Avalanche
- Slab Width
- 61 m
- Weak Layer
- Density Change
- Trigger Type
- Skier
- Avalanche Type
- Hard Slab
- Slab Thickness
- 46 cm
- Trigger Detail
- Unintentionally Triggered
- Start Zone Aspect
- North
- Avalanche Problem
- Wind Slab
- Start Zone Elevation
- 2987 m
With the goal of skiing the N facing, sheltered trees on the W side of the Spire / Dog Dish, my party was skinning up the lower portion of boulder basin along a wind affected subridge. S winds had been moderate, gusting to strong all day, transporting snow at mid and upper elevations. Alpine areas were wrecked by the winds, featuring a medley of sastrugi, hard wind slabs, breakable wind crust, breakable MFC, orange peel, and scoured areas with loose graupel. Treed northerlies held weakened settled powder.
As the second person ascending, I unintentionally triggered a hard wind slab avalanche (HS-ASu-R4-D1) that broke a foot below my skis, propagating 200' across on a convex roll below me and running an estimated 75'. The debris was piled an estimated 2-3' in a gully below. I was standing on 22 degree wind loaded slope and my partner ahead of me was standing on 25 degree terrain. The hard slab breaking at my feet slid a few inches before it self arrested due to low slope angle. Where the slope angle steeped 4-6' down the slab fully released. The angle of the crown where it fully released below me was 28 degrees and 10 feet away where it propagated to the slope angle steepened to 35 degrees. The 18" slab was a pencil hard wind slab failing on 1" of F hard snow (that I suspect was lightly faceted grains). The bed surface was a 4F hard old wind slab a few inches thick overlying a 1F hard graupel layer.
Fortunately, just prior to this avalanche my partners and I had measured slope angles, rerouted to low angle terrain, and moved back and away from the convex loaded roll. This demonstrates the unmanageable nature of hard slabs. They can wrap up into low angle terrain as the slab is so hard and connected.