Wet snow (slab avalanches)

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  4. Wet snow (slab avalanches)

Avoid staying for a longer period in avalanche release and run out areas. For danger Level 3: Avoid avalanche release and run out areas.

Wet snow avalanches tend to release spontaneously.

Stability decreases when the snow surface gets wet and soft.

Timing is important.

Characteristics

  • The avalanche problem is related to wetting and weakening of the snowpack due to the presence of liquid water. Water infiltrates the snowpack due to melt or rain. 
  • The problem can occur due to rain, sun, warm temperatures or lack of refreezing of the snowpack at night.
  • Avalanches can vary in size from small to very large. Runout length can be very long with high water content.
  • Mainly natural release
  • Requires a snowpack with clear layering.
  • The probability for wet slab avalanches is at it's highest at first time wetting of a previous dry layered snowpack occurs.
  • Predicting the release of wet slab avalanches is difficult. It takes time for the water to penetrate into the snowpack and weaken the slab/weak layers enough for an avalanche to release.

 

Spatial distribution

  • When sun is the main cause, distribution of the problem is mostly depending on aspect and elevation.
  • All aspects are affected in the event of rain on snow.

 

Release characteristics

  • Weakening and failure of pre-existing weak layers in the snowpack or release at layer interfaces where water accumulates.
  • Rain is also an additional load on weak layers.

 

 

Location of weak layer in the snowpack

Avalanches releases at pre-eisting weak layers or in layers where water accumulates.

Possible weak layers:

  1. Water pooling in/above snow layers
  2. Buried weak layer of surface hoar
  3. Buried weak layer of faceted snow near surface
  4. Buried weak layer of faceted snow above a crust
  5. Buried weak layer of faceted snow beneath a crust
  6. Buried weak layer of faceted snow near the ground

 

Duration

  • Hours to days, depending on temperature, precipitation and radiatio
  • Rapid loss of stability possible
  • Especially critical as water infiltrates deeper down for the first time after the snowpack has warmed up to 0 °C.
  • Natural avalanches can become more likely in the course of the day, depending on aspect (unless rain is dominating factor).

 

Identification of the problem

  • Usually easy to recognize, the snow surface becomes wet.
  • Onset of rain, snowballing, pin wheeling and small wet slabs or wet loose-snow avalanches are often precursors of natural wet-snow slab avalanche activity.
  • Deep foot-penetration is another sign of increased wetting.

 

Wet slab avalanche. Photo: HåvardT/regObs