Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Likely
- Size
- 1.5–2.5
A dangerous snowpack structure exists in all three forecast zones at and below 3000'. A stout rain crust forms an excellent bed surface, and buried surface hoar sits above it. What varies by zone is the slab on top.
The Maritime zone is the primary concern today because the snow above the weak layer is deep and cohesive enough to act as a slab. In the Intermountain zone, including Thompson Pass, we’ve found roughly 10–12 inches of new snow above the weak layer, but the snow remains relatively low-density and uncohesive, making triggering less likely than in the Maritime zone. In the Continental zone, 4–8 inches of new snow covers the weak layer and is not yet sufficient to create a major problem.
This layer is not expected to heal quickly. As the snow above it settles, stiffens, and deepens with additional snowfall and wind redistribution, sensitivity is expected to increase. Because the weak layer is still relatively close to the surface, signs of instability—shooting cracks and collapsing—may be present where triggering is likely. Identifying where this problem exists can be straightforward: if you can probe a firm rain crust beneath the new snow, assume buried surface hoar is present above it. Avoid steep or complex terrain below 3000' where you find a cohesive slab at the surface, especially in the Maritime zone.
Wind Slab: North winds increased overnight on Thompson Pass. Watch for small, fresh wind slabs in specific wind-loaded terrain features. Be especially cautious if you encounter wind slab formation below 3000', where it may combine with or add stress to the buried surface hoar problem.
1/26 BSH found under 1 foot of new snow. Girls/ S/ 2300'/2mm grid
Photo: Gareth Brown