Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 2–2.5
A 2 to 3-foot-thick cohesive slab overlies two buried surface hoar layers. One got buried at the end of January (1/28) and the other in mid-February around Valentine’s Day (2/14). The slab above has stiffened and settled due to wind, warmer temperatures, and natural settlement. As the slab has strengthened, the likelihood of human-triggered avalanches has decreased, and natural avalanches have become unlikely. Although the snowpack is trending in the right direction, the potential consequences remain significant if one of these persistent weak layers is woken up and an avalanche occurs.
Snowpack feedback and signs of instability in the form of audible collapses, shooting cracks, or worrisome test results have diminished, and there will likely be little to no warning before you trigger a large avalanche. Shaded, more wind-protected slopes that favor the northern half of the compass are the more likely places where looming persistent weak layers will be more intact. Areas with a thinner or softer overlying slab may act as trigger point and the slide could expand to where the slab is deeper. Navigate around slab margins, rocky slopes, and steep convex rolls where snow depths are likely shallower if traveling through avalanche terrain.
Annotated photo showing the depths of our primary weak layers of concern - two surface hoar layers (4 to 6 mm in size). They now sit below a mostly pencil hard slab around the slope-specific tree line. Selkirk Mountains. March 5, 2026.
Photo: Chris Bilbrey