Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Unlikely
- Size
- 1.5–2.5
Although triggering an avalanche involving buried weak snow layers has become unlikely, it can still occur, especially above 8000 feet in terrain connected to very steep (>35°) slopes. This problem type can appear inactive - the last reported avalanche in this zone occurred 9 days ago on January 9th in Cliff Creek - but should remain a concern to backcountry users due to its difficult-to-predict nature. You're most likely to trigger a large hard slab avalanche 1-3 feet deep on the north half of the compass - where snowpack depth is variable. These avalanches are easiest to trigger in spots where the snowpack is thin. Signs of a thin snowpack include wind-scoured snow near ridges, cliff bands, and rocks sticking out of the snow. Signs of instability, such as collapsing and cracking, might not be present before triggering a persistent slab avalanche.
Larger triggers, such as groups, snowmobiles, or falling cornices, will increase the odds of impacting these weak layers. This avalanche problem is currently a low-likelihood but high-consequence scenario. Snowpack evaluation and safe travel practices remain the best methods for avoiding triggering an avalanche involving the old snow layers.
(1/13/2025)
This photo shows an example of where you can trigger persistent weak layers.