Bitterroot

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1–2

  Skiers and riders are likely to trigger large, growing wind slabs at upper elevations today. Heavy snowfall and moderate west winds are forecast today, with accumulations of 3 to 6 inches during the day and 5 to 10 inches tonight. This is on top of the 5 to 10 inches that fell since Friday. 

  Cornices, wind lips, and drifts all indicate wind direction and loading. Wind slabs often appear as smooth and rounded pillows of drifted snow. Expect to find wind-drifted snow just below ridgelines, and in cross-loaded gullies. Wind slabs may be especially sensitive where they lie on weak near surface facets that formed earlier this week. Stay alert for signs of instability, like shooting cracks and recent avalanches. If you see these, stick to slopes less than 30 degrees in steepness to avoid avalanches. Wind slabs also have the potential to step down and trigger larger persistent slab avalanches. 

  Storm slabs could become possible in wind protected areas on all aspects by this evening. If you see signs of instability, stick to low angle slopes.

• 

  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 3-5 feet deep. The persistent slab problem has been trending towards better stability, but the additional weight of heavy snowfall over the next few days could make the buried weak layers more sensitive. The 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. 

  Check out this video from last weekend that talks about conditions from early January until early February and how they relate to the persistent slab problem. The problem has transitioned to low probability, high consequence conditions. In these conditions, there may be no red flags before a large avalanche occurs. 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.