Arctic air and strong to very strong northeast winds have set in.  Winds slabs 3 feet deep were reported yesterday in wind channeled terrain that were sensitive to  human triggers on wind loaded rollovers.  As wind speeds continue to increase, areas further away from the top of Thompson Pass will begin to form new wind slabs on lee aspects.  High elevation ridgelines that are experiencing winds of 60+ MPH  will likely see snow blown into the air and sublimating (turning from solid to gas state) before landing on lee aspects.  Slabs that do form off high elevation ridge lines will likely be located further down from ridges than you may expect.

 Wind slab avalanches will have the potential to step down into deeper faceted layers in our snowpack, (see problem 1) and produce larger avalanches.  Avoid recently wind loaded slopes in terrain steeper than 32°.  The hazard for this avalanche problem will be increasing over the next 48 hours, especially in areas away from the top of Thompson Pass where new wind slabs are forming

There is a 2-3+ foot deep slab sitting on the November facets.    These facets have continued to show propagation in ECT and PST stability tests as recently as 12/11.  Very strong northeast winds are loading lee aspects, further stressing facets in these areas.    It is possible that we will see some natural avalanche activity on southerly aspects in the next 48 hours.

 On 12/8 a remote triggered/sympathetic avalanche event (see observation section) is a clear sign that our persistent slab avalanche problem is hanging on by its fingernails.  This is especially true in wind sheltered areas.  In locations were old wind slabs exist above the November facets, weak layers may be more stubborn to triggers, due to bridging.  But the same weak layer exists and dangerous human triggered avalanches are possible in these locations as well.  

The 24 hour rule will not apply for our current setup.  A common question to this problem is “What will it take to heal the snowpack?”.  To understand the answer to this question it is important to understand how this problem formed.  Without getting too in depth, a faceted snowpack forms when the snowpack is  shallow and temperatures are cold for an extended period of time.  Unfortunately, snow can turn to facets (weak snow) much quicker than it can turn back into rounds (strong snow).  There are a few things that need to occur for our snowpack to heal.  The quickest fix would be a very large warm and wet Pacific storm that delivers significant amounts of dense snowfall to our region in a short period of time (> 3 inches of SWE in < 24 hours).   The result would be a widespread natural avalanche cycle that would clean out the areas where the snowpack is the weakest.  This is commonly referred to as a (Crush and Flush event).  Another way for our snowpack to heal which is much slower and requires a lot of patience, would be for incremental storms to build the depth of our snowpack along with an increase in temperatures.  This combination would promote sintering (creation of rounds or strong snow) in our snowpack, this is a very slow process.  Unfortunately, neither one of these scenarios is in the short, or long term forecast.  Our dangerous snowpack is here to stay for now…

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.

Some important things to remember about a persistent weak layer problem is that tracks are not a sign of stability.  A slope can fail after many tracks have been laid down.  Remotely triggered avalanches are possible (triggering a slope above, below or adjacent to your position).  Pay attention to the terrain above you and what the consequences would be if that slope were to fail.  

Photo of Remote triggered/sympathetic avalanches on 12/8 

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