Continental

Lawinenprobleme

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  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Hochgebirge
  • Waldgrenze
  • Unterhalb der Waldgrenze

  Wahrscheinlichkeit

  Possible

  Größe

  1–2

  North outflow winds that occurred for about an 8 hour period on the evening of the 15th through the early morning of the 16th have come to an end.  Winds have already switched around to the South with the approach of the next weak system.  Wind slabs up to a foot in depth formed during this period in specific areas and will be our main concern today.  The sensitivity of these will be decreasing from yesterday but will still be possible for a person or machine to trigger.  Light snowfall today is not expected to increase the avalanche hazard.

  Watch for signs of previous wind loading like textured snow surfaces, and fresh windlips that will point towards wind loaded slopes.  Wind slabs are easier than other avalanche to avoid because they can be visually identified unlike persistent slabs and they only exist in specific areas unlike storm slabs.  Wind slabs often have a rounded pillowed look, or may have a rippled texture.  Using small inconsequential wind loaded test slopes is a good way to test the sensitivity of this problem.  Look for cracking on these test slopes that would indicate wind slabs would have the energy to produce an avalanche on a larger slope.

  There is a small chance that wind slab avalanches could step down to deeper layers in the snow in isolated areas and create very large avalanches. (see problem2)

  

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  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Hochgebirge
  • Waldgrenze
  • Unterhalb der Waldgrenze

  Wahrscheinlichkeit

  Unlikely

  Größe

  2.5–3.5

  Prior to last weekends snowfall event there were multiple persistent weak layers that were being tracked, including a couple of layers of buried surface hoar and a rain crust sandwich with facets in the middle.  These exist primarily in the mid elevation band and a couple of them were beginning to show propagation in stability tests as snow steadily accumulated from the 4th-10th.  The 11th-12th brought 100" of snowfall to Thompson Pass in 48 hours which overloaded these layers in many areas with natural and artillery triggered avalanches the result.  

  These layers are now deeply buried by 1.5-2 meters or more of settled storm snow.  As precipitation has taken a break, the snowpack is allowed to settle and slowly accept the weight of this new snow.  Over time, this decreases the likelihood of human triggered avalanches at these layers due to their depth and an overall decrease in sensitivity.  

  It is now unlikely to affect these deep layers and trigger one of these avalanches.  We are leaving this problem on our list for the time being as a conservative approach until we can test a wider variety of areas to confirm better structure and lower sensitivity than what existed prior to last weeks storm.

  If this problem were to arise it would probably be in an area where the recent storm snow was thin due to wind redistribution during the storm, with steep convex slopes in the mid elevation band.  Thin, rocky areas in steep terrain could be a place where trigger points exist as well.