NewsFebruary 24, 2012

Avalanches Taking Toll; Foreshadowing the Future?

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Andrew Freedman

By Andrew Freedman

Already this winter, 20 people have been killed by avalanches in the Western U.S, with the most recent loss of four skiers and snowboarders in Washington State on Feb. 19. Colorado alone has recorded six fatalities—its average annual total—in just the past month.

As counterintuitive as it may sound, less snow and thinner snowpacks may lead to more avalanches in some years. While it may seem like more snow would lead to more avalanches, experts say that in many areas, the high avalanche danger this year is tied to the thin, weak snowpack that was established earlier this winter, and is now failing to hold on to new snow that falls.

Experts say that in many areas, the high avalanche danger this year is tied to the thin, weak snowpack that was established earlier this winter. Credit: Jupiterimages Corporation.

If climate change leads to more warm winters with less snowfall, as some models project, then this year could foreshadow avalanche risk in years to come. “While the winter is not over, and many years have had high March snowfall, the climate future will probably be for additional declines in snowfall, and snow duration in the West,” said Steve Running, an ecologist at the University of Montana.

In Utah, where avalanches are the most dangerous natural hazard, there have already been four fatalities this year, the state's  annual average. Snow depth in the Utah mountains is running at about 40 percent below normal, according to Larry Dunn, the meteorologist in charge of the National Weather Service’s Salt Lake City office.

Dunn said that much of the snow that has fallen has turned into what he called “sugary snow,” with weak bonds between the crystals. “We have a much more dangerous snowpack this year with far below average snow than last year when we had above average snow,” he said. “We think there are going to be a lot more avalanches this year.”

Officials in Utah, Montana, and Colorado all reported that the same basic recipe has led to the dangerous conditions. In each of those states, early season snows were followed by a lengthy dry spell, during which time the thin snow pack underwent structural changes, eventually forming a long-lasting, weak-base layer. Similar dynamics—involving weak layers of snow—are at work in the Pacific Northwest, although there is a thicker snowpack there.

In an average year, there are 25 avalanche-related deaths in the U.S., and there has been an upward trend in fatalities over time as back-country activities such as skiing and snowmobiling have become more popular.

The dangerous, weak snow layer conditions are expected to continue for the rest of the winter, meaning more risky days for back-country skiers and snowboarders. Credit: Canadian Press.

Spencer Logan, an avalanche forecaster at the Colorado Avalanche Information Center, said the unusual boom-or-bust cycles of winter storms has resulted in ideal conditions for forming weak snow layers. “Winter started fairly normal and then went really, really dry,” Logan said, noting that the dry weather helped form a base of what avalanche experts call “depth hoar” that can take months to stabilize.

New snow falling on top of weak layers can be easily dislodged by back-country skiers and snowmobilers, setting off deadly slides.

“It’s hard to build a strong building on a very poor foundation,” Logan said, adding that these types of avalanche risks are relatively common in Colorado, but that the problem is worse than normal this year.

According to Doug Chabote, director of the Gallatin National Forest Avalanch Center in Southwest Montana, once snow has fallen, snow crystals can undergo structural changes that make them more likely to help trigger an avalanche. Montana has seen four avalanche-related fatalities this winter, including two deaths on New Year’s Day.

Chabot said that when there is a thin snow pack and clear, cold nights, there can be a large difference in temperature between the top of the snow layer and the bottom. This temperature gradient causes water molecules within the snowpack to move, and this process creates small, angular snow grains that don’t bond well, instead forming snow grains that resemble raw sugar—hence the term “sugary snow.”

It’s the type of snow that falls apart easily, and as kids all across the country know all too well, from which it’s nearly impossible to make a snowball.

Once these weak layers are buried by new snow, the weakness is preserved. “As soon as we started to get snow on top of this stuff it got incredibly unstable,” Chabote said. Recent snowstorms in Montana have added several feet of snow to the mountains, which is leading to more avalanche alerts there.

According to Chabote and other avalanche specialists, the dangerous, weak snow layer conditions are expected to continue for the rest of the winter, which means more risky days for back-country skiers and snowboarders, and potentially more injuries and fatalities as well.

Snow cover in the U.S. is running well below average this year, and far below what it was last winter. The only avalanche-prone region that has consistently seen above average snow cover is the Pacific Northwest, due to a more active storm track there, although there have been recent snowstorms in Montana, Wyoming, Utah and Colorado. (The mountains of California have been so snow-starved that it would be difficult to set off an avalanche there.)

The warmer temperatures and below average snowfall seen this year are consistent with some climate change projections. However, the combination of natural variability and global warming will still allow for more heavy snowfall years in the West as well.