Continental

Avalanche problems

• 

  Storm Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1–2

  Overnight snowfall totals look to be fairly consistent at 7-10 inches as of 5:30 am across our forecast zone.  Another 4-8 inches is expected during the short daylight hours today.  This snow came in with strong North winds that switched to South around midnight and are forecasted to be decreasing.  Temperatures are also expected to rise, but a strong spike is yet to be seen.

  Expect new snow to be sensitive today, especially in areas where North winds have created deeper and stiffer slabs.  Increased cohesion (stiffness) can often be subtle during storm events but can make a big difference in stability.  Picture the terrain that will be loaded today, will be similar to what we see during outflow (North) wind events.  Cross-loaded gullies, convex terrain and slopes steeper than 30° where new snow has been loaded by wind will be the most likely place to trigger an avalanche.  

  This new snow is loading a weak snowpack, triggering a deeper and more destructive avalanche will be possible in specific areas( see problem 2).  Today, conservative terrain choices and guarded route finding will be important for safe travel in avalanche terrain.  The hazard is expected to remain elevated as moderate snowfall continues to build the depth of storm slabs.

  

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  2–3

  We have two distinct persistent weak layers that are separated by elevation.  The first is the 12/1 rain crust that goes up to 3000'-3500'.  The second is weak snow (December facets) that exists beneath rock hard wind slabs above 3000'.  I'll discuss them separately, as their distribution, load thresholds, and type of avalanche they could produce differ.

  12/1 Rain Crust: This weak layer has been recently responsible for numerous signs of instability, including shooting cracks and large-scale collapses.  It exists in all areas below 3500', except for those that received the strongest winds in December, like the terrain in close proximity to Thompson Pass summit.

   So far, there has not been a cohesive slab above this weak layer except for areas of isolated old wind slab.  There is now around 2 feet of new snow from last week.  This layer is reaching threshold and is a major concern, but the elevation that it exists in puts it in thick alders in the majority of locations.  This thick vegetation acts as anchors and breaks up the slab, preventing wide propagations in a lot of terrain.  Areas of concern are slopes below 3500' that are free of vegetation and hold a cohesive slab.  A good example of this terrain is the buttresses between the Gullies 1,2. and 3 on the east side of Thompson Pass.  

  December Facets:  Above 3000', where the rain crust does not exist, the December winds had a much greater scouring effect on the snowpack.  Large areas of terrain were blown to the ground, while lee slopes collected thick wind slabs that are now knife hard.  December was not just windy but dry and very cold.  This weather promoted faceting beneath these hard slabs and we are left with an extremely variable snowpack, including some areas with very poor structure.  When we mention the December facets we are not talking about a specific weak layer, but rather the entire snowpack is comprised of weak snow beneath the old hard wind slabs.  This weakness is being capped by these very strong wind slabs and will be difficult to assess and to trigger, but if intiated would produce very dangerous hard slab avalanches.  Trigger points may exist along the periphery of these hard slabs were they are thinner and a person could penetrate to the weaker snow.  There is significant uncertainty regarding this avalanche problem, but we can assume that the more snow that accumulates on top of this setup, the more trigger points will arise.