Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 1.5–2.5
What and where is it? 1.5-2.5 foot thick slabs lie above persistent weak layers: crumbly facets or crust + facet combinations. Until proven otherwise, assume the old, weak layers exist near the bottom of the snowpack on all upper elevation slopes and on shadier middle elevation slopes (W-NW-N-NE-E aspects). Slopes where the wind recently formed drifts are especially suspect.
How can you recognize it? On shadier aspects, you can easily feel the old crusts and sugary facets with your hands, ski pole, or snowmobile track (see photos and videos below). On sunnier aspects, the weak layers present as a series of crusts and facets and are not as obvious.
How can you manage it? Sadly, the only surefire option is to avoid avalanche terrain where denser slabs rest above the weak, older facets and crusts. Short of that, you can reduce your risk by avoiding recently wind-loaded slopes, large avalanche paths, and those with ugly consequences if you are caught in a slide. Persistent Slab avalanches can break wider and higher on slopes (above you!) than you might expect. Several skiers or riders can descend a slope, only to have the next person trigger an avalanche.
ADDITIONAL DISCUSSION: Watch for thin, fresh wind slabs near ridgelines in exposed terrain. In the craggy Sawtooth Mtns, slabs also commonly form on the aprons below steep cliffs and couloirs. Although small, these slabs could overlie facets, crusts, or surface hoar that make the slabs easy to trigger. Small avalanches can carry you where you don't want to go and can be effective triggers for much larger avalanches failing on the weak snow near the ground.