Persistent Slabs
- Alpin
- Limite forestière
- Sous la limite forestière
- Probabilité
- Possible
- Taille
- 1.5–2
Several weak layers are buried in the upper 1-2.5 feet of the snowpack. On slopes that face away from the sun, you'll find some surface hoar and facets at these interfaces. On slopes that face the sun, you'll find crusts with some small facets here. Moderate to strong winds this week created a batch of stiff wind drifts in terrain exposed to the wind, building slabs on top of these buried weak layers. Thursday, skiers on Titus Ridge found this combination of unstable slab+weak layer when they remotely triggered a small wind slab from 50 feet away (photo here). Wednesday, a wind slab released naturally south of Della Mountain, west of Hailey (photo here). Cracks shooting out from your machine or skis are clear indicators of an unstable slab. Small slabs that knock you off your feet and send you downslope can have high consequences in the wrong terrain.
Further down in the snowpack, you'll find layers of weak snow near the ground. Triggering an avalanche on these layers is most likely in wind-affected alpine terrain where scouring has produced thinner areas of the snowpack (watch Ethan's video at right/above). Here, you'll have better access to this weak snow, and triggering a slide involving it remains a concerning possibility. Any avalanche that fails on these deeply buried weak layers will be large and destructive. In the mountains above the Wood River Valley, the majority of the snowpack has faceted and triggering an avalanche on deeper weak layers is unlikely.
Avalanches failing on persistent weak layers can break well above you on the slope and may break wider than you expect. You can significantly reduce your risk by steering clear of wind-affected, rocky, alpine terrain steeper than about 35 degrees.
ADDITIONAL DISCUSSION: Small loose snow avalanches will become possible as the sun warms up the snow surface and melts existing crusts. Pay attention to the snow you are traveling through and look for roller balls and pinwheels as indicators that the surface snow is heating up. These slides start small but may pick up speed and snow quickly as they travel downslope.