Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Likely
- Size
- 2–3
Slab depths over fragile persistent weak layers are generally 2 to 3 feet deep. This means, avalanches breaking in any one of our suspect weak layers below - multiple surface hoar layers or faceted crusts - are capable of producing an avalanche large enough to bury, injure, or kill a person. These dangerous slabs slowly formed during incremental snowfall in February, and have more recently reached a critical mass threshold. The most recent snow load did not provide the major gut punch to the snowpack we had hoped for to provoke a widespread natural cycle. Instead, the snowpack on many slopes was pushed toward the edge without completely falling apart, and now teeter there waiting for a rider or machine to tickle it in the wrong spot.
As the overlying slab got thicker and became more connected across terrain features, we observed an uptick in size for both natural and human-triggered avalanches. Clear warning signs like long-shooting cracks and audible collapses have returned and these clues of unstable snow are also being reported well away from the slope that avalanches. This classically describes the unpredictable, remote-trigger behavior of Persistent Slab avalanches. Because of this, it’s not enough to simply avoid traveling on steeper slopes, but it’s also imperative to avoid traveling in lower-angled terrain with steeper, bigger hazards directly above.
This large avalanche that failed on a buried persistent weak layer was triggered by throwing a block of snow over the ridge and onto the start zone. This speaks volumes to the fragile nature of the snowpack, and the sensitivity will only increase with new snow and strong winds.
February 22, 2026.