Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 1–2
Slabs 1 to 2 feet thick rest on buried weak layers of surface hoar, faceted snow, and hard crusts. Incremental snowfall since January 28 has gradually overloaded these weaknesses, creating a concerning snowpack structure. The character and distribution of the buried weak layers vary somewhat between the Silver Valley-Bitterroots and the northern ranges, including the Selkirks and West Cabinets. These differences reflect variations in rain–snow elevation, storm totals, aspect, and periods of warm, sunny weather.
As additional snow accumulates this week, the added weight will push these weak layers closer to their tipping point. Because loading has occurred slowly and incrementally, pinpointing when storm slab danger will rise to a more serious level is challenging. A practical benchmark is storm totals: if you encounter more than about 6 inches of new snow, it’s time to reassess your plan and give avalanche terrain a wide margin.The potential to trigger a large, dangerous avalanche on these persistent weak layers should guide decision-making. Persistent slabs are notoriously difficult to evaluate and predict, especially during periods of changing conditions, and warrant a conservative approach.
Surface hoar on top of a crust with weak facets below the crust, all sitting below a consolidating slab. This is a bad recipe for recreating in steeper terrain. New snow today will fall on a weak surface.
West Cabinet Mountains. February 5, 2026.
Photo: Chris Bilbrey