Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Likely
- Size
- 2–3.5
Triggering thick, meaty slabs remains likely on wind-affected middle and upper elevation slopes, and is possible in sheltered terrain. Slabs may fail on either weak, sugary facets buried 2-5 feet deep or on crusts in the upper 3 feet of the snowpack.
The facets: Faceted snow is buried near the ground on W-NW-N-NE-E-SE-facing slopes at middle and upper elevations, and the facets need time without snowfall or wind-loading to strengthen. Many large to monstrous-sized avalanches released naturally in the past few days (view a list of recent avalanches and photos here). Many of these avalanches would have been unsurvivable. On Wednesday in the Boulder Mtns, a skier remotely triggered a very large avalanche involving this layer. The slide broke ~1/4 mile wide and averaged 3-4 feet thick. You're most likely to poke this beast 1) in windy and/or rocky areas where the snowpack depth is variable and 2) in more sheltered terrain where the snowpack is about 3 feet deep.
The crusts: Several rain and temperature crusts formed in the past 10 days and are now buried about 1-3 feet deep. Triggering avalanches involving these crusts (photo) is most likely on slopes where the wind drifted snow or stiffened the snow surface.
How can we manage this problem? Sadly, avoidance is the best option to reduce your risk. Assume a slab + weak layer setup exists on all slopes. You may or may not experience obvious signs of instability—like snowpack collapsing or cracks shooting out from your skis or sled—before triggering an avalanche. You may be able to trigger avalanches remotely—from flat terrain below, above, and to the sides of steep slopes.
(12/24/2025) This very large avalanche was remotely triggered from low-angle terrain adjacent to a large alpine bowl. Spring Ck in the Boulder Mountains. NW @ 10,200'. Photo: Ben VandenBos
