Mean annual temperature has increased approximately 1.5°F since the beginning of the 20th century. Winter warming has been characterized by a far below average number of occurrences of extremely cold days since 1990. Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century.
Rising temperatures will lead to earlier melting of the snowpack, which plays a critical role in spring and summer water supplies. Along with more precipitation falling as rain instead of snow, this may also lead to an increase in springtime flooding.
Wildfires during the dry summer months are of great concern, and the frequency of wildfire occurrence and severity is projected to increase in Washington.
Washington’s location in the heart of the middle latitudes exposes it to frequent storm systems associated with the mid-latitude jet stream. Its climate varies widely from the western to the eastern parts of the state due to the physical barrier of the Cascade Mountains. The Pacific Ocean provides abundant moisture, causing frequent precipitation west of the Cascade Mountains that is orographically enhanced in some places. The eastern side generally receives less precipitation due to blocking of moisture by the mountains. Temperatures in the central and eastern portions of the state are not as strongly moderated by the ocean and exhibit a greater annual range compared to the western side of the state.
Since the beginning of the 20th century, temperatures have risen approximately 1.5°F and temperatures over the past three decades have been warmer than any other historical period (Figure 1). The year of 2015 was the hottest on record with a statewide average temperature of 50.0°F, 3.9°F above the long-term average. The overall warming trend is evident in the number of days with extreme nighttime temperatures. Since 1990, the state has seen below average numbers of very cold nights (nights with minimum temperature below 0°F) and below freezing days (days with maximum temperature below 32°F) (Figure 2a). Warm nights (days with minimum temperature above 60°F in Eastern Washington and above 65°F in Western Washington) have been above average since 1990 (Figure 3). The number of very hot days (days with maximum temperature above 95°F) has been quite variable, with numbers well above average occurring in the first decade of the 21st century, but below average numbers occurring over 2010–2014 (Figure 2b).
Annual precipitation exhibits wide regional variations across the state. Portions of the Olympic Peninsula receive upwards of 150 inches of precipitation annually, while annual totals average less than 10 inches along the Columbia River in eastern interior Washington. Statewide annual precipitation has ranged from a low of 26 inches in 1929 to a high of 54.95 inches in 1996. The driest multi-year periods were the late 1920s, early 1940s, and late 1980s while the wettest periods were the 1950s, early 1980s, and late 1990s (Figure 2c). The driest 5-yr period on record was 1926–1930 with an annual average of 34.6 inches while the wettest was 1995–1999 with 51.0 inches. Washington has not experienced any long-term trend in the number of extreme precipitation events (Figure 2d).
Most of Washington’s precipitation falls during the winter months and the Cascades can receive upwards of 400 inches of snowfall annually. Snowpack in the mountains provides an important source of water during the drier summer months (Figure 4). Precipitation falling as rain instead of snow can have negative impacts on critical industries, such as the timber and agricultural industry, which are also vulnerable to extreme temperatures.Wildfires during the drier summer months are of particular concern. The 2015 wildfire season was the most destructive in Washington’s history, with over 1 million acres burned, more than 6 times the average.
Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century (Figure 1). Even under a lower pathway of greenhouse gas emissions, temperatures are projected to most likely exceed historical record levels by the middle of the 21st century. However, there is a large range of temperature increases under both pathways, and under the lower pathway, a few projections are only slightly warmer than historical records (Figure 1). Overall, warming will lead to increases in heat wave intensities but decreases in cold wave intensities. Unlike in other parts of the United States, Seattle and other urban areas are rarely exposed to very high temperatures. Future heat waves, particularly an increase in the frequency of hot nights, could stress these communities, which are not well adapted to such events.
Temperature increases will affect basins with significant snowmelt contributions to streamflow. Projected rising temperatures will raise the elevation of the snow line in the mountains. This will increase the proportion of precipitation falling as rain instead of snow, leading to less snow accumulation during the winter. Rainfall is expected to be the dominant form of precipitation across the majority of the state by the end of the 21st century. Higher spring temperatures will also result in earlier melting of the snowpack, with average snowpack projected to decline by up to 70% by the end of the 21st century. This will further decrease water availability during the already dry summer months and increase the risk of spring flooding due to earlier spring peak flows. Projected increases in heavy rainfall events by mid-century could further increase flood risk. Reductions in summer flow (projected to occur in 80% of the state’s watersheds) will have important ecological implications, and are of particular concern in some areas from hydropower and irrigation water supply perspectives.
Increasing temperatures raise concerns for sea level rise in coastal areas. Since 1880, global sea level has risen by about 8 inches. It is projected to rise another one to four feet by 2100 as a result of both past and future greenhouse gas emissions due to human activities (Figure 5). Sea level rise has caused an increase in tidal floods associated with nuisance-level impacts. Nuisance floods are events in which water levels exceed the local threshold (set by NOAA’s National Weather Service) for minor impacts. These events can damage infrastructure, cause road closures, and overwhelm storm drains. As sea level has risen along the Washington coastline, the number of tidal flood days has also increased, with the greatest number occurring in 2010–2011 (Figure 6).
Although projections of overall annual precipitation are uncertain, summer precipitation is projected to decrease (Figure 7). Drier conditions during the summer could increase reliance on diminishing snowmelt for irrigation. Additionally, drier summers, along with higher temperatures and earlier melting of the snowpack, would tend to increase the frequency and extent of wildfires.