Persistent Slabs
- Hochgebirge
- Waldgrenze
- Unterhalb der Waldgrenze
- Wahrscheinlichkeit
- Possible
- Größe
- 2–3
Snowpit observations and tests still show multiple layers of weak sugary snow buried 4 to 5+ ft below the surface. In wind-loaded areas, even more snow exists above the weak layers. Persistent weak layers have been seen on several aspects sometimes as low as 6800 ft, but they are most prevalent on NW-N-NE aspects at higher elevations. Strong snow exists above these persistent weak layers making it difficult to transmit force down to them. No obvious signs of instability like persistent slab avalanches, deep shooting cracks, snowpack collapse, or whumpfing have been reported since 12/14. Overall data and observations indicate that triggering a persistent slab avalanche gets more difficult each day. Still, uncertainty exists about what it would take to trigger one of these avalanches. A larger trigger like a cornice failure, another avalanche, or multiple people on a slope might produce enough force to cause a persistent slab avalanche. A person finding a trigger point where less snow exists above the weak layers may be able to start a fracture in the weak snow. If something does cause those weak layers to break, the resulting fracture could travel along the layer producing a large avalanche with serious consequences. These kinds of avalanches often break wider and travel farther than expected. They can also be triggered from lower angle terrain, from the bottom of a slope, or from adjacent terrain.
The uncertainty and potential consequences of this low-likelihood, high consequence problem create challenging decision-making situations. Dialing back terrain choices can help make those decisions easier. Traveling on terrain where the weak layers do not exist or in terrain less steep than 30° not connected to steeper slopes represent ways to deal with the uncertainty surrounding this problem.