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Avalanches In The Northwest

As Forecasting Improves, Human Judgments Are Still Unpredictable

Avalanches arise from a set of simple ingredients: a slope, layered snow, and a trigger.

Triggers include natural factors, such as rapidly falling wet snow, and human factors, such as someone walking, skiing, or snowmobiling.

The Pacific Northwest has all of these ingredients, making it a natural laboratory for the study of avalanches, as well as a place where avalanche fatalities inevitably make the headlines in winter. They are a common hazard of which backcountry travelers and even drivers on I-90 should be aware.

Avoiding headlines was on the mind of Howard Conway, research professor in earth and space sciences at the University of Washington, when he decided to commit to avalanche research. A native of New Zealand, Conway frequently skied and climbed in New Zealand's Southern Alps, and worked in a national park. "I figured I'd probably die unless I learned more about avalanches," he says.

Conway has developed avalanche prediction models in collaboration with transportation departments in New Zealand and Washington state. In April 2004, Conway attended the International Symposium on Snow Monitoring and Avalanches in Manali, India, to present the latest results of his snow slope stability model.

"It predicts when avalanches will start during a storm," says Conway. "It's kind of an odd model because it works so well. It's very simplified." The model is fed data that can readily be provided by automated weather stations, including precipitation, air temperature, and new snow density. It was developed at Snoqualmie Pass in Washington and has also been tested at Milford Sound in New Zealand, where it has been used by the transportation department for several years. In case studies, it has predicted the timing of avalanches to within a couple of hours.

Craig Wilbour, an avalanche control supervisor for the Washington State Department of Transportation (WSDOT), says the model is a "good tool" for avalanche prediction in the mountain passes, but notes that the model is still in the experimental stages here. Wilbour and his team are working to get the model into a format that can be run daily for prediction of avalanches at Snoqualmie Pass.

Although model results are encouraging, Wilbour says that there is no "magic bullet" in the evaluation of avalanche potential in the mountains. The model may help to prevent extended highway closures where avalanche zones are well known. However, it is not intended for backcountry adventurers, who must consider snow, terrain, and weather conditions specific to where they go.

Avalanche experts in the Northwest say that human judgment is the key to avoiding avalanche accidents. "People in the back country need to remember to assess conditions for themselves," says Garth Ferber, an avalanche meteorologist at the Northwest Weather and Avalanche Center (NWAC) in Seattle.

Part of that assessment involves understanding how the strength of a snowpack can change with time. Conway's model has some basic take-home messages. The stability of a snow slope reflects the competition between the strength of the old snow layers and the amount and rate of new snowfall from a storm. When the old snow cannot support the weight of the new snow, avalanches occur. Specialized meteorologists at the NWAC use the same principles in providing general avalanche forecasts for the Cascade and Olympic mountains twice daily during the winter. The avalanche forecasts contain code words which correspond to a five-level color scale. Avalanche probabilities range from the green "low" to the black "extreme."

Specific conditions leading to avalanches in the maritime climate of the Cascades and Olympics are relatively well understood. "Heavy snowfall and a warming trend: that's guaranteed to produce avalanches," says Ferber. "We base our avalanche forecasts on current weather conditions and current snow conditions as they are reported." They then assess what effect forecasted weather will have on the snowpack conditions.

The NWAC is housed at the National Weather Service (NWS) office at Sand Point in Seattle, Wash., and makes use of NWS forecasts and observations. The center also maintains its own network of automated weather stations throughout the Cascades.

To obtain an idea of the actual snowpack situation, NWAC forecasters maintain contact with local ski patrols and highway crews, and they solicit backcountry reports from anyone who will provide them. They also drive the mountain passes frequently and do their own testing of the snow. Digging snowpits, they can look for weaker layers which will fail if too much stress is placed on top either from new snowfall or from a person. Simple stability tests are performed with skis, hands, and shovels.

Human-triggered avalanches pose the largest hazard to winter recreationists. About 90 percent of all avalanches in which people are involved are triggered by the victims themselves, or members of their party, according to statistics of the U.S. Forest Service National Avalanche Center in Bozeman, Mont. When the forecast calls for considerable or higher avalanche danger, avalanche accidents are most likely to occur, and people should consider alternate activities if they are not highly skilled in assessing avalanche risk for themselves on a local level.

The forecasts of the NWAC and other avalanche centers are believed to be effective and have a proven track record. Since the 1950s, avalanche fatalities have been on the rise in the U.S. and have exceeded 20 per year since 1994-95. However, the increasing number of fatalities is small in proportion to the rising number of people using the backcountry in winter, says Karl Birkeland, an avalanche scientist at the Forest Service National Avalanche Center. In Washington and Oregon, the fatality rate has been relatively stable, although last season was a particularly bad one with seven avalanche-related deaths in Washington.

Three of these fatalities occurred within a week in December of 2003. The hazardous snow conditions followed a large storm system that had dumped a thick blanket of snow in the Cascades. These incidents involved snowshoers at Mt. Baker, snowmobilers at Navajo Peak near Blewett Pass, and a snowshoer near the Alpental ski area. Prior to these accidents, NWAC forecasts called for considerable or high avalanche danger. In many instances, though, travelers don't bother to check the forecast. A Seattle Times account of the Mt. Baker accident reported that trip leaders had never made the call to the NWAC or visited its Web site.

A survey of snowmobilers at a trailhead in Montana found that half of the snowmobilers said they had called for an avalanche advisory prior to their trip. Of the people who hadn't called, most were going into a low-risk area on trails, according to Birkeland. Nonetheless, overall statistics are not in snowmobilers' favor: From 1985 through 2003, snowmobilers accounted for 25 to 30 percent of all U.S. avalanche fatalities–more than any other group, according to NWAC data. Avalanche experts attribute this trend to powerful snow machines that can cover a lot of ground in one day, the practice of "high-marking" on steep slopes, and less avalanche education among snowmobilers compared with other winter recreationists.

It is difficult to gauge just how many accidents the forecast system may be preventing by causing people to reschedule their trips or to become more aware of safe travel techniques. "We hope that the use of forecasts is sufficient to prevent an increase in the fatality rate in the Northwest," says Mark Moore, director of the NWAC. The forecasts on the Web site of the NWAC received about 2.65 million hits during 2003-04, up from 1.1 million in 2000-01.

Colorado leads the country in the average number of avalanche deaths, a statistic which may be due to patterns of backcountry use and to the continental climate, Ferber suggests. Cascade and Olympic snow is typically different than that found farther east.

"In this [coastal] climate, things tend to strengthen with time," says Conway. "The most hazardous time is during a storm."

Birkeland divides Western mountains into three broad zones: coastal ranges including the Cascades, Olympics, and Sierra Nevada, the intermountain zone encompassing Idaho and parts of Montana, Wyoming, and Utah, and the continental zone, primarily in Colorado. Storm-related, or "direct-action" avalanches are of primary concern in the coastal zone. In the continental zone, "unstable conditions can persist for weeks or even months," says Birkeland. The intermountain zone sees a mixture of snow conditions and avalanche types.

Weak snow layers that result in unstable conditions arise from two primary mechanisms. The first is the deposition of large, angular snow crystals on the surface by the atmosphere, much like dew forms on blades of grass. These "surface hoar" crystals give a snow surface a sparkly appearance on a cold moonlit night, Birkeland says. If this layer is buried intact, it is weak in shear strength and creates an avalanche waiting to happen.

In continental climates, the second mechanism often occurs. Cold winters can cause recrystallization of buried snow layers, especially when cold air moves over a warmer snow pack creating a temperature gradient. The new layer of "depth hoar" crystals weakens the snowpack.

The evolution of a snowpack is a substantially more complex problem to model than the loading of snow during a storm, Conway says. His slope stability model has not been applied to the more continental climates of the West.

Within a mountain range, spatial differences in the strength of snow are difficult to judge, even on a single slope. Just after graduating with his Ph.D., which focused in part on this problem, Conway got caught in an avalanche while skiing in the backcountry. "I made an assessment of the slope that was wrong." He was lucky to survive.

Birkeland, who is also an adjunct professor at Montana State University-Bozeman, is now focusing his research on the spatial variability of snow on the slope scale. He aims to develop rules of thumb that can be used to determine the relative strength of snow across a slope and through time.

Not all avalanche victims are engaged in recreational activity when accidents happen. In January 2004, University of Washington graduate school dean Marsha Landolt and her husband were killed when an avalanche raced down the mountain behind their cabin and filled the first floor where they were sleeping with dense snow. Heavy snowfalls and strong winds led to the naturally occurring avalanche, which took place just outside Idaho's Soldier Mountain ski area.

Within ski areas and along mountain roads, technicians go to great lengths to significantly reduce the likelihood of such disasters.

Wilbour began his work as an avalanche control technician in the 1970s at a ski resort and has worked with WSDOT since 1975. His crew of four intentionally triggers numerous avalanches per season at Snoqualmie Pass with skis, artillery, and explosives. They are also responsible for clearing Chinook Pass each spring, working with the maintenance crews. Another crew works at Stevens Pass and with opening the North Cascades Highway. If all goes as planned, the highways need not be closed for very long. WSDOT estimates that each hour Snoqualmie Pass is closed costs local businesses $480,000.

Most experts advise that winter backcountry travelers should carry avalanche beacons, probes, and shovels. Would-be adventurers should remember to check the forecast and most importantly, says Ferber, to take simple, standard precautions.

For more information, visit www.nwac.us or www.avalanche.org

David Schneider is working on a Ph.D. in earth and space sciences at the University of Washington.

Top: Controlled-release avalanche along the Trans-Canada Highway in British Columbia, 14 miles west of Rogers Pass. In addition to economic and safety benefits, avalanche control work provides an avenue for avalanche research and an opportunity for photographers to capture an event that people seldom see safely in nature. Photo: Parks Canada, Bob Greyell

Low-temperature Scanning Electron Microscope image of a stellar snow crystal. Photo: Electron Microscopy Unit, Beltsville Agricultural Research Center, Md.

Bottom: An M60A3 tank has been used on Stevens Pass by WSDOT for avalanche control work. Photo: WSDOT

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