Deep Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 1
Despite the lack of new snow the snowpack continues to change as it sits on the ground. The layer of weak sugary snow (facets) below the surface continues to lose strength. In most places, little to no slab layer exists above this persistent weak layer and avalanches remain unlikely. However, in some specific areas where a slab layer does rest on top of the persistent weak layer, evidence indicates that the snowpack has lost enough strength for small human-triggered persistent slab avalanches to become possible. These areas include places on NW-N-NE aspects where hard wind slabs exist in near and above treeline terrain and some isolated areas in below treeline terrain where smaller softer slabs may exist. Persistent slabs avalanches can be triggered from the bottom of a slope or other connected slopes. They often break above the person who triggers them and may fail after several people have used a slope. They often do not fail until the trigger has fully committed to the slope.
Clues like whumphing and collapsing, shooting cracks, hollow sounding snow, or digging into the snowpack to find a loose weak layer of snow below a harder layer of snow can indicate that conditions conducive to persistent slab avalanches may exist. Sometimes these clues may be difficult to find. Despite the fact that these kinds of avalanches should remain small, they could still have serious consequences in areas with terrain traps or if they carried a person into/over the numerous shallowly buried or exposed obstacles. Avoid steep slopes where slabs may exist on top of loose weak snow. Choosing sheltered terrain where no slabs exist will avoid this problem and provide softer snow to recreate on.