Sawtooth & Western Smoky Mtns

Avalanche problems

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  2–3

  Yesterday, a rider in the Beaver Creek drainage remotely-triggered an avalanche large enough to bury a person and snap a large dead tree (photo at right/above). They triggered the slide from 100 feet away on lower angle terrain. Numerous large, natural avalanches occurred in this zone during the prolonged storm last week. Many of these failed within the new snow while others appear to have failed on deeper weak layers buried in the snowpack. It will take time to observe the pattern of avalanche activity, but it is clear that the snowpack here did not hold up well to the loading from the storm.

  To simplify, there are two main weak interfaces in the snowpack:

  1. Weak snow buried underneath the recent storm snow. The character of this layer varies by aspect (the direction a slope faces). On slopes that face the sun, a crust with some weak facets exists (culprit weak layer in the slide pictured at right/above). Other slopes are holding weak snow without a crust underneath and still others are harboring feathery crystals of surface hoar here.
  2. Weak snow buried near the base of the snowpack. This is the layer that, up until this past week, had been responsible for almost all of the avalanches. We've observed over 400 avalanches on this weak layer this year. It produced numerous avalanches during the storm and will continue to do so. It is now buried 3-4' down in this zone and any avalanche that fails on this layer is likely to be unsurvivable.

  If this all sounds complex and dangerous then I am doing a good job of describing what you will encounter today in the backcountry. Persistent weak layers are challenging problems to deal with, particularly when they are irregularly distributed. They fail in ways that surprise even seasoned backcountry travelers. The pattern of these weak layers is complex, but the bottom line is simple: if you choose to enter avalanche terrain today, there is certain to be weak snow underneath your feet and you are likely to trigger an avalanche on that snow. 

  

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1–2

  Strong winds blowing throughout the storm last week, easily building thick drifts in upper elevation terrain. While these slabs are hazardous in their own right, they are also adding stress to the weak snow described above. Any wind slab you trigger has a good chance of triggering a slide on one of those layers of weak snow.

  ADDITIONAL DISCUSSION:Today will be warmer than yesterday, with a good chance for extended periods of sunshine. This combination may produce another round of natural loose snow avalanches within the recent storm snow. These slides are particularly common in the steep, rocky terrain in the Sawtooths. As these sluffs run downhill, they could trigger larger (and much more dangerous) avalanches on the weak layers described above.