Bitterroot

Avalanche problems

• 

  Loose Wet

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Very Likely

  Size

  1–2

  Wet snow avalanches are very likely today on sun-affected slopes. Temperatures today have the potential to reach the low 40s at upper elevation. Warm temperatures, combined with the strong spring sun, will cause the snowpack to rapidly lose strength. Overnight, clear skies helped with a refreeze, but this refreeze is likely superficial and will break down rapidly.

  The typical progression of wet loose avalanches involves rollerballs, and small point releases, leading to larger wet loose avalanches. If you encounter rollerballs or small wet loose avalanches, expect larger ones to follow.

  We are trending towards conditions where wet slab avalanches are possible across the forecast area. In isolated terrain today, wet loose avalanches have the potential to break out into a slab as they move down the slope. If you see this, be especially cautious around avalanche terrain.

  When dealing with wet snow:

  - Making conservative terrain choices when dealing with a warm, wet snowpack is essential.

  - Be alert for rapidly changing conditions. Morning is often faster and safer, so plan your day so you are out of avalanche terrain before the snow becomes wet and weak.

  - Aspect is important. Generally, southeast slopes warm and destabilize first, followed by the south and then the west.

  - Changes can occur quickly, sometimes as fast as minutes, but definitely within an hour

  - Avoid steep sun affected slopes with wet snow

  - Warm temperatures and sun can cause cornices to fail naturally, so be cautious of overhead cornice hazards

  - In the evening, even if temperatures are cool, it can take hours for the heat to escape the snowpack. Avalanche danger can remain elevated into the evening.

  

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Unlikely

  Size

  1.5–2.5

  A poor snowpack structure with facets surrounding a crust in the middle of the snowpack still lingers. This problem has been unreactive and had not produced avalanches in weeks, until this week when a group of skiers triggered a persistent slab avalanche in the southern Bitterroot in an area where the snow above the crust is especially weak.

  Today, you are unlikely to trigger a persistent slab avalanche, except on specific slopes where this problem is more developed. You will most likely trigger a persistent slab avalanche on convexities, rollovers, or near rock bands. Before triggering, you will unlikely see obvious signs of instability, such as cracking and collapsing.

  Identify this problem by digging in the snowpack. Look for a crust buried by 2-4 feet of snow with a thin layer of weak snow above or below the crust. If you find this, use slope angle to avoid the problem by sticking to slopes under 30 degrees in slope angle. If you choose to ride in terrain with this persistent weak layer, reduce exposure by choosing a smooth planar slope free of terrain traps or trees in the runout.