Northwest Washington Climate

Explorations in Environmental StudiesSpring 2004

Western Washington University©Bach

NORTHWEST WASHINGTON CLIMATE

The northwest section of Washington state extends in a north-south direction from the Canadian border to near Seattle, and lies between the Cascade and Olympic Mountains. Most of the air masses reaching Puget Sound have their origins over the Pacific Ocean. The maritime air has a moderating influence on the climate in both summer and winter. In late spring and summer, the prevailing westerly and northwesterly flow of air into the state results in a dry season beginning in May and reaching peak in midsummer. In the late fall and winter, the prevailing southwesterly and westerly flow of air produces a wet season beginning in October, reaching a peak in November, then decreasing in spring. Annual precipitation increases from less than 20 inches in the "rain shadow" of the Olympic Mountains to 200 inches on the wettest slopes of the Cascade Mountains. Winter season snowfall ranges from less than 20 inches along the shore of Puget Sound to 500 inches in the mountains. One-half of the annual precipitation falls in the four-month period, October through January, and three-fourths in the six months, October through March. The diurnal range in temperature is approximately 10°F in winter and 20°F in summer.

Factors Influencing the Regional Climate

Terrain, position and intensity of the semi-permanent high and low pressure centers over the north Pacific Ocean, westerly winds, distance and direction from the ocean (or Sound) have an influence on our climate. Sharp contrasts characterize the topography of this area once covered by the continental ice sheet. Elevation varies from sea level at Puget Sound to over 7000' at the crest of the Cascade and Olympic Mtns. To the east and north, the Cascade and Rocky Mountains shield western Washington from winter season cold air masses traveling southward across Canada. To the west, the Olympic Mountains and the Coastal Range on Vancouver Island are effective in protecting this area from the more intense winter storms reaching the coast. The Strait of Juan de Fuca, Strait of Georgia and the Chehalis River valley provide low level passages for maritime air moving inland (Figure 3). Infrequently, dry continental air from the north or east reaches Puget Sound.

Moisture laden Pacific air masses generally follow a northeasterly storm track over the Puget Lowlands and into the Cascades. The jet stream directs cold front after cold front through the area during the winter season. These storms are slowed by the Cascade ranges, producing low intensity and long duration precipitation by orographic uplift (Schermerhorn 1967). Snow level varies throughout the winter season and from year to year, but elevations above 2000 m commonly accumulate 8-10 m of snow. Occasional warm southwestern winds elevate ambient temperatures during winter storms, leading to rapid melting of snow that sharply increases runoff (rain-on-snow events). During the summer months the NW is influenced by the Hawai’ian (North Pacific) High. Dry winds blowing from this anti-cyclone produces clear skies and mild temperatures, moderated by the cool Pacific Ocean surface.

Due to the similarity characteristics of the polar and tropical air masses that meet along the Polar Front over the Pacific Northwest, intense frontal lifting generally does not occur and the classic cold front thunderstorm activity is absent. The air masses mix well along the frontal boundary, due to the similar temperatures and densities, as well as the turbulence introduced by the topographic influences of the mountains, gaps and islands. While the general weather conditions (i.e. wind directions, pressure tendency, and temperature trends) associated with fronts occur, the precipitation is generally different. Precipitation associated with cold fronts is light and generally long in duration as a result of the mixing. It can be said that Pacific Northwest cold fronts behave more like warm fronts in their precipitation patterns.

Occasionally the region does receive an air mass from the tropics. Typically this occurs when the Tropical Jet Stream meanders northward and joins with the Polar Jet Stream. This phenomenon is known as the Pineapple Express, when abundant tropical moisture is fed into the Polar Front. Exceptionally heavy precipitation is often a result. These events occur about once each winter season, but are more frequent during La Niña conditions.

Temperature

The annual range of temperature for the region is typified by the climograph from Bellingham (Figure 1). During the warmest summer months, afternoon temperatures over the Islands and near the Sound are in the lower 70's, increasing to mid 70's near the foothills, then decreasing into the 60's in the mountains. Maximum temperatures reach 85° to 90° on a few days. Extreme maximum temperatures between 92° and 109° have been recorded (94°F in B’ham). Highest temperatures, lowest relative humidity and the greatest danger of forest fires generally occur during periods of easterly winds (Cascade Chinooks). These conditions seldom prevail longer than one to four days before cooler moist air moves inland. Over the lower elevations, minimum temperatures are in the 50's, decreasing into the 40's in the mountains during summer.

In an average winter, afternoon temperatures over the lowlands range from 35° to 45° and nighttime readings from 28° to 35°. Below freezing temperatures are recorded on 50 to 90 nights, depending on air drainage from Cascade valleys and distance from the Sound. Almost every winter, minimum temperatures drop to between 10° and 20° on a few nights and below zero readings have been recorded in most localities (-2°F in B’ham). In the mountains, below freezing temperatures are recorded on most nights between October and April. The coldest weather occurs when the Pacific Northwest is under the influence of air masses from over the continent. Outbreaks of cold air through the Fraser River Canyon and strong northeasterly winds are observed each winter. Temperature of the water in the Strait of Juan de Fuca and northern Puget Sound ranges from 45° in February and March to between 55° and 58° in August. Shallow and protected bays are several degrees warmer.

Precipitation

The annual distribution of precipitation for the region is typified by the climograph from Bellingham (Figure 1). Precipitation is light in summer, increases in fall, peaks in November, then decreases through spring with a slight increase in May and June followed by a sharp drop near the first of July. The driest regions receive 18 to 30 inches of precipitation annually. This area, often referred to as the "rainshadow" of the Olympic Mountains, extends eastward from Port Angeles to near Everett and northward into the San Juan Islands (Fig. 2). One feature that distinguishes this belt from other localities in the Puget Sound region is the rate of rainfall. Frequently drizzle or light rain falls in this area, while other localities are receiving light or moderate rainfall. In an easterly direction from the Sound annual precipitation ranges from 35 to 50 inches over the lower elevations (35.6" in B’ham), increasing to 75 inches or more in the foothills and 150-200 inches on the wettest slopes of the Cascades due to orographic uplifting (Fig. 2).

Thunderstorms occur on 5 to 10 days each year and are much more common in the mountains due to orographic uplifting. The greater number is reported in summer; however, they have been recorded each month of the year. Hail of sufficient size or intensity to cause crop or property damage is rarely, if ever, reported. Each summer lightning starts some forest fires.

Influence of Topography on Temperature and Precipitation

The climate of the Pacific Northwest is strongly influenced by a winter precipitation maximum and a steep topographic gradient from west to east that accounts for relatively abrupt changes in the quantity and form of precipitation from rain to snow. The cold, very snowy environment of the high elevations contrasts markedly with the mild, rainy climate of the lower elevations to the west. Mean monthly temperature and precipitation at Clearbrook and Glacier illustrate these contrasting energy and moisture conditions in the Puget Lowlands and Cascades. Clearbrook is located near Sumas on the U.S. Canadian border, and Glacier is in the Cascades along Mt. Baker Highway. The linear distance between these two weather stations is 31 km, but Glacier is 264 m higher in elevation. Its average annual temperature is 1.2°C cooler, and it receives 524 mm more precipitation, mostly in the form of snow, than Clearbrook.

Climate Variability

Variability of climatic phenomenon is an important NATURAL component of earth’s climate system. Climatic variability is different than climatic change, which is a permanent change in climatic conditions. However, changes in climatic variability (occurrence of certain climatic events) is a likely impact of climatic change. Some regions, like the Pacific Northwest, inherently have very variable climates as we are influenced by the convergence of two very different parts of the global circulation system- the polar and tropical regions, with the polar regions experiencing large seasonal changes in energy. Other regions, like the equatorial region, experiences little variability, as they have nearly the same energy fluxes year round. Reflecting the complexity of our climate system, most regions show different patterns and magnitudes of variability and trends through time. Among the regional differences, the following consistent trends emerge in data sets over the last century: the number of extremely warm summer temperatures has increased a small amount, the number of extremely cold winter temperatures has clearly decreased (with fewer frost days), and mean summer season precipitation has increased, especially an increase in heavy precipitation events. All of these general trends have temporally reversed during the period of record.

While the lack of inter-regional consistency in climate records might frustrate those looking for a global climate change signal, regional inconsistency to a forcing mechanism (i.e. global greenhouse gas increase) is the hallmark of climatology. Local climatological factors are going to respond differently in each geographic setting to global warming, thus we should fully expect to see different responses by the climate system in different regions. Climate change will not merely be a global increase of 1°C, but will be experienced very differently in every region of the globe- some warming, some getting wetter, while others experience drought.

Variability is an important and fundamental characteristic of Western U.S. weather and climate. Climate and weather vary geographically and fluctuate on daily, seasonal, annual, and decadal time scales. Climatic processes follow regular seasonal patterns. For example, there are regular seasonal changes in the global distribution of solar radiation, leading to predictable patterns of seasonal change in global atmospheric circulation. However, within those seasonal patterns, there is a great deal of intraseasonal and interannual variability. The variability is, to some extent, the random outcome of complex nonlinear interactions among independently varying components of the climate system. Climatic regimes in the Pacific Northwest are prone to interannual and secular variations related to large-scale patterns of climate variations over the Pacific, the El Niño/Southern Oscillation and Pacific Decadal Oscillation (Lins, 1999; Nigam et al. 1999; Parson, 2001). The predominate pattern is that warm years tend to be relatively dry, with light snowpacks, lower spring streamflows and higher summer streamflows, while cool years tend to be relatively wet with heavy snowpacks and high streamflows. Warmer years tend to have water shortages, increase glacier melt and less abundant salmon runs (Mantua et al. 1997; Hulme et al. 1999). The Pacific Decadal Oscillation has been in the warm, dry phase since 1977, but appears to be moving towards the cool, wet phase (Nigam et al. 1999). Global warming is likely to increase the frequency and magnitude of these climatic variations and their impacts on the hydrological system (Trenberth, 1999).

Infrastructure and patterns of use are tuned to the expected range of variation in surface runoff and groundwater availability. Floods and droughts are part of this natural range of variation, although the probabilities of such extreme events may be difficult to discern from limited historical experience. Prospective climate change complicates long-term water resources planning because it will alter streamflow probability distributions and the characteristics of aquatic ecosystems in ways that are not yet entirely clear. The available evidence suggests that global warming may lead to substantial changes in mean annual streamflows, the seasonal distribution of flows, and the probabilities of extreme high or low flow conditions.

Several climate models agree that the Pacific Northwest will likely experience a 1.7-2.8°C (3-5°F) temperature increase and 1-10 cm (0.5-4.1 in.) increase in winter precipitation during early 21st century (Parson,2001). Such warming will be accompanied by changes in precipitation, evaporation, and runoff, but those changes cannot yet be forecast reliably at the watershed scale. In the near term, however, the effects of global warming are likely to be masked by ongoing year-to-year climatic variability (Trenberth, 1999). Higher temperatures would cause both a higher percentage of annual precipitation to fall as rain (i.e. higher snowlines), as well as accelerate summer melt.

Past Climates: The Ice Age

Over the last two million years, the climate has repeatedly changed, resulting in the formation of ice sheets. Repeated advances and retreats of continental ice sheets from Canada have occurred into the Puget Sound. After the Continental ice sheets retreated, valley glaciers advanced down out of the Cascades. It was the action of the ice and its meltwater that shaped the Puget Sound landscape we see today. While ice may have advanced and retreated as many as 22 times, it is the last major advance which we see the most evidence.

This advance, called the Vashon in Washington, reached a maximum extent about 16,000 years ago (see map on next page). At this time ice was about 6000 ft. thick over Bellingham, and it extended southward past Olympia and through the Strait of Juan de Fuca to the Pacific Ocean. About 14,000 years ago, the climate began to warm and the ice sheet began to retreat into Canada, by 11,000 years ago the ice rested at the Canada-U.S. border. At this time, with the Puget Sound free of ice, the mountain glaciers of the Cascades could grow. Because the climate was cold enough to allow glaciers to grow, the vegetation during the ice age was very different from that of today. Analysis of pollen in bogs and lakes suggests that grasslands with spruce and pine species replaced the lowland forests we know today in the NW. A number of paleoenvironmental records (such as pollen, soils, tree-rings etc.) have been used to reconstruct the regional climate since the last ice age, a time period called the Holocene. Most records show the general record shown on the next page. Prior to 7000 year ago during the early-Holocene, climate was much drier and warmer than present and many xeric (arid) species dominated, including many species we find in NW prairies today. It is generally thought that those xeric species became established during this time and prairies covered (continued in box on next page).

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