Persistent weak layer - slab avalanches

At danger level 2-moderate: Keep distance to each other and to release areas. NB, remote triggering is possible.

At danger level 3-considerable: Avoid steep terrain (more than 30 degrees) and in runout zones. NB, remote triggering is likely.

Make very conservative route choices, especially in unknown terrain, after snowfall and if temperatures rise. 

Identifying areas where the weak layer is present might be difficult and requires experience.

Danger signs are whoumpf-sounds, cracking and recent avalanches. However, absence of signs does not mean it is safe.

Characteristics

  • The avalanche problem is related to the presence of persistent weak layers in the old snowpack.

  • These weak layers form in cold weather and typically include buried surface hoar, depth hoar or faceted crystals

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

  • Avalanches can become very large

  • Fracture propagation over long distances is common; remote triggering is possible. Avalanches can release above the triggering person.

 

Spatial distribution

  • The avalanche problem can be widespread or quite isolated. 
  • Can often be found in specific elevations, expositions or terrain features.
  • Faceted crystals and depth hoar often exist in all aspects.
  • Buried surface hoar is more frequent on shady, wind sheltered slopes.
  • Will typically release in terrain between 30-40 degrees.

 

Trigger mechanisms

Avalanche releases when loading exceeds the strength of the weak layer. This can happen due to:

  • Fracture in the weak layer occurs either due to increased slab weight (loading), i.e. due to new snow, wind loading or rain.
  • Weakening of the weak layer. This can happen because the faceted crystals keep evolving in cold weather or if melting water penetrates into the weak layer.
  • The slab above the weak layer starts to creep. This causes increased stress on the weak layer (increased shear force).

 


 

Location of weak layer in the snowpack

The weak layer can be found anywhere in the old snowpack, often deep inside the snowpack. When deeply buried, however, triggering becomes less likely, but the avalanches will become bigger.

Possible weak layers:

  1. Buried weak layer of surface hoar
  2. Buried weak layer of faceted snow near surface
  3. Buried weak layer of faceted snow above a crust
  4. Buried weak layer of faceted snow beneath a crust
  5. Buried weak layer of faceted snow near the ground

 

Duration

  • Weak layers can persist for weeks to months; possibly most of the winter season.
  • Can exist in specific terrain features/pockets even after most other areas have stabilized.
  • Can often have "sleeping" or "hibernating" periods and become active again, typically in spring when the overlying slab becomes soft.

 

Identification of the problem

  • Persistent weak layers are very challenging to recognize.
  • Signs of instability such as whumps are typical but not necessarily present. Lack of such signs does NOT mean it is safe.
  • Typical danger signs:
    • Whupmf sounds
    • Shooting cracks
    • Remote triggering
    • Recent avalanche activity
  • Information on snowpack history is critical and reference to the published avalanche report is important.
  • Stability tests can be helpful to detect the persistent weak layers. However, to interpret the test and to find a good place to dig requires a lot of knowledge. 

 

Soft slab on top of persistent weak layer. Photo: NVE