Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 1.5–2.5
We're in the realm of low likelihood, high consequence avalanches. Areas where the snowpack remains unstable are isolated. Due to this isolated distribution, the vast majority of backcountry travelers will not encounter areas of unstable snow today.
If you do wander into an area of where instability remains, you might trigger snowpack collapse, whumpfing, or a large avalanche with no preceding signs of instability. Keep in mind that triggering a collapse is nearly the same as triggering an avalanche. The difference being that the slope was either not steep enough or the slab was sufficiently anchored to not avalanche at the time. This was the case yesterday where an experienced party reported triggering several large, loud collapses between 8,300’ and 9,400’ while ascending NE to ENE aspect terrain in the North Fork Galena Creek drainage, NV.
Most people will not see signs of unstable snow today, nor will they trigger avalanches. Still, triggering a persistent slab remains possible in isolated areas. These spots are most likely to exist on NW-N-NE aspects, especially in areas where a shallower snowpack exists (think northeastern, eastern, and southern portions of the forecast area). Triggering one of these avalanches would likely be unexpected, surprisingly large, and consequential. Choosing simpler terrain and wide safety margins may help reduce the consequences, while lower-angle terrain could help avoid the problem.
(12/18/2024)
Pointing at the lingering persistent weak layers of concern, Dec 12 facets and basal facets/depth hoar. All the snow above the right hand was accumulated during the last storm cycle. N aspect, 8,960', Price Peak NV
Photo: Sierra Avalanche Center