In the avalanche bulletin a distinction is made between "wet snow avalanches" and "wet snow avalanches during the day", and these phrases are used to describe the two different avalanche problems.
The avalanche problem is related to weakening of the snowpack due to the presence of liquid water. Water infiltrates the snowpack due to melt or rain.
Expected avalanche types
- Wet‐snow slab avalanches
- Wet loose snow avalanches
- Mainly natural avalanches
When melting due to solar radiation is the main cause, distribution of the problem is mostly depending on aspect. The elevation is mainly depending on air temperature and humidity. All aspects are affected in the event of rain on snow.
Position of the weak layer in the snowpack
Anywhere in the snowpack
Wet‐snow slab avalanches:
- Weakening and failure of pre-existing weak layers in the snowpack or release at layer interfaces where water is ponding.
- Rain represents also an additional load on weak layers.
Wet loose snow avalanches:
- Loss of cohesion between snow crystals
- Hours to days
- Rapid loss of stability possible
- Especially critical as water infiltrates for the first time deeper down, once the snowpack has warmed up to 0 °C.
- Natural avalanches might be more likely in the course of the day, depending on aspect (unless rain is the dominating factor).
How to manage?
Identification of the problem in the field
The wet snow problem is usually easy to recognize. Onset of rain, snowballing, pin wheeling and small wet slabs or loose wet avalanches are often precursors of natural wet-snow slab avalanche activity. Deep foot-penetration is another sign of increased wetting.
If the wet snow surface freezes overnight due to clear skies and cold temperatures and develops a strong supporting crust, favourable conditions will usually be present in the morning. After warm, overcast nights, the problem often exists already in the morning. Normally rain on fresh snow creates this problem almost immediately. Good timing and trip planning are important. Consider avalanche runout zones.