On 3/9-10 very strong north winds affected our forecast area.  Prior to the latest wind event very little snow was available for transport due to strong outflows on 3/2-4.  This has limited the depth and extent of fresh wind slab development. The snow that was available for transport was mostly small amounts of near surface facets that had formed as old wind damaged surfaces have slowly broken down due to diurnal recrystallization.  Expect to find shallow wind slabs (less than 1 foot) on the lee side of ridges (SE-NW) and cross loaded terrain, deeper slabs(> 1 foot) may exist in isolated locations.  These shallow wind slabs have been deposited on near surface facets, which may make them sensitive to human triggers in isolated-specific areas.  As mentioned above the limited mass of redistributed snow has prevented extensive new slab development.  On 3/10 strong winds were observed on Thompson Pass with very little visible flagging present. 

Human triggered avalanches are possible up to 1 foot in depth where fresh wind slabs are present.  Watch for signs of instability such as shooting cracks and avoid slopes that have been recently loaded by north winds.

The recent temperature inversion from the last few days has broken down allowing temperatures to drop below freezing at mid and upper elevations.  This will decrease the sensitivity of persistent weak layers near the surface. The 2/23 layer of buried surface hoar exists in isolated locations and is most likely to be encountered on south aspects at mid elevation.  This has been shown by the recent natural avalanche activity that has been occurring on south aspects in the mid elevation band over the last week.  This layer is not likely to exist in close proximity to Thompson Pass or other wind channeled terrain, as it was likely stripped away by outflow winds before it could be buried on 2/23.

Other persistent layers of concern exist in the mid snowpack, but are unlikely to be affected by human or machine triggers due to their depth. Large triggers such as cornice and ice fall have been responsible for producing very large avalanches over the last two weeks.  It is important to be aware that these weak layers exist and realize the most likely place to affect them will be in areas where the snowpack is thin and these layers are closer to the surface.  The weak layers mentioned above include the 1/14 buried surface hoar which is likely the responsible failure plane of some very large recent avalanches triggered by cornice fall and has been observed at a depth of 70-150cms.

Another layer we are tracking in the mid snowpack is the 1/25 rain crust/ facet combo.  This layer exists below 2500′.  Collapsing in the Intermountain and Continental zone has been observed at low elevations failing at this layer.  Natural and human triggered avalanches have not been reported at this layer and stability tests continue to show a lack of significant results and rough shears.  Even though human triggered avalanches are unlikely at this layer, it is still worth investigating if choosing to travel in steep consequential terrain at low elevations.

 Several large avalanches resulting from cornice fall have been reported and observed over the last 2 weeks.  Cornices have been observed and reported as being larger than normal and poorly attached.  Cracks on ridge tops showing cornices separating from ridge lines have been observed and reported.  Cornices have continued to grow in size with strong winds.

Cornices are an objective mountain hazard that should always be a concern.  Although, recent activity strongly shows that we need to keep an extra cautious approach to avoiding overhead exposure to cornices.  Avoid traveling below or on top of cornices.  The avalanches that cornice fall has recently produced have been very large and likely unsurvivable.

Recent outflow winds have stripped many areas, which has led to an overall decrease in the mass of our snowpack.  Some areas did see depositions add to the depth of the pack, but the strength of winds we received has done more scouring, stripping and sublimating than redistributing and depositing.  One effect of this, other than damaging riding conditions is that depth hoar is now closer to the surface.  Depth hoar has been dormant nearly the entire season and has not been a major concern except during major loading events.  The most likely place to affect these deep layers are in areas where the snowpack is thin and a person or machines weight can directly effect this weak snow.

 The overall snowpack has become thinner over the last 2 weeks in most locations.  This may increase the possibility to see full depth avalanches once significant loading events occur, or the suns energy is able to penetrate deeper into the snowpack on south aspects.  Another affect of depth hoar being closer to the surface is that these grains that are currently rounded may be moving toward faceting as temperature gradients increase.   Currently affecting this layer remains unlikely.  Although, may become an issue later in the season especially if our snowpack continues to be eroded by strong wind events, and an extended period of clear and cold.

This weak snow mentioned above was created by early season cold temps, dry conditions and strong north winds.  Poor structure near the base of the snowpack has been observed since late November.  Mild temperatures and consistent snowfall during mid winter allowed this weak snow to slowly round and gain strength.  This weak snow has been dormant through the majority of the season with human triggered avalanches being unlikely at this layer.  As the sun has come out on 2/20 for the first time in weeks several very large natural avalanches  occurred with cornice and ice falls being the triggers.  Human triggers remain unlikely at this layer, although these events show that large triggers can affect this weak snow.  Cornice fall will be the most likely trigger to affect this weak snow, although it is possible that a large group could have the same affect.  Avoid overhead exposure to cornices.

In most location facets near the ground have been found to be rounding and unreactive in stability tests.  In thin areas of the snowpack basal facets have been found to be more developed.  If you find it is possible to push a ski pole to the ground in areas you travel, assume that a weak faceted snowpack exists in that location. These areas could act as a trigger point.