Wet snow (slab avalanches)

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.