Thompson Pass, Alaska - On February 6, two separate skier-triggered avalanches occurred in Thompson Pass near Valdez. Both resulted in a skier or snowboarder being caught and partially buried. Both of the people involved were strong, fast athletes who attempted to outrun the slides but were overtaken. Fortunately, both were also successfully rescued by the other members of their party.
The avalanches occurred in two popular areas for backcountry skiing and snowmachining: Girls’ Mountain and Cracked Ice. Both parties, one a local member of the public, and one a Valdez Avalanche Center forecaster, reported they had not observed obvious signs of avalanche danger before dropping into their lines.
Full reports of these incidents are posted on the Valdez Avalanche Center website: www.alaskasnow.org/valdez
Avalanches occur when a cohesive layer of snow, often called a slab, loses its grip on an underlying weaker layer or the ground and slides down a slope due to the pull of gravity. They often occur naturally after a storm because of the added weight of additional snow.
If a layer of snow is close enough to the point of failure from the snow on top of it, the stress of a skier or snowmachiner can be enough to trigger an avalanche.
According to the report submitted by Valdez Avalanche Center forecaster Gareth Brown, the most likely cause of the avalanche on Cracked Ice was failure between surface hoar (frost) that formed during a clear period in late January and snow that had fallen afterwards.
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Every year, the snowpack in an area will be different due to the layering created by frost, storm snow, wind deposits, and rain. Avalanche forecasters and recreationists interpret these layers to learn more about avalanche risk. The layers in the snowpack contribute not only to the likelihood of an avalanche, but to the likely size of an avalanche if it does occur.
The snowpack in the Thompson Pass area has developed a “persistent slab” problem, which is both difficult to interpret and potentially very dangerous.
Brown explained, “A persistent slab issue involves a layer of snow beneath the surface that has the potential to remain reactive over long periods of time. Interpreting the likelihood of an avalanche to release on a particular slope can be challenging even for experienced users. There are tests that users can perform on the snow to test its sensitivity, but even these can have results which are subjective and misleading. The depth and density of the slab changes across terrain sometimes dramatically, further complicating assessment.”
A persistent slab fails when it is hit in just the right spot. These trigger points could be a slightly shallow layer of snow, buried rock, or a change in terrain shape. From the recreationist’s standpoint, that point may not look remarkable. Multiple people can ride a slope without hitting a weak spot. According to Brown, “Forecasters call this negative feedback as users can gain a false sense of security when they see tracks, when sometimes dangerous conditions can still exist.”
As of February 8, the avalanche hazard was described as “considerable” on the Valdez Avalanche Center website. The forecast identified stress from recent snowfall on weak layers as a source of concern. The forecast also noted that if an avalanche does occur, it could propagate to cover a wide area, which would make it difficult to escape.
Avalanche forecasts for Thompson Pass are updated Wednesday through Sunday through the winter recreation season. Check the forecast regularly at www.alaskasnow.org/valdez to learn about trends and patterns, and always check the forecast before heading into avalanche terrain.
Disclosure: Gareth Brown reviewed a draft of this article prior to publication.
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