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What?

Characteristics

The avalanche problem is related to the presence of persistent weak layers in the old snowpack. These weak layers typically include buried surface hoar, depth hoar or faceted crystals.

Expected avalanche types

  • Dry‐snow slab avalanches
  • Mostly human triggered avalanches; natural avalanches are rare, mainly in combination with other avalanche problems

Where?

Spatial distribution

The avalanche problem can be widespread or quite isolated. It can exist in all aspects, but is more frequent on shady, wind sheltered slopes.

Position of weak layers in the snowpack

Anywhere in the old snowpack, often deep in the snowpack. However, when deeply buried triggering becomes increasingly hard.

Why?

Release characteristics

Release of avalanche when loading exceeds the strength of the weak layer.

When?

Duration

Weak layers can persist for weeks to months; possibly most of the winter season.

How to manage?

Identification of the problem in the field

Persistent weak layers are very challenging to recognize. Signs of instability such as whumps are typical but not necessarily present. Stability tests can be helpful to detect the persistent weak layers. Information on snowpack history is critical and reference to the published avalanche report is important. Crack propagation over long distances is common and remote triggering is possible.

Travel advice

Travel conservatively and avoid large steep slopes. Consider the history of weather and snow cover processes in the area. Be extra cautious in areas with a thin snowpack and at the transition from thin to deep snowpack. This problem is a major cause of recreational avalanche fatalities.

Typical accident

Monte Rosa refuge, Zermatt (VS), 20 March 2011