Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 1–2.5
The dry weather in January resulted in Near Surface Faceting on many shaded slopes at and below treeline throughout much of the Central Cascades. In these areas, recent storm snow fell on this weak faceted layer. This has resulted in a poor snowpack structure on shaded slopes (primarily NW - NE). Structures vary across terrain and even across the same slope, making this problem difficult to assess. Where the weak layer is present, it is more likely to lack the strength to support the stronger new snow above it. With more snow coming through Wednesday, we will see this layer get burried even deeper and receive additional loads on top of it. This is worrisome. By end of day Tuesday, observations show that this layer is likely to be anywhere from 18" to to 24" below the snow surface in these areas. With close to a foot of new snow by end of day Wednesday, this preserved weak layer could result in large, potentially fatal avalanches. It is important to remember that it may be easier to initiate a persistent slab avalanche where the new snow (ie slab) is shallower, and it will be more consequential where the slab is deeper.
Persistent slab problems act differently than our typical wind slab and storm slab problems, with increased spatial variability, greater likelihood to be triggered remotely, and often wider propagation. This demands a wider safety margin and careful consideration when recreating in the backcountry. Collapses, or audible "whoomfs" are clear warning signs this problem exists, along with shooting cracks on a slope. The most reliable way to manage Persistent Slabs is through conservative decision making with terrain choices. Avoid travel on or below steep terrain where this problem exists. Unlike direct action avalanche problems (i.e. wind slab, storm slab), persistent slabs may even get weaker over time and more likely to trigger.
