About Pacific Northwest Climate

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Introduction

Pacific Northwest (PNW) climate and ecology are largely shaped by the interactions that occur between seasonally varying atmospheric circulation (i.e., weather) patterns and the region’s mountain ranges.

Approximately two-thirds of the region’s precipitation occurs in just half the year (October-March) when the PNW is on the receiving end of the Pacific storm track. Much of this precipitation is captured in the region’s mountains, influencing both natural and human systems throughout the PNW. From late spring to early fall, high pressure to the west generally keeps the Northwest fairly dry. These seasonal variations are related to changes in large-scale atmospheric circulation occurring over the Pacific Ocean, including the Gulf of Alaska.

Fast Fact

Major PNW mountain features include: Washington's Olympic Mountains; Oregon's Coast Range; the Rocky Mountains in Idaho; and, most significantly, the Cascade mountain range, which extends from British Columbia into southern Oregon.

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Topo map, major PNW mountian features.

Figure 1 Major PNW mountain features.

Spatial contrasts in PNW climate can be stark owing to the region’s mountains (Figure 1), especially the Cascades, which create a barrier between the maritime climate influences to the west - where temperatures are generally mild year-round - and the continental climate influences to the east, with more sunshine and larger daily and annual ranges in temperature. Figure 2 shows the resulting extreme contrasts in precipitation around the region.

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Map, Annual mean precipitation for the Pacific Northwest

Figure 2 Annual mean precipitation for the Pacific Northwest. Figure courtesy of Oregon Climate Service (Oregon State University).

World record snowfalls

Washington State’s Mount Baker Ski Area, located in the north Cascades near the U.S./Canada border, set a new world record in 1998-99 for the highest ever recorded annual snowfall (October-September) with a total of 1,140 inches or 95 feet (29 meters). The previous record, 1,122.5 inches (93.4 feet or 28.5 meters), was set 150 miles south of Mt. Baker at Mt. Rainier National Park between July 1971 and June 1972.

Typical Climate West of the Cascades

Climate in the low-lying valleys west of the Cascades is characterized by mild year-round temperatures, abundant winter rains, and dry summers. Average annual precipitation in most places west of the Cascades is more than 30 inches (75 cm) (Figure 2). Precipitation in the mountains is much higher, however. The western slopes of the Olympic and Coast mountain ranges - the first recipients of winter storms - typically receive about 118 inches (300 cm) per year, with some locations on the Olympic Peninsula exceeding 200 inches (500 cm) per year. Average annual precipitation in the Cascades typically exceeds 100 inches (250 cm) or more. The Cascades are often among the snowiest places on Earth (see sidebar).

Typical Climate East of the Cascades

Climate east of the Cascade crest is more continental, creating a sharp contrast to the maritime climate of the western PNW. Sunshine and dry conditions become more common as the recurring winter storms brought to the PNW by the Pacific storm track drop their rain over the mountains. Average annual precipitation is generally less than 20 inches (50 cm) in the interior PNW, with some places receiving as little as 7 inches (18 cm). A greater fraction of precipitation also falls in the warm half of the year, particularly in May and June (Figure 3).

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Graph, Monthly mean precipitation for the PNW for 1900-1998

Figure 3 Monthly mean precipitation for the PNW for 1900-1998: east and west of the Cascade crest, and the average monthly precipitation for the region as a whole. (Data source: NOAA Climate Services Division. Figure source: Climate Impacts Group, University of Washington)

Annual and daily temperature ranges east of the Cascades are considerably greater than those in the west. Winters are colder, with snow more common at lower elevations, and summer days are hotter (though nights are cooler) than areas west of the Cascades. Typical summertime high temperatures east of the Cascades are 15-25° F (8-14°C) warmer than those of coastal stations (Table 1).

Coastal Sites Summer High Temp Interior PNW Summer High Temp
North Bend, OR 66°F (19°C) Bend, OR 82°F (28°C)
Long Beach, WA 66°F (19°C) Richland, WA 90°F (32°C)

The mountains east of the Cascade crest – portions of the Rockies in Idaho, the Okanogan Highlands in Washington, and the Blue Mountains of northeastern Oregon and southwestern Washington – receive much less precipitation than the western Cascade and Olympic Mountains. Higher elevations in the Rocky Mountains, for example, typically receive 60-80 inches (150-200 cm) of precipitation per year.

Variations in PNW Climate

Despite these spatial contrasts, when PNW climate departs from normal, it tends to do so consistently across the region. Correlations tend to be quite high (>0.8) between anomalies in any part of the region (say, southern Idaho) and those in any other part of the region.

Some fluctuations in regional climate are related to the El Niño/Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) phenomena. In their warm phases (i.e., El Niño conditions for ENSO), both ENSO and PDO increase the odds for a warmer-than-average PNW winter and spring and decrease the odds for a wetter-than-average winter. The opposite tendencies are true for cool phase ENSO (La Niña) and PDO: they increase the odds that PNW winters will be cooler and wetter than average. While tending to be warmer than average, very strong El Niño winters often have near-normal precipitation.

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The relationships between ENSO and PDO and PNW winter/spring climate allow us to monitor for and, in some cases, predict variations in these patterns to provide information about future PNW climate a few seasons to years in advance. For example, because temperature and precipitation often covary in predictable ways (PNW winters are often either warm and dry or cool and wet), these climate forecasts can be used to make skillful predictions about PNW snowpack, streamflow, and other resources sensitive to the water cycle. The CIG annually translates the ENSO forecast and PDO state into climate and resource forecasts for PNW resource managers.

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Past and Future Trends in PNW Climate

Both PNW temperature and precipitation have increased over the 20th century (Figure 4 and Figure 5). On average, the region warmed about 1.5°F (0.83°C); warming was largest west of the Cascades during winter and spring. The largest relative increases in precipitation occurred in eastern Washington and southern British Columbia, mainly in spring. There is good reason to expect warming to continue as a result of climate change, with a likely warming rate of about 0.5°F (0.27°C)/decade. While future changes in precipitation are less certain, overall, precipitation is projected to increase in the PNW. These changes have significant implications for the natural resources of the PNW, as well as the human systems that depend on them. Much of the CIG’s efforts focus on identifying both the likely nature of these impacts and strategies for decreasing the region’s vulnerability to these likely changes.

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Graph, 20th century trends in average annual temperature (1920-2000)

Figure 4 20th century trends in average annual temperature (1920-2000). Increases (decreases) are indicated with red (blue) dots. The size of the dot corresponds to the magnitude of change.

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Graph, 20th century trends in average annual precipitation (1920-2000)

Figure 5 20th century trends in average annual precipitation (1920-2000). Increases (decreases) are indicated with blue (red) dots. The size of the dot corresponds to the magnitude of change.