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Know the Snow: Different tools for different avalanches

Posted: January 25, 2013 - 1:02am

Over the last few years, avalanche folks have tried to identify the avalanche problem they are likely to face as part of their forecasting process before they commit to venturing too far into the mountains. Different avalanches call for different risk management techniques and different forecasting tools. Avalanche-problem characterization goes back to 2006 (Roger Atkins). Different problems typically develop in different environments, form differently, have different release mechanisms and call for different forecasting methods. Recently, avalanche forecast centers across the country have adopted a standard list of avalanche-problem descriptions to consistently convey the hazard and risk management methods that are likely to be most effective for the conditions.

Several months ago a group of avalanche forecasters, lead by Brian Lazar, Ethan Green and Karl Birkland, worked together to create a nationwide uniform avalanche-problem description list. The list includes nine different avalanche types, how they release and a short description of the conditions that lead to these avalanches. The avalanche problem types in the list are:

• Dry and Wet Loose Avalanche — Typically occur within a layer near the top of the snowpack. They start at a point, collect snow and form a fan shape as they travel. In some cases these avalanches can trigger avalanches involving deep snowpack layers.

• Wind-Slab Avalanche — Release of a soft or hard cohesive layer (slab) of snow that forms on leeward slopes as result of wind transport. Wind slabs are often smooth and rounded and sometimes sound hollow. Wind slabs that form over facets, depth hoar or surface hoar can be also labeled as Persistent Slabs.

• Storm-Slab Avalanche — Release of soft cohesive layer (slab) of new storm snow. These avalanches break within the snowstorm snow or on the interface between the new and the old snow cover. Storm-Slab problems typically last between a few hours and few days.

• Wet-Slab Avalanche — Release of a cohesive, typically moist or wet layer of snow (slab). These avalanches occur when the flow of liquid water weakens the bond between the slab and the surface below. They often occur during prolonged warming events and/or rain-on-snow events.

• Persistent-Slab Avalanche — Release of a cohesive layer of soft to hard snow (slab) over persistent weak layers like facets or surface hoar. These avalanches involve the middle to upper snowpack and typically occur during or shortly after a loading event. Persistent-Slab problems can persist for days, weeks or even months, making them especially dangerous and hard to forecast. As the snow cover over a weak layer grows thicker, this avalanche problem may develop into a Persistent, Deep-Slab.

• Persistent, Deep-Slab Avalanche — Release of a thick cohesive layer of hard snow (slab), when the persistent weak layer deep in the snowpack or near the ground collapses under the load above it. Persistent, deep- slab avalanches are typically hard to trigger, but also very destructive and dangerous. These deeply buried weak layers can persist for months, waiting for the right trigger once developed. These avalanches are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast and manage. They commonly develop when persistent slabs become more deeply-buried over time.

• Cornice / Cornice Fall — Release of an overhanging snow that forms by the wind as it transports over terrain features, like ridges. Cornices can become large and hard overhanging masses of snow and exceed 30 feet in height. They can break off the terrain suddenly and pull back onto ridge tops, catching people by surprise. Cornice fall can trigger any type of slab avalanche on the slope underneath it.

• Glide Avalanche — Release of the entire snow cover as result of gliding over the ground. They are often preceded by full depth cracks (glide cracks), though the time between a crack appearance and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.


Since the mechanisms and the conditions that create these avalanche problems are different, it only makes sense that the forecasting tools and the risk management strategies of choice to forecast and manage these problems will also vary.

Typically, weather and obvious signs of instability are effective predictors of storm snow avalanches. Storm snow avalanches typically release on hard-to-detect layers that do not break cleanly with most stability tests (recently Ned Bair showed the Exceeded Column Test is a powerful predictor of storm snow avalanches). I usually deal with these avalanche problems by observing loading patterns, ski cut testing on small or flat slopes, looking for shooting cracks in front of my skis or giving the new snow a day or two to bond with the old snow. In any case, when no signs of instability appear, I go ahead and dig a snowpit to confirm my assessment with a harder look at the snowpack and stability tests.

As avalanches include deeper and harder portions of the snow cover, I pay close attention to loading patterns and other obvious signs of instability, but I also tend to dig more and more pits in the snow. I look for persistent weak layers and put more weight into stability tests.

When the avalanche problem is too deep to expose with stability tests or snowpits, I tend to rely on historical knowledge and terrain management. Deep-Slab, Glide and Cornice Fall Avalanches are low frequency / high consequence avalanches; they are difficult to forecast and very destructive. I try to find out if there are persistent weak layers buried deep in the snowpack, if the ground underneath it is wet and slick and when, where and why these avalanches occurred. If these problems seem to exist, I carefully choose the terrain I travel on.

The best way to deal with wet slab avalanche is good time management. The conditions that lead to wet avalanche activity can progress rapidly but also rather obviously. In many cases, the snow surface conditions are wet, slushy, too warm, and rainy and generally speaking, miserable when wet slab instability becomes a significant problem. When the on-snow traveling conditions start to decrease, it is time to leave to where conditions are better and wet slab problem does not exist.

The bottom line is: Recognizing and understanding the avalanche problem ahead is a powerful tool to effectively managing the avalanche risk that may exist along the path ahead. Mastering this tool is an effort worth taking.


• Ron Simenhois is an avalanche forecaster who lives in North Douglas; contact him at simenhois.ron@gmail.com.

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