Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 1.5–3
Expect to find 1-2 foot thick slabs in areas sheltered from the wind. The slab may be twice as thick in terrain affected by the wind. These avalanches have failed on various weak layers: facets and/or surface hoar on shadier aspects and crust + facet combinations on sunnier slopes. Large natural and human-triggered avalanches have occurred on several aspects in recent weeks. Assume this problem exists on all upper and middle elevation slopes and on shadier, lower elevation aspects.
Avalanches on this uncommonly widespread and fragile layer will break widely and in surprising ways. Don't expect obvious signs of instability, like snowpack cracking and collapsing. Your first sign of trouble may be the destructive avalanche you trigger. These slides can be triggered from long distances away—from flat or gentle terrain below, above, or to the sides of steep slopes.
(1/24/2024) This slope collapsed and then returned unstable snowpack test scores. This was a mostly wind-sheltered location. Snowpacks like this produced nearly continual collapsing. In wind-affected terrain, the slab was harder and thicker (up to 2') and more difficult to collapse. When initiated, collapses traveled hundreds of feet. Sydney Butte, Soldier Mtns, N aspect at 6,900'.