Mountain Safety

Yellowstone is home to some sick terrain, just watch out when it warms up, photos by Ralph Backstrom

Red Flags in reality is a Jones Avalanche awarness Month series detailing Live-From-The-Field stories about five of the most common signs of avalanche danger - The Red Flags. Next up we’ll learn about the dangerous effects of rapid warmingfrom an experience Ralph Backstorm had in Yellowsotne National Park.

A couple years back I went on a spring splitboard mission to Yellowstone National Park with Bryan Iguchi, Alex Yoder and Jeff Hawe. The first day of the trip we rode some smaller lines close to the trailhead. The snow had gotten really warm that afternoon so we bailed early hoping to come back the next morning and ride some of the bigger objectives we had scouted. On the way out we noticed small roller balls and point release slides but nothing too out of place for spring conditions. The spring snow just felt sticky and slow as usual.

So we left the park that night with the plan to come back early the next day. We never made it back in the park. Half-hour after we left the park gates a massive slide came down and buried the road to the park entrance! For more reasons than one, the slide caused by rapid warming was a trip ending red flag.” – Ralph Backstrom

Rapid Warming is one of the most prominent causes of snow instability and a major avalanche danger red flag. Warming causes instability in the snowpack due to a process known as ‘Creep Tension’. When the temperatures spike, the top layers in the snowpack go through a process of viscous deformation and become a separate layer that’s effected by gravity differently than the rest of the snow pack. The warm layers will ‘creep’ down the slope as they are pulled by gravity faster than the rest of the snowpack. This difference in motion between the surface and the base introduces tension.

This diagrma shows the effects of gravity on a snowpack, the red arrows are creep, the blue arrows are settlement and th eblack arrows are the movement of the entire snowpack and it’s base, known as glide, diagram by www.avalanche-center.org

When the warm surface layers start to ‘creep’, this added tension in the snowpack makes it easier to set off weaker under layers. Surface loads and stresses also penetrate deeper into the snowpack when the surface layers are warm making skier-triggered avalanches more likely.

Backstrom, Iguchi and the crew dig a snow pit in Yellowstone National Park.

Convex features are especially prone to ‘creep’ as they are already under tension. If you detect any signs of rapid warming such as roller balls, point release slides off rocks, trees shedding snow or an obvious temperature swing, convex slopes and other known avy paths should be carefully avoided. One of the other important things to remember about the rapid warming red flag is that it might be the only red flag you see that day. Unlike the other red flags that tend to appear simultaneously (new snow, winds, natural avalanches), rapid warming may be the only red flag observation you make before the avalanche danger increases significantly.

Rapid warm up caused an avalanche that buried the road into Yellowstone, Red Flags don’t get much more obvious.

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Author
Seth Lightcap
Category
Mountain Safety
Published on
21 December 2012

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