Triggering a wind slab avalanche 2- 3 feet deep is likely today on lee aspects (SE-NW) of terrain recently exposed to wind.  Wind slabs that were 2 feet deep were found to be very reactive on lee aspects of test slopes in the Catchers Mitt area on 12/14.  These were failing on faceted snow, meaning they will likely stay reactive longer than your typical windslab.  Visibilities were limited in the area on 12/14.  Although, several D2-2.5 natural avalanches were observed on southerly aspects with average crown depths of 2-3 feet with max depths of 6 feet observed.  Windslabs were generally found to be 4 finger to 1 finger in hardness with shooting cracks traveling up to 20 feet in length.  The hazard for this avalanche problem will be less in areas that were protected from the recent outflow wind event beginning 12/11.  In these areas triggering a persistent slab avalanche (see problem 2) will be possible in terrain steeper than 32°.

 Wind slab avalanches will have the potential to step down into deeper faceted layers in our snowpack, (see problem 2) and produce larger avalanches.  Avoid recently wind loaded slopes in terrain steeper than 32°.  The hazard for this avalanche problem will be slowly decreasing as wind speeds decrease and the snowpack adjusts to this change..

There is a 2-3+ foot deep slab sitting on the November facets.    These facets have continued to show easy to moderate propagation results in ECT and PST stability tests.  Red flags such as shooting cracks, large collapses and natural avalanches have been present in the Thompson Pass area lately.  This is a bullseye indicator that our persistent avalanche problem is still active.

 Very strong northeast winds have recently loaded lee aspects, further stressing facets in these areas.  In areas exposed to wind, the slab overlying the November facets have variable depths.  These range from scoured down to the facets (little to no slab) on windward aspects to 2-4 feet deep on lee aspects (dense wind slabs).  In areas exposed to recent winds the avalanche problem is confined to specific areas (lee aspects).

 In areas sheltered from the wind the slab thicknesses are more uniform.  In these area the persistent avalanche problem is widespread.  A skier or snowmachine may be able to directly affect these weak layers and could trigger avalanches in terrain steeper than 30°.  With this type of avalanche problem it is also possible to trigger avalanches remotely from flat terrain.  At the present moment this problem layer is 2-3 feet deep in most locations.  Red flags such as collapsing and shooting cracks are a sign that slopes steeper than 30° could produce an avalanche.

The only successful mitigation practices for a persistent slab avalanche problem are patience, and a conservative approach in choosing terrain.  Good protocols are very important as well during this type of avalanche problem ie: only exposing one person at a time to an avalanche prone slope, good communication and having an escape route should a slope fail.

Photo of Remote triggered/sympathetic avalanches on 12/8 

1cm+ chained facet found buried 40 cms (16″) on Catchers Mitt 3500′ SE aspect.