Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Likely
- Size
- 2–3
What and where is it? The recent snowfall, rain, wind, and warm temperatures built thick + dense slabs above weak, faceted snow and crusts on many middle and upper elevation slopes.Since Saturday, we and other observers saw several natural and human triggered slab avalanches big enough to bury people (view Avalanches, view Observations). Some avalanches were over 2 feet thick and up to 1000 feet wide. All of the observed avalanches occurred on N-NE-E-SE facing slopes above 9000'. Most occurred in obviously wind-loaded terrain. The recent weather probably created unstable slopes both below and above 9000', on other aspects, and in terrain sheltered from the wind.
How can you recognize it? Triggering a large avalanche is most likely where 1) the wind has formed drifts or stiffened the surface and 2) old, weak snow exists below the recent wind-drifted snow. Be on the lookout for stiff, smooth pillows of snow on the more sheltered sides of ridgelines and large terrain features. You can use your boots, hands, or poles to quickly check for "old snow"—crusts and / or weak, sugary facets—beneath the more recent storm or wind-driven snow.
How can you manage it? Avoid steep, wind-loaded terrain. Cracks shooting out from you and "whumpfing" sounds as the snowpack collapses are red flags that indicate unstable conditions. Avoid all steep slopes where you experience these obvious signs of instability. You may be able to trigger avalanches remotely—from gentler terrain above, below, or to the sides of steep slopes.
(12/8/2025) Large avalanches on E facing upper elevation slopes on McDonald Peak in the southern Sawtooth Mtns.
Photo: Ben VandenBos
