Continental

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1–2

  Strong North winds with gusts to 80 mph are expected today.  There is very little snow available for transport in our forecast area, which lowers the impact of this wind event compared to last week's outflow that triggered a widespread natural avalanche cycle.  Most areas received a trace- 2 inches of new snow yesterday that will act as the ammunition for todays windslabs to form.  Expect these new slabs to be shallow but sensitive to human triggers.

  Pay attention to the depth and sensitivity of freshly deposited snow and avoid terrain traps and slopes with exposure.  An additional concern revolves around the uncertainties of the reactivity of the 1/26 layer underneath old wind slabs (see problem 2).

   

   

  

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  2.5–3

  The upper snowpack has drastically changed over the last week driven by extreme North winds.  The 1/26 persistent weak layer that has produced multiple rounds of avalanche activity has gone from a widespread problem with a fairly even slab depth distribution of 2-3 feet, to one that is now more of a specific-isolated problem due to wind stripping and loading.  On windward slopes the overlying slab has been completely removed, and the weak layer destroyed in areas that received the highest winds.   On lee slopes, deep wind loaded deposits have built the depth of slabs over this weak layer.  These wind slabs at this point are rock hard and stubborn to unreactive to triggers.  During a typical wind event these slabs would be very unlikely to trigger, however with weak faceted snow beneath old hard slabs, triggering an avalnche remains a possibility.  Several observations of significant collapses over thick wind slabs corroborate the potential that this layer is not yet dead.

  Not only has the character of the slab above the 1/26 PWL changed but also its sensitivity.  As described above wind slabs that formed last week are now pencil to knife hard.  This bridges a skier or snowmachines weight to some extent, but a high degree of uncertainty exists with the sensitivity of the weak layer moving forward.  It is possible that a person or machine could still effect this weak layer where wind slabs are thinner or near their periphery.  

  The main points we would like to convey to users is that:

  1- The problem has moved from widespread (exists everywhere) to specific (lee slopes or terrain that was protected from outflows)

  2- Sensitivity has decreased due to hard wind slabs capping the weak layer.

  3- Consequences have increased.  Triggering an avalanche at this layer would result in a dangerous hard slab.

  Users can reduce their risk by avoiding large connected alpine faces and slopes that have been previosly loaded by wind.

   

   

   

   

  

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Unlikely

  Size

  2.5–3.5

  A deeply buried weak layer near the ground remains a low-likelihood, high-consequence problem—especially in steep, rocky terrain and areas where the snowpack is thin or highly variable. Triggering this layer directly is unlikely at this time, but avalanches that start in the upper snowpack could step down to the basal weak layer.  There is a high degree of uncertainty regarding poor structure at the base of the snowpack.  What we can be sure about is that this layer is less of a concern in the Maritime zone and more so as you move towards the Continental zone where the snowpack is thinner and the weak layer is more developed.

  Watch for thin spots near rock outcrops, scoured features, and along ridge tops. Wind loading adds stress to these shallow areas, and we don’t know where the tipping point is.