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

Though you wouldn't guess it by looking at recent 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, UW researchers predict.

While snowpacks currently are at, or slightly below, normal levels for this time of year, El Niño 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 Niño years. By June, however, the average snowpack in Snoqualmie Pass drops to less than 30 inches during El Niño years compared with over 60 inches during non-El Niño years.

"Looking at data back to 1916, we compared El Niño years with non-El Niño years and found that low snowpack in late spring is much more likely in El Niño 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 Niño 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 Niño does not always follow this pattern. During the other big El Niño 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 Niño 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 Niño 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.

In addition to smaller snowpacks, conditions related to El Niño 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 Niño forecasting capability in order to mitigate some of these negative impacts. ¶

Greg Orwig, News and Information