Bitterroot

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

  Large wind slabs are possible on upper elevation north and east aspects today. With up to 4 inches of new snow forecasted and moderate wind from the southwest, anticipate fresh small wind slab formation. Triggering a 1-2 ft deep wind slab formed within the last 36 hours is also possible. Lingering wind slabs could still be found lower down on slopes than normal or in middle elevation cross loaded terrain from very strong southwest winds that blew on Thursday and Friday night. A wind slab avalanche could step down and trigger a larger persistent slab avalanche as described below.

  Wind slabs can be found on or just below the downwind side of exposed ridgelines, behind rocky outcrops, on rolls or convexities, or in cross loaded terrain features such as gullies. They can look smooth, rounded, and pillowy.  Snow will feel more dense and consolidated underneath your skis or sled than surface snow that has not been wind affected. It may even be hard and sound hollow, like a drum. Be on the lookout for rippled, uneven snow surfaces, indicating that wind has transported snow, and wind slabs may be lurking nearby.

  Red flags include cracking, collapsing, and recent slab avalanches. Cracks shooting out from your skis or machine clearly indicate that you've found an unstable slab, however, if the snow is very hard, you may not see cracking before its too late. Stick to wind sheltered slopes less than 35 degrees if you find signs of instability.

  

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  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Hochgebirge
  • Waldgrenze
  • Unterhalb der Waldgrenze

  Wahrscheinlichkeit

  Unlikely

  Größe

  1.5–2.5

  Large persistent slab avalanches are unlikely on all upper elevation aspects today. There is a variety of persistent weak layers buried in the snowpack, including rounding facets near the ground, small facets above and below a crust in the middle of the snowpack, and buried surface hoar. 

  Recent snowfall and wind loading throughout the week added stress and tested the strength of the buried weak layers. Additional load from new snow and wind today could be what finally tips the scale. You will unlikely see obvious signs of instability, such as cracking and collapsing, before triggering an avalanche. Smaller avalanches, such as fresh wind slabs described above, can step down and trigger a larger avalanche on persistent weak layers.

  Dig down into the snow, and if you find weaker snow around hard crusts, or loose sugary snow below a firmer slab, recognize the potential for a large and dangerous avalanche. Use your individual or group risk tolerance to dictate how steep of a slope you are willing to expose yourself to. You can reduce your risk by defaulting to shorter, planar slopes with runouts free of surface obstacles and terrain traps. Use slope angle to avoid the problem altogether by sticking to slopes less than 30 degrees.