
0.86
1.49
3.23
3.32
18.86
465
5.13
3.70
3.56
2.24
1.43
0.83
0.43
0.73
1.24
2.23
5.36
5.47
32.35
1220
12.79
10.24
9.42
6.54
4.87
3.27
0.90
1.60
3.56
6.42
13.21
13.98
86.80
Government Camp
3980
13.65
10.01
8.92
7.15
4.75
3.42
1.13
1.83
3.90
6.13
11.92
14.01
86.82
McKenzie Bridge RS
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
66.50
Crater Lake
6475
9.66
7.78
8.09
4.60
3.01
1.98
0.68
1.29
2.38
4.75
10.56
10.84
65.62
Silver Lake RS
4380
0.89
0.68
0.76
0.63
0.86
0.85
0.50
0.61
0.58
0.63
1.20
1.10
9.29
500
4.56
3.59
2.50
2.47
1.10
0.91
0.44
0.27
0.64
2.69
5.95
5.28
30.40
Moro
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
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
Bend
3660
1.83
0.97
0.92
0.60
0.77
0.86
0.49
0.58
0.47
0.65
1.57
1.99
11.70
Burns
4140
0.81
1.17
1.60
0.80
1.07
0.67
0.52
0.43
0.62
0.78
1.41
1.41
11.29
Klamath Falls
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
Enterprise
3880
1.26
1.17
1.65
1.55
2.15
1.72
1.00
1.40
1.19
0.84
1.50
1.00
16.43
John Day
3063
1.15
0.82
1.12
1.21
1.56
1.40
0.53
0.95
0.84
0.92
1.47
1.40
13.37
La Grande
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
2145
1.33
0.87
0.82
0.63
0.73
0.72
0.20
0.44
0.53
0.61
1.30
1.50
9.68
Rome
3410
0.58
0.50
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
328
22
19
21
18
15
12
8
8
10
15
21
22
190
471
20
18
20
16
12
9
4
6
8
12
20
21
167
1055
17
16
18
13
9
6
4
5
6
11
18
18
141
Newport
6032
20
18
21
17
14
11
7
8
10
15
21
22
187
6073
19
17
20
16
12
9
4
6
7
12
20
20
160
Tillamook 1 W
8494
21
19
21
18
15
10
7
7
11
15
21
23
187
Bonneville Dam
897
20
18
20
17
14
10
5
6
9
13
20
21
172
2709
17
15
16
12
10
6
3
4
6
10
17
18
134
2997
19
17
17
14
12
8
4
4
7
12
19
20
154
6751
18
15
17
14
12
9
4
5
8
12
19
19
151
7500
17
15
17
13
11
7
3
4
7
11
18
19
143
Silver Creek Falls
7809
19
16
19
16
13
9
4
5
8
13
19
19
163
Grants Pass
3445
15
14
16
11
7
4
2
3
5
9
16
17
118
5429
13
11
12
10
7
5
2
3
4
7
14
14
101
Riddle
7169
16
15
16
13
8
6
2
3
5
9
17
17
126
7331
18
15
17
14
9
6
2
3
6
10
18
18
137
Toketee Falls
8536
17
16
19
16
11
8
3
4
7
11
18
18
149
Belknap Springs
652
18
16
19
17
13
9
4
5
8
13
19
19
160
2292
20
18
20
18
15
10
5
5
9
13
21
21
176
Government Camp
3402
20
18
20
18
15
10
5
6
9
13
20
20
172
Santiam Pass
7559
20
18
18
16
12
10
5
6
9
12
18
21
164
Crater Lake
1946
17
16
19
14
10
8
3
5
6
10
17
18
142
Fremont 5 NW
3095
8
7
9
6
5
6
2
3
3
5
9
9
74
Wickiup Dam
9316
13
11
12
8
7
6
4
4
5
7
13
13
103
Antelope 1 NW
197
10
9
11
8
7
6
3
4
4
6
12
11
91
Hermiston 2 S
3847
11
9
9
7
6
5
3
4
4
6
12
12
87
4003
16
14
14
12
8
6
3
4
6
10
18
17
128
Moro
5734
11
9
11
8
7
5
3
4
4
7
13
11
91
6546
12
11
11
9
7
6
3
4
5
6
12
12
96
The Dalles
8407
12
12
11
7
5
4
2
4
4
7
14
13
94
Bend
694
10
7
8
6
5
5
3
4
4
5
9
10
75
Burns WSO
1175
9
9
13
8
9
6
3
4
5
6
12
10
99
Hart Mountain Refuge
3692
7
7
9
8
8
7
3
4
5
6
9
8
80
Klamath Falls
4506
11
9
11
7
6
5
2
3
3
6
11
12
86
4670
11
10
11
9
8
6
3
3
4
6
11
11
93
5139
8
8
7
6
5
5
3
3
4
5
9
9
73
Ochoco Ranger Station
6243
9
8
6
6
6
6
3
5
4
6
9
9
75
Sisters Ranger District
7857
10
8
9
7
5
4
3
3
4
6
11
10
82
Squaw Butte Exp. Stn.
8029
8
6
8
5
7
6
3
4
4
5
8
7
69
Austin
356
14
12
13
9
10
8
4
6
6
8
14
15
115
Elgin
2597
14
13
14
11
11
9
5
6
6
9
14
15
130
Enterprise 2 S
2675
11
11
14
12
13
11
7
7
7
7
14
10
120
John Day
4291
11
9
10
10
10
9
3
5
5
7
11
12
101
LaGrande
4622
12
9
12
11
10
9
4
5
6
8
11
11
108
Seneca
7675
10
9
10
8
8
7
3
4
4
6
10
12
88
Ukiah
8726
13
10
11
10
9
8
3
5
5
7
12
12
104
Burns Junction
1174
7
8
7
7
6
5
3
4
4
4
8
7
68
Malheur Branch Exp Stn
5160
9
8
8
6
6
6
2
3
4
5
10
10
76
Rome 2 NW
7310
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
328
47.8
51.0
53.2
56.0
60.1
64.1
67.5
68.8
67.8
61.1
53.5
48.2
58.3
471
53.4
55.1
55.6
57.1
60.4
63.7
66.1
67.0
67.1
63.1
57.4
53.5
60.0
1055
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
50.8
53.9
55.8
58.7
62.9
67.0
70.3
70.9
70.8
64.2
55.4
50.5
60.9
6073
51.8
54.0
54.8
56.5
60.2
63.8
66.3
67.1
66.9
63.0
56.9
52.4
59.5
Seaside
7641
51.1
54.1
55.6
58.0
61.9
65.3
68.2
69.2
69.9
64.4
56.3
51.4
60.5
Tillamook
8494
50.0
53.0
54.6
57.0
60.9
64.8
67.3
68.6
68.8
62.9
54.7
49.8
59.4
Corvallis OSU
1862
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
2709
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
2997
45.2
50.7
55.6
60.8
67.8
74.8
80.9
81.9
75.9
64.9
52.2
45.2
63.0
Hillsboro
3908
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
46.6
51.8
56.0
60.8
67.2
74.2
81.5
82.1
75.8
65.0
52.5
46.2
63.3
North Willamette Exp. St.
6151