Weather

Changing weather patterns are already severely impacting the PNW.

Temperature

Since the beginning of the 20th century, temperatures in Washington have risen almost 2°F  and since 1986, all but 5 years have been above the long-term (1895–2020) average. The hottest year on record was 2015, with a statewide average temperature of 50.0°F – was 3.7°F above the long-term average. June of 2021  set records for the hottest days on record  – 120°F at Hanford, and 108°F in Seattle. Another record was set on October 16, 2022 for the hottest late-season day on record – a blistering 83 degrees -over 25 degrees above normal. It just keeps getting hotter and summers seem even longer.

Under a higher emissions pathway, historically unprecedented warming is projected to continue through the end of this century. Even under a lower emissions pathway, temperatures are projected to most likely exceed historical record levels by the middle of this century. Overall, warming will lead to increases in heat wave intensities but decreases in cold wave intensities. Unlike other locations in the US,  Puget Sound cities have rarely been exposed to very high temperatures. Future heat waves, particularly an increase in the frequency of warm nights, could stress our communities, which are not well adapted to such events.

Projected rising temperatures will raise the elevation of the snow line—the average lowest elevation at which snow falls. This will increase the likelihood that precipitation will fall as rain instead of snow, reducing water storage in the snowpack, particularly at those lower mountain elevations that are now on the margins of reliable snow accumulation, like the Snoqualmie Pass region.

Rainfall is expected to be the dominant form of precipitation across the majority of the state by the end of this century. Higher spring temperatures will also result in earlier melting of snow, with average snowpack projected to decline by up to 70% by the end of this century. This will further decrease water availability during the already dry summer months, and due to earlier spring peak flows, it will increase the risk of spring flooding. Projected increases in heavy rainfall events by midcentury 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 a particular concern for hydropower and irrigation water supplies.

Glacial Melt

Washington State is one of the nine contiguous states that has mountainous glaciers. These glaciers of the Olympic Range and the Northern Cascades produce 30 billion cubic feet (850,000,000 m3) of water every year. These glaciers are losing their size rapidly. As an example, the Southern Cascade Glacier in Darington has lost two-thirds of its volume. The glaciers in these ranges have, on average, decreased by 31 feet (9.4 m) and between 18 and 32% of their volume of water. An increase of 3.6 °F (2.0 °C) will cause 65% to 75% of the glaciers to disappear in 40 years.

Glaciers reflect the incoming light from the sun so with less glacial cover, the rocks on mountainsides will heat up, causing the surrounding ice to melt even faster. Drainage basins that use glacial runoff will also be affected. Glaciers contribute to a base level of water that runs off after all of the new snow cover has melted.

Southern Cascade Glacier
Darrington, WA

Southern Cascade Glacier, Darrington WA

Snowpack

Over 75% 0f Washington’s precipitation falls as snow during the winter months, and the Cascades can receive upwards of 400 inches of snowfall annually. The hydrology of the PNW is particularly sensitive to changes in climate because the snowpack in the mountains provides an important source of water during the drier summer months. Even the slightest temperature changes have a significant impact on the balance of precipitation falling as rain or snow.

As snow accumulates in upper elevations it forms a natural reservoir that stores water during times when demands are relatively low. As the climate has warmed due to the burning of fossil fuels, more precipitation in Washington’s mountain ranges falls as rain and less as snow, leaving less water naturally stored in the snowpack and glaciers. Furthermore, we’re observing snow melts occur more rapidly—with peak runoffs taking place one to four weeks earlier than in the 1950s—and so less water is available to feed streams in the late summer when water demands are highest. 

These patterns are expected to continue and further alter the hydrologic behavior of many watersheds in Washington. Under a moderate emissions modeling scenario, spring snowpack levels across the state are projected to decrease 29 percent by the end of the 2020s, 44 percent by the 2040s, and 70 percent by the 2060s, relative to the 1971-2000 average.

The Columbia River is a snow-dominated basin; as temperatures increase we are likely to see reduced peak spring streamflow, increased winter streamflow, and reduced late summer flow.
The Yakima River is a mixed rain-and-snow dominated basin; as temperatures increase it will likely shift to a rain-dominated basin type. Peak streamflow will occur earlier in the spring with late summer streamflows declining.
The Chehalis River is a rain-dominated basin in which increasing temperatures are unlikely to change streamflow timing. However, the basin will likely experience higher winter streamflow due to the potential for more winter precipitation.

Rain

Projected rising temperatures will raise the elevation of the snow line—the average lowest elevation at which snow falls. This will increase the likelihood that precipitation will fall as rain instead of snow, reducing water storage in the snowpack, particularly at those lower mountain elevations that are now on the margins of reliable snowpack accumulation. Rainfall is expected to be the dominant form of precipitation across the majority of the state by the end of this 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 this century. This will further decrease water availability during the already dry summer months, and due to earlier spring peak flows, it will increase the risk of spring flooding. Projected increases in heavy rainfall events by midcentury 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 a particular concern for hydropower and irrigation water supplies.

Increasing temperatures raise concerns for sea level rise in coastal areas. Since 1900, global average sea level has risen by about 7–8 inches. It is projected to rise another 1–8 feet, with a likely range of 1–4 feet, by 2100 as a result of both past and future emissions from human activities.  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 at Seattle

Although projections of overall annual precipitation are uncertain, summer precipitation is projected to decrease. Drier conditions during the summer could increase reliance on diminishing snowmelt for irrigation. Additionally, the combination of drier summers, higher temperatures, and earlier melting of the snowpack would tend to increase the frequency and extent of wildfires.

Information on this page collected from: NOAA NATIONAL CENTERS FOR ENVIRONMENTAL INFORMATION, Washington State Climate Summaries, https://statesummaries.ncics.org/chapter/wa/

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