Bitterroot

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1–2

  Moderate west winds are drifting 4 to 8 inches of new snow into fresh reactive wind slabs at upper elevations. Skiers and riders are likely to trigger these wind slabs near ridgelines and along the edges of gullies. These wind slabs will likely be small in most places but could be large enough to bury you in bigger upper elevation terrain. The wind slabs may be especially sensitive where they lie on weak near surface facets that formed earlier this week. Some older wind slabs could linger from shifting winds earlier this week, but they will be increasingly stubborn. 

  Cornices, wind lips, and drifts all indicate wind direction and loading. Stay alert for shooting cracks and smooth pillows of drifted snow. If you see these, stick to slopes less than 30 degrees in steepness to avoid avalanches. Remember that even small avalanches can knock you off your feet and carry you into or over hazards. A small wind slab also has the potential to step down and trigger a larger persistent slab avalanche. Wind slabs will continue to grow tonight and tomorrow as another storm impacts the region.

  

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1.5–2.5

  It remains possible to trigger a large persistent slab avalanche that fails on weak layers buried 2-5 feet deep. The buried weak layers include surface hoar, near surface facets, and crust/facet combinations. 

  • The surface hoar is the most concerning of all the weak layers. Take some time to understand surface hoar formation to better assess where buried surface hoar is possible. Likely areas are wind sheltered terrain, lighly gladed slopes, and areas lower on the slope than what you might expect as the classic start zone. It often presents as a thin grey line across a snowpit wall. 
  • Areas with a thin or breakable crust with facets below is the second concern. The sensitivity of this problem is more uncertain than the surface hoar. 
  • The buried near surface facets have shown the best bonding with the new snow and the least reactivity in tests but are still suspect. 

  You may not see any red flags before triggering an avalanche on these layers. The problem has transitioned to low probability, high consequence conditions. Observations have shown intermittent propagation in tests, but don’t let that fool you into thinking there is no danger. You are more likely to trigger one of these persistent slabs in areas where the snowpack is thinner, like around rock bands or convex rollovers. 

  To identify the problem, you can dig down in the snow and look for poor snowpack structure with a slab above weak snow. Even if snowpit tests are unreactive, poor snowpack structure should be enough to give you pause before committing to avalanche terrain. Stick to slopes under 30 degrees in slope angle and without steep terrain overhead if you simply want to avoid the problem.