"Avalanche season upon us’ forecasters say | VailDaily.com

"Avalanche season upon us’ forecasters say

Jane Stebbins/Special to the Daily

SUMMIT COUNTY – The avalanche season is upon the High Country.

Recent storms that dumped up to 2 feet of snow – and more than 4 feet in the Wolf Creek area – in the past week have finally covered the rocks and stumps that threaten to damage skis and ligaments.

Now, it’s the snow’s turn, said Scott Toepfer, a weather and avalanche forecaster for the Colorado Avalanche Information Center.

Avalanche danger in the north and central mountains is considerable above treeline on northwest and southeast slopes, and moderate on southern and western aspects. Below treeline, those dangers drop to moderate and low.

Much of the potential danger lies in the fact that so much snow has fallen and skiers can now reach good ski terrain – some of which is also prime avalanche terrain.

“All the necessary ingredients of a slab now exist in our backcountry,” Toepfer said.

Sliding slopes

So far this season, 15 avalanches have been reported to the Avalanche Information Center. Most, including a few on Loveland Pass, were triggered by explosives, although the No. 6 slide of the Seven Sisters on Loveland Pass broke loose Tuesday and ran just above treeline.

“I suspect that other natural slides have run as well,” Toepfer said. “And with the present winds, these most likely would be on northwest to southeast aspects as well as cross-loaded slopes and gullies near and above treeline in all mountains.”

Avalanche Information Center forecaster Brad Sawtell, who ventures into the backcountry to determine avalanche danger, said he spotted a natural slide on Hoosier Pass Wednesday.

“Our snow, although there’s not much to it on one hand, on the other hand, in wind-loaded areas, there’s some stored energy there,” he said.

New snow, measuring from a trace to 2 inches, followed by anticipated winds Wednesday night, won’t likely reduce the danger, forecasters said.

Avalanche ingredients

Three factors – snowpack, weather and terrain – go into making an avalanche.

Humidity is responsible for the initial shape of snowflakes. But early-season snow and temperature fluctuations melt and smooth the pointed snowflake crystals, making them less likely to bond to one another. The weight of additional snow on old snowpack also breaks down the snow crystals, making them into a compact grain.

When the next storm moves through, new snowfall doesn’t affix to the old, rotten snow, said forecaster Nick Logan. Snowpack then awaits a trigger – be it wind, a human or sheer stress on the snowpack – to slide.

Clues that an area is prone to avalanches include previous slide paths, flag trees – those with broken, uphill branches – and cracks along the snow surface. Backcountry enthusiasts also should pay attention near rocks, Logan said. While some might be safe refuges, snow in the area can be weak because the sun heats rocks, causing snow to rot.

Temperatures, humidity and wind also affect avalanches. Changing temperatures melt and refreeze snowpack, making it less stable.

“If the temperatures increase fast enough, they’ll destroy the bonds between the old and new layers,” Logan said. “Snow’s just like Silly Putty. If the temperatures change slowly, it’ll adjust. But if they change fast, it gets real brittle.”

Wind wicks moisture from the snow, and humidity controls the types of crystals that land on the slopes. And snowfall of an inch an hour for several hours increases the possibility of avalanches.

Wind also pushes snowfall to the leeward side of the mountain, where it builds thick cornices and drifts that are prone to slide.

Surviving a slide

Avalanches typically slide from a single point and progress outward, or in a slab from a horizontal fracture line. Some merge with other slides in neighboring chutes, and others trigger avalanches nearby.

When the snowpack finally gives way, the snowflakes are pulverized into tiny grains that pack tightly as the snow roils down the slope. And if someone is trapped in the slide, the snow can embed him or her in what feels like setting concrete. A Breckenridge man rescued from an avalanche near Arapahoe Basin late last month said he couldn’t even open his eyes.

“The human factor is huge in avalanches,” Logan said. “You think, it took you hours to get there; you’re going to ski it. People are often unwilling to bend to these factors. You have to think, “Will it avalanche?’ and, “If it does, where will it take me?'”

The results of being trapped in an avalanche aren’t pretty, either, he said.

“You get compressed,” he said. “It squeezes you extremely tight. It’s not fun for you, not fun for your family, not fun for your partners.”

Someone trapped in a slide for less than 15 minutes stands a 90 percent chance of survival. At 30 minutes, however, that drops to 50 percent.

Avalanche forecast information

Summit County and Vail 970) 668-0600

Aspen (970) 920-1664

Durango (970) 247-8187

Crested Butte (970) 349-4022

Denver, Boulder (303) 275-5360

Fort Collins (970) 482-0457

Colorado Springs (710) 520-0020

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