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Deadly Colorado avalanches underscore dangers of unstable snowpack

Fickle early-season storms and increased backcountry usage are factors behind a deadly winter week in Colorado and across the country. Here’s how hydrologists view the state’s current snowpack and avalanche danger.

February 8, 2021

By Lindsay Fendt

Avalanches across the United States killed 15 people during the first week of February, making it the deadliest seven-day period for snowslides since 1910, according to the Colorado Avalanche Information Center.

In Colorado, more than 500 avalanches have been reported since Jan. 29, according to the CAIC. Two were fatal: An experienced backcountry skier was killed in an avalanche in backcountry terrain east of Vail Ski Resort on Thursday, and a slide near Silverton killed three experienced backcountry skiers on Feb. 1, making it the deadliest Colorado avalanche since 2013.

It’s a grim start to a month in what is shaping up to be a bad year for avalanches. Colorado snowslides have claimed the lives of eight skiers this winter, and it’s still early in the backcountry season, which usually extends well into April.

Though Colorado has seen less snow than normal this winter, the number of fatalities has already surpassed that of the snowy last season. According to snow hydrologists, the number of deaths in a dry year reveals the complexity of forecasting avalanche danger, which goes beyond how much snow is on the slopes.

“It’s not just how much snow there is,” said Tom Bellinger, Ph.D., a hydrologist at Metropolitan State University Denver’s Department of Earth and Atmospheric Sciences. “We look at how the snowpack changes with time, how stable or unstable it is.”

 
 
 
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This season, instability has been one of the key drivers of avalanches. Most of Colorado’s mountain regions are in drought, with the snowpack levels hovering between 70% and 84% of normal.

The late start to the snow season created an unstable layer of snow that is sitting beneath the more substantial snowfalls that have come in the past few weeks. The weak snow layers make it more likely that new snow will topple when trod on.


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“It’s a dry period, so even if you get some snow, it sits on top of icy snow,” said Tom Cech, co-director of MSU Denver’s One World One Water Center. “How often we get snow in a winter – even if it’s every week or once a month – can make a big difference on avalanche danger.”

Though fatalities have been high this season, the total number of accidental avalanches has, so far, been unremarkable relative to years past. Cech said droughts like the current one are not uncommon.

Warning signs are posted at Loveland Pass. Photo by Amanda Schwengel
Warning signs are posted at Loveland Pass. Photo by Amanda Schwengel

“Colorado has large variations in rainfall and snowfall from year to year,” he said. “It’s just the way it is out here with our weather and climate. It’s not good; it’s pretty dry, but things have been worse.”

One main difference this year is that the sketchy snowpack has built up during a season when many are looking for a break from pandemic isolation. Backcountry ski-equipment sales hit an all-time high this past fall, suggesting that more newbies are leaving resorts for the isolated, unpatrolled mountain sections where most avalanches happen.

“More people going into the backcountry usually equates to more accidents,” Bellinger said.


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Historically, Colorado is the most dangerous state for avalanches. The high volume of backcountry visitors is likely one factor leading to the number of slides, but there are other factors. Colorado has high winds that can move snow around. Also, the state’s sunny winter weather melts snow during dry periods. This melted snow often becomes part of the unstable layers of slippery snow that can easily slide.

Chris George, an MSU Denver graduate, is a global mountaineer and snow-science pioneer. George, 82, is retired, but he spent decades monitoring the snowpack in Colorado and teaching avalanche-safety courses. Though avalanche safety has come a long way since he began his career, George still advises those headed to the backcountry in winter to be careful.

“I’ve climbed in Afghanistan and the Andes and the Alps,” he said. “There’s one thing I’ve learned: The snowpack in southern Colorado or in the southern Rockies is probably the most unstable snow anywhere.”


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