UW News

February 13, 1998

El Nino expected to shrink spring snowpack, may reduce water supply

Though you wouldn’t guess it by looking at current conditions, snowpacks in the Cascade Mountains are likely to fall significantly below normal levels by late spring, which may affect water supplies, fisheries, agricultural operations and hydroelectric plants which depend on the runoff, University of Washington researchers predict.

While snowpacks currently are at, or slightly below, normal levels for this time of year, El Nino increases the likelihood of unusually warm spring temperatures that would lead to earlier snowmelt runoff and a longer period of low stream flows in the summer. Average snowpack levels at Snoqualmie Pass in February are about 50 inches during normal years and 60 inches during El Nino years. By June, however, the average snowpack in Snoqualmie Pass drops to less than 30 inches during El Nino years compared with over 60 inches during non-El Nino years.

“Looking at data back to 1916, we compared El Nino years with non-El Nino years and found that low snowpack in late spring is much more likely in El Nino years,” says Philip Mote, research scientist with the Joint Institute for the Study of the Atmosphere and Ocean, a combined research and public policy initiative of the UW and National Oceanic and Atmospheric Administration.

“The typical pattern for El Nino years is for the snowpack to be normal until early March, then the accumulation we would typically see through mid- to late-April doesn’t happen. And it’s that late spring snowpack that usually gets us through the summer in terms of water supply.”

The Pacific Northwest’s response to El Nino does not always follow this pattern. During the other big El Nino event of this century, in 1982-83, temperatures and snowpack levels remained near normal throughout the spring. However, Mote says, temperatures have been above normal this year and scientists are expecting a typical El Nino pattern rather than a repeat of 1982-83.

Climate researchers at the UW and elsewhere have been able to make unusually accurate predictions about the current El Nino event, expected to be the strongest on record, thanks to new understanding about the recurring weather phenomenon and more sophisticated equipment to monitor the atmospheric forces that drive it.

El Nino events, which occur around Christmas time every two to 10 years, begin with a warming of sea-surface temperatures in the Pacific Ocean off the coast of South America. This often prompts a shift in oceanic tradewinds that can disrupt normal weather patterns around the globe. In the Pacific Northwest, El Nino typically brings warmer, and sometimes drier, weather through the winter and spring months. In addition to smaller snowpacks, conditions related to El Nino can increase populations of insects that infest trees, heighten the risk of forest fires, impact salmon runs, dislocate some fish and bird species and increase coastal erosion.

The challenge, Mote says, is to take advantage of the heightened El Nino forecasting capability in order to mitigate some of these negative impacts. In dealing with the fallout from smaller snowpacks, however, this is easier said than done.

“The difficulty is that reservoir managers have to balance several competing demands such as flood control, salmon runs, hydroelectric power, irrigation and water supplies,” Mote explains. “While the smaller spring snowpacks threaten to reduce late summer water supplies, reservoir managers can’t hold back water now without risking spring floods. At this point they don’t have a lot of options.”

Nonetheless, UW researchers say, awareness of the heightened probability for early snowmelt runoffs and a longer period of low stream flows in the summer can be very useful to fisheries managers, hydroelectric plant operators, agricultural interests and other water resource users in the Pacific Northwest.


For more information, contact Mote by phone at (206) 616-5346 or by pager at (206) 994-6945; and Nathan Mantua, research associate in the UW Department of Atmospheric Sciences, by phone at (206) 616-5347 or by email at mantua@atmos.washington.edu.