Continental

Avalanche problems

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  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1–2

  Moderate NE winds have been active in mid to upper elevations over the last few days.   A combination of older, more settled snow surfaces along with moderate wind speeds has not allowed deep wind slab development, but rather formed shallow wind slabs on the lee side of terrain features, high elevation ridgelines and cross loaded gullies.  These slabs have been found to be stubborn to unreactive to triggers over the last couple of days.  

  The formation of wind slab adds cohesion to the surface snow which can increase the sensitivity of a weak layer where it is present (see problem 2).  There is a good example of this from a report yesterday of a skier triggered avalanche that occurred in the mid elevations of Loveland Basin on a South aspect.  The avalanche was around 2 feet in depth and about 50 feet wide and, judging by the depth and aspect, likely failed on the weak layer described in problem 2 prompted by increased cohesion of the slab above caused by wind.  More on this in problem 2.  Direct observation and photo not available.

  Expect for these wind slabs to gain strength over the coming days and become less and less reactive in places where they do not overlie a persistent weak layer.  Watch for signs of instability such as shooting cracks that would indicate wind slabs still have the energy needed to produce an avalanche.

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1.5–2.5

  For the last 10-plus days, we have been tracking a layer of buried surface hoar covered by 2 feet of snow near the coast that decreases to only 4 inches at the far northern end of our forecast area.  Numerous snowpack observations have found that this layer is laid over and mixed with other grain types on shaded aspects along the Thompson Pass road corridor and not reactive in stability tests.  This allows for a trend towards lower likelihood for avalanche propagation where this process is occurring.  This has been reiterated by the hundreds of tracks that have been laid down in our forecast area over the last week with very little human triggered avalanches observed or reported.  However, isolated pockets of reactive surface hoar remain possible in our core forecast area.

  A skier triggered avalanche on a solar aspect described in problem 1 shows that triggering this problem is still a possibility along the Thompson Pass road corridor.  There is a melt freeze crust beneath the 3/16 buried surface hoar layer on South aspects that likely acted as an effective bed surface and may have prevented the absorbtion and mixing of the layer with the snow below that has been observed on W-N-E aspects in other locations.  It is reccomended to avoid steep south facing terrain this time of year as solar input is weakening the snow surface (problem 3), they have recently recieved wind loading (problem 1), and some slopes may harbor buried surface hoar above a bed surface.

  We remain concerned about the buried surface hoar layer, primarily in terrain outside of our forecast area, where there have been reports that this layer is more preserved and has shown signs of being reactive in snowpits and human triggered avalanches.  Unfortunately, details on how recent human triggered avalanches have occurred are unavailable. However, poor stability results in snowpits continue to be reported East of Marshall Pass as recently as 3/29.

  Buried surface hoar can persist and remain active for an extended period. Any area where well-preserved crystals are found below the overlying slab may be capable of producing avalanches for quite some time. Remain diligent and assess the snowpack on a slope-by-slope basis in remote or unfamiliar terrain.  Natural settlement of the overlying slab, temperature increase, and wind can all increase cohesion, which can increase the sensitivity of this weak layer where it is present.

   

   

   

  

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  Loose Wet

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1–2

  Wet loose hazard is becoming more of a concern in the Maritime zone.  Temperatures have remained above freezing at sea level for the past 3 days.  Natural wet loose activity is gaining mass on the steep south terrain that is above town.  Expect these avalanches to continue to grow in size and distribution as warm temperatures penetrate deeper into the snowpack.  It is recommended to avoid steep solar aspects during the heat of the day especially in the Maritime zone.  The degree of wet loose activity will depend on if and how quickly the sun breaks through the clouds today.

  Sun and temperature crusts have formed on solar aspects that will initially prevent wet loose activity.  Once the heat of the day breaks down these crusts, wet loose activity will become possible.  The first area to become active will be the steep rocky terrain where the sun's energy is more effectively absorbed by the dark color of rocks and then transmitted into the surrounding snow surface.  Pay attention to where the sun is in the sky; wet loose hazards will peak on a slope when the sun becomes perpendicular.

  Steep terrain that produces wet loose activity will also have the potential for slab avalanches to fail naturally or be human-triggered (problem 2).   Avoid steep terrain that is being heated up by the sun.  Even small slopes can be dangerous if they are above terrain traps or exposure.  

  Pay attention to what effect the sun is having on the snow surface.  Avoid exposure to areas with wet snow during the heat of the day, this includes the terrain above you, not just the slope you are travelling on.