Deep Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 1
The prolonged dry period combined with a shallow snowpack has allowed a layer of loose weak sugary snow (facets) to form on northerly aspects across the forecast area. In most areas, no slab layer exists above this persistent weak layer. However, isolated slab layers do exist on some NW-N-NE aspects in the form of hard wind slabs in near and above treeline terrain and some softer slabs in below treeline terrain. Observations over the last week have indicated that in those isolated areas where the combination of persistent weak layer with a slab on top of it exists, the weak layer has weakened to the point that it can barely support the snow above it. Human triggered persistent slab avalanches may be possible in those areas today. Despite the light precipitation amounts associated with this storm, some of these isolated persistent slabs may become easier to trigger during the night as rain and snow add a bit more weight to the snowpack.
Signs that conditions conducive to persistent slab avalanches may exist include: human-triggered cracking, collapsing or whumpfing, a hollow sounding snowpack, feeling a softer weaker layer of snow below the surface with a probe or pole or hand pit, unstable snowpit tests, and any other signs that a slab may exist above weak faceted snow. Clues to instability may be difficult to find unless just the right combination of slab over weak layer exists, and an isolated human-triggered avalanche may represent the first clue to an unstable slope. Use safe travel protocols when traveling in and around avalanche terrain. A shallow snowpack with numerous barely covered and still exposed obstacles would magnify the consequences of any size avalanche.
