Northeast winds reached speeds of 40 mph on Thompson Pass 12/18 and are currently losing strength.  There was a small amount of loose snow available for transport that built fresh shallow slabs atop older/ denser wind slabs.  Fresh slabs that formed on 12/18 will have the possibility to be triggered by a person, although should be relatively small in size.  Although, even small slides will have the potential to step down to deeper layers in our snowpack and create larger avalanches.

There is still the possibility that older wind slabs that are overlying facets could be reactive.  The chances of affecting these larger wind slabs is decreasing, but small localized collapses have still been reported so these deeper slabs need to stay on our radar.  Digging snowpits can be a good way to assess the snowpack structure in areas where old dense wind slabs are overlying facets.

There is a 1-3+ 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.  Recent benign weather has allowed our snowpack to slowly adjust to its load and triggering a persistent slab avalanche is becoming less likely, although remains a dangerous possibility.  

 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 has 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 persistent avalanche problem is confined to specific areas (lee aspects).

 In areas sheltered from the wind the slab thicknesses are more uniform.  In these areas 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 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.