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
46.1
51.0
55.3
59.7
66.3
73.3
79.8
80.3
74.5
63.9
52.8
46.2
62.4
6751
45.4
51.0
56.0
60.6
67.1
74.0
79.9
80.3
74.6
64.0
52.6
45.6
62.6
7500
46.4
51.5
55.7
60.4
67.0
74.5
81.6
82.0
76.0
64.2
52.4
46.4
63.2
Silverton
7823
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
46.0
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
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
47.6
54.9
60.5
67.0
74.6
82.9
90.1
89.8
83.1
70.3
53.8
46.2
68.4
5424
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
5429
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
48.5
53.4
57.8
62.9
69.3
76.5
83.6
84.1
78.1
67.0
54.3
48.0
65.3
Toketee Falls
8536
42.0
48.1
53.5
60.8
69.5
78.0
85.7
85.4
77.0
62.9
47.8
41.3
62.7
Belknap Springs
652
39.1
44.4
49.6
56.4
64.5
72.8
80.7
80.7
73.9
62.7
47.1
39.0
59.2
Government Camp
3402
35.4
38.5
40.6
45.1
52.2
60.0
67.7
68.1
62.0
53.4
40.7
36.2
50.0
Crater Lake N.P.
1946
34.5
35.2
36.5
41.7
49.2
58.1
68.0
68.5
61.7
52.0
38.5
34.4
48.2
Wickiup Dam
9316
37.7
42.0
46.0
53.2
62.3
71.2
79.9
79.8
71.8
60.9
44.9
37.9
57.3
Antelope
197
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
40.6
47.6
55.1
62.0
70.3
78.0
85.0
84.5
76.6
64.4
48.7
40.5
62.8
Heppner
3827
41.3
47.6
53.7
60.4
69.0
77.9
85.7
84.8
75.4
64.3
50.1
42.0
62.7
Hood River Exp. Stn.
4003
40.7
46.8
53.7
60.0
67.5
74.2
80.1
80.5
74.0
63.4
49.3
41.2
61.0
Moro
5734
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
40.0
46.9
54.1
61.3
69.5
78.8
88.3
87.2
77.3
65.2
49.8
41.1
63.3
6546
39.7
46.9
54.2
61.3
70.0
79.5
87.8
86.2
76.3
63.7
48.9
40.5
62.9
Bend
694
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
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
39.2
41.9
45.0
52.9
62.4
71.5
81.2
80.0
71.4
60.9
46.1
39.6
57.7
Klamath Falls
4506
38.8
45.0
50.3
58.0
67.1
76.2
84.6
83.4
75.5
63.8
47.1
39.0
60.7
Klamath Exp. Stn.
4510
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
4670
37.9
42.3
47.4
55.7
65.0
74.1
83.5
82.0
73.8
62.6
46.6
39.0
59.2
5142
43.2
49.8
56.2
62.8
71.2
79.8
87.4
87.0
78.4
66.5
51.4
43.2
64.7
Malheur NWR HQ
5162
37.3
43.9
50.1
58.5
67.6
76.1
84.9
83.3
75.0
63.4
47.6
38.0
60.5
Ochoco Ranger Station
6243
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
41.6
47.0
51.3
58.9
67.5
76.0
84.6
83.7
76.1
65.4
49.4
41.7
61.9
Squaw Butte Exp. Stn.
8029
35.2
40.8
46.4
54.7
63.6
73.7
82.8
82.3
72.6
61.2
45.0
36.4
57.9
Whitehorse Ranch
9290
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
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
32.8
37.2
47.1
56.1
61.6
70.6
77.6
78.2
68.6
59.0
42.6
31.3
55.4
John Day
4291
40.4
47.0
52.7
60.0
68.7
78.3
88.1
87.2
77.6
66.0
50.1
41.8
63.2
Union Exp St
8746
36.4
43.0
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
33.4
42.4
54.0
63.4
73.0
82.3
91.1
89.2
78.3
64.7
47.5
35.5
62.9
6294
35.2
44.5
56.2
66.0
76.2
85.9
95.4
93.0
81.5
67.2
49.1
36.8
65.6
Rome 2 NW
7310
39.8
47.9
54.9
63.4
72.9
82.5
92.6
90.7
80.4
68.2
51.5
41.0
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
328
35.9
37.3
38.1
40.3
44.8
49.5
52.4
52.6
49.0
44.0
40.2
36.6
43.4
471
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
1055
40.9
41.9
41.9
42.8
46.1
49.5
51.2
52.1
51.1
48.2
44.7
41.2
46.0
Honeyman State Park
3995
37.0
38.4
38.6
39.8
43.2
47.9
49.7
50.8
48.8
45.2
41.3
37.6
43.2
6073
38.9
40.5
41.1
42.4
46.6
50.7
52.5
53.0
50.5
46.7
43.2
39.7
45.5
Seaside
7641
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
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
33.0
35.1
37.0
39.2
43.1
48.6
51.0
51.2
47.8
41.7
38.0
33.9
41.6
2709
35.2
37.0
38.9
40.6
44.5
49.7
52.8
53.2
49.3
43.5
39.7
35.9
43.4
2997
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
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
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.St.
6151
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
6751
33.7
36.1
38.6
41.3
47.0
52.9
56.5
56.9
52.0
44.9
39.5
34.8
44.5
7500
32.7
34.1
35.6
37.7
42.2
48.3
50.9
51.4
47.1
41.1
37.5
33.6
41.0
Silverton
7823
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
28.7
31.1
33.1
36.0
41.4
47.8
51.6
51.5
45.3
38.7
33.3
29.5
39.0
Drain
2406
33.5
35.4
36.8
38.7
42.8
48.0
49.9
50.2
46.1
42.4
39.0
34.7
41.5
Grants Pass
3445
32.7
34.4
36.0
38.4
43.6
49.7
53.1
52.7
46.7
41.2
37.9
33.7
41.7
5424
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
5429
30.4
32.2
35.4
38.0
43.4
50.7
55.2
55.1
48.2
40.4
35.5
31.2
41.3
Roseburg KQEN
7331
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
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
27.2
29.0
30.6
33.4
38.4
45.0
48.3
48.3
43.5
37.6
32.8
28.4
36.9
Government Camp
3402
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
17.5
17.9
18.1
21.1
27.0
33.8
39.8
40.2
35.5
30.1
22.5
18.3
26.8
Wickiup Dam
9316
16.9
19.9
23.5
27.8
33.6
40.3
43.6
42.1
35.0
29.0
25.3
18.9
29.7
Antelope
197
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
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
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
28.3
31.2
34.4
38.4
43.8
50.0
53.4
52.8
45.8
38.1
34.4
29.4
40.0
Moro
5734
24.0
28.3
31.8
35.7
41.5
48.5
53.7
53.2
45.4
36.7
31.0
25.0
37.9
Pendleton Br. Exp. Stn.
6540
24.8
29.1
32.4
35.7
41.4
47.5
51.4
51.0
42.9
34.4
31.4
25.6
37.3
6546
27.2
31.6
35.4
39.4
45.8
52.9
58.0
57.7
49.9
41.0
34.1
27.9
41.7
Bend
694
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
13.0
19.3
24.9
29.0
35.9
41.6
47.2
45.0
36.3
28.1
22.0
15.1
29.8
Hart Mountain Refuge
3692
18.0
21.0
22.2
26.0
32.2
39.1
43.9
43.6
37.0
30.6
24.2
18.4
29.7
Klamath Falls
4506
20.3
24.8
27.7
31.0
38.2
45.6
50.8
49.7
42.7
34.4
27.4
21.6
34.5
Klamath Exp. Stn.
4510
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
4670
19.0
23.1
26.3
30.6
37.3
44.6
50.2
48.2
41.1
33.2
26.4
20.6
33.4
5142
23.6
26.3
28.2
31.0
36.9
43.2
46.4
45.7
39.2
32.3
28.8
23.7
33.8
Malheur NWR HQ
5162
17.3
22.4
25.8
30.0
37.4
44.4
49.3
47.6
38.5
29.9
24.5
17.8
32.1
Ochoco Ranger Station
6243
15.9
19.5
23.0
26.2
31.6
37.7
40.2
40.0
33.7
28.2
23.8
17.5
28.1
Paisley
6426
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
18
22.5
25.5
29.4
35.8
43.9
50.2
49.7
42.1
34.3
26.0
19.6
33.1
Whitehorse Ranch
9290
17.8
22.0
25.4
30.1
37.4
44.9
50.8
49.9
41.2
33.4
25.0
18.4
33.0
Elgin
2597
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
12.2
14.3
23.3
27.9
33.4
39.6
41.5
41.1
33.0
26.5
20.8
10.1
27.1
John Day
4291
21.0
25.0
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
23.5
27.4
29.9
33.8
39.4
45.9
49.5
48.7
41.0
34.0
30.6
24.8
35.7
Malheur Br. Exp. Stn.
5160
17.9
24.1
30.8
37.0
45.0
52.9
58.1
55.8
46.0
35.6
28.1
20.3
37.6
6294
19.1
24.8
30.6
36.2
44.0
51.6
57.4
54.4
44.2
34.3
28.3
20.5
37.1
Rome 2 NW
7310
17.3
23.3
25.8
30.1
38.4
46.3
51.3
48.6
39.0
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
Year
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
Year
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
Year
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
Year
Astoria WSO AP
2.2
0.4
0.4
0.1
            
0.2
1.4
4.7
Brookings
0.2
0.3
0.1
                
0.1
0.4
Corvallis
2.2
1.3
0.2
              
0.2
1.6
5.7
Eugene WSO AP
3.1
0.9
0.2
              
0.2
1.7
6.1
Portland WSO AP
1.8
0.9
0.1
              
0.5
2.0
5.4
Ashland
2.7
1.2
1.0
0.5
            
0.3
2.4
8.4
Medford WSO AP
3.0
0.8
0.7
0.2
            
0.4
2.3
7.5
Roseburg KQEN
2.3
0.6
0.1
              
0
0.7
4.0
Belknap Springs
24.4
15.9
13.3
2.7
0.2
        
0.3
6.8
22.3
88.0
Detroit Dam
7.9
3.7
2.2
0.4
            
1.0
3.5
18.2
Government Camp
61.6
44.6
49.2
29.8
7.5
0.2
    
0.3
6.4
34.1
52.7
278.2
McKenzie Bridge
13.2
6.9
4.3
0.4
          
0.3
2.6
8.8
40.1
Crater Lake
85.3
73.2
87.5
43.0
19.3
4.0
0.5
0.2
4.1
21.3
69.1
84.0
495
Silver Lake
5.0
2.7
3.1
1.4
0.7
        
0.3
2.2
6.4
18.6
Hood River Exp Stn
14.0
6.4
1.5
              
3.4
9.9
36.5
Moro
6.0
2.7
1.2
0.2
          
0.2
2.5
6.2
19.3
Pendleton WSO AP
6.1
2.1
1.0
0.2
          
0.2
2.2
5.2
17
Bend
10.0
3.9
4.1
2.0
0.3
        
0.2
5.6
9.5
34.8
Burns WSO
6.6
6.9
4.6
1.0
0.3
0.1
      
0.6
6.4
10.4
42.3
Klamath Falls
9.3
4.7
3.7
0.9
0.1
        
0.5
4.5
10.0
34.9
Enterprise 2 S
12.0
8.2
8.1
5.0
1.4
      
0.2
1.3
7.6
9.4
52.6
John Day
6.2
3.7
3.2
1.2
0.1
        
0.3
2.5
7.1
24
LaGrande
9.0
4.7
1.7
0.7
          
0.2
3.0
7.4
28.7
Ontario
7.2
2.8
0.7
0.1
          
0.1
2.2
6.7
20.3
Rome 2 NW
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.