Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Likely
- Size
- 2–2.5
We are seeing our third avalanche cycle that is failing on the same Jan 2 persistent weak layer (PWL). Depending on location, this layer is composed of buried surface hoar, faceted snow, and/or a crust/facet combo. This layer is buried 1 to 3 feet below the snow surface in wind sheltered areas and up to 6 feet deep in wind loaded areas. Seven persistent slab avalanches were reported to occur yesterday, all of which were triggered by backcountry travelers moving through low angle non-avalanche terrain adjacent to steeper avalanche terrain. This type of remote triggering could easily occur again today.
There are also signs that the weak faceted snow and depth hoar that exist near ground level at the bottom of the snowpack and has been dormant since late December is waking up due to the amount of new snow load received over the past week. There is a slight chance today that an avalanche that fails higher up in the snowpack on the Jan 2 PWL could step down to this deeper weak layer. Additional new snow loading tomorrow will increase the likelihood of avalanches failing near ground level in the snowpack.
Recent avalanches, snow surface cracking, snowpack collapse, or whumpfing sounds will indicate unstable persistent slabs in the immediate area. These avalanches could be remotely triggered from low angle terrain above, below, or off to the side of a steeper slope. Conventional wisdom would suggest that there would be less instability today than yesterday. This particular weak layer combined with wind drifting of snow leading to continued post-storm loading will challenge that convention. Choose backcountry terrain and route selection very carefully today. Very conservative terrain choices that maintain extra wide margins of safety are recommended.
(1/11/2024) This avalanche was remotely triggered by snowmobilers riding in adjacent low angle terrain. NE aspect near 7,900', in an area NW of Independence Lake, CA
