Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Likely
- Size
- 2
A dense, cohesive slab exists on top of a layer of weak, faceted snow at the base of the snowpack. This layer exists at all elevations on slopes facing W-N-E, and on SE aspects near and above treeline that still held snow prior to the storm. The danger is greatest on steep slopes near and above treeline that face NW-N-E, but human triggered avalanches are possible on all aspects where this structure is present. This problem will be with us for awhile even though signs of instability are decreasing. In fact, it now becomes even more scary as you aren't sure which slopes will slide and which won't. In some cases, a slope can be skied or ridden multiple times before it releases. Avoidance of slopes steeper than 30 degrees remains the best course of action, especially those slopes that have a northerly aspect.
Photos below illustrates the structure of a strong, dense slab over weak, faceted snow.
Photo below illustrates the results of an extended column test performed on a west aspect at 10,300' on November 27. (ECTP 11)