Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Unlikely
- Size
- 1.5–2.5
Slabs of cohesive snow 1-3 feet thick overlay weak, faceted snow that formed during a dry period in November. While unlikely, the potential exists to trigger an avalanche at this interface on northwest through northeast aspects above 8,500’. Collapsing the weak layer in a shallow portion of the snowpack, especially in very steep and unsupported terrain, is the best way to trigger one of these avalanches. Keep in mind that a slope can be ridden several times before a particularly thin, weak spot is impacted.
The snowpack in the Greys River forecast area varies significantly between the Salt River, Wyoming, and Snake River Ranges. As a result, slopes where the persistent slab problem can be found will differ slightly from one range to another. What remains consistent is the fact that avalanches can happen anywhere weak, sugary snow sits beneath a stiff slab. Digging into the snowpack is the only way to know for sure if this problem exists. Trenching your snowmobile or probing with a pole to feel for hardness changes (hard snow over soft snow) are easy ways to identify this layer. Remember that a slope 50 yards away from the one you just dug on will likely hold a different snowpack.
Yesterday’s 2-4” of snow combined with moderate winds out the west have created very shallow soft slabs (3-6”) on leeward aspects above 10,000’. These slabs are mostly harmless to humans except in extreme terrain where being knocked off one's feet could carry severe consequences.
(12/21/2023)
Here is a good example of the terrain where triggering a persistent slab is most likely. These avanches occured during out last storm cycle.
Photo: Alex Drinkard