Persistent Slabs
- Alta montaña
- Límite del bosque
- Bajo el límite del bosque
- Probabilidad
- Possible
- Tamaño
- 2–3
What is it? 1-3 foot thick slabs overlie persistent weak layers (sugary facets, surface hoar, and/or crusts) on many slopes. Recently wind-loaded slopes are the most dangerous. The photo to the right/below is a great example of a medium-sized avalanche that you might be able to trigger in very steep terrain that is somewhat sheltered from the wind, especially in the western half of this zone.
Where is it? Most slopes harbor at least one of several weak layers in the upper part of the snowpack, but you're most likely to trigger avalanches on slopes where the wind has formed drifts or slabs or stiffened the snow at the surface (example photo). Wind-exposed bowls and slopes near ridgelines, prominent gullies, and chutes are most likely to produce large avalanches. Triggering avalanches is far less likely in terrain sheltered from the wind.
How can you recognize it? Assume this problem exists until proven otherwise. You may or may not experience snowpack collapsing and/or cracking while traveling. These red flags are obvious signs of instability. Wind-loaded areas feel stiffer under your skis or sled and keep you from easily sinking into the snowpack.
How can you manage it? Unfortunately, avoiding slopes steep enough to slide is the only surefire way to manage this problem. Persistent slab avalanches are tricky. These avalanches may break wider than expected or higher on the slope above you. In the western half of this zone, you may be able to trigger avalanches remotely—from gentler terrain below, above, and to the sides of steep slopes.
ADDITIONAL DISCUSSION: While triggering slides on faceted snow deep in the snowpack is unlikely, it can't be ruled out in steep, rocky, wind-affected alpine terrain. Slopes like these have variable snow depths, and thinner snowpack areas allow our weight to more easily impact deeply buried weak layers.
(1/1/2023) Debris from a very large avalanche that was remotely triggered above Hyndman Creek in the Pioneer Mountains. This slide failed in a heavily wind loaded area and very likely involved a persistent weak layer. It covered the summer trail with a pile of debris that was 100-150' wide and 4' deep.
