Sawtooth & Western Smoky Mtns

Avalanche problems

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  2–3

  Our snowpack is plagued by two prominent persistent weak layers:

  1. Facets and isolated surface hoar buried 1-2' down: The train of storms that brought significant snowfall across our region buried a weak layer that has produced avalanche activity in all of our zones. It was the likely culprit in a remotely-triggered slide in Beaver Creek last weekend. Thanks to the nature of these layers and the ongoing wind-loading, triggering an avalanche on these layers is likely today.
  2. December facets, now buried 3-4' down: As this layer becomes more deeply buried it becomes harder to impact. Obvious signs of instability (like collapsing of the snowpack) become less frequent, the first sign of instability may be a large, unsurvivable avalanche. Unfortunately, it has shown few signs of healing. This layer is trending towards a deep persistent slab problem: difficult to trigger with terrible consequences if you do.

  We are in the midst of a multi-day wind event that is producing strong W to NW winds. These winds are loading many slopes at middle and upper elevations, keeping these weak layers on edge. Avoiding slopes over 35 degrees and/or those that have been affected by the wind will greatly reduce your chances of triggering a persistent slab avalanche. Keep in mind that these slides can be triggered remotely—from below, above, and to the sides of steeper terrain.

  

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1–2

  If you were outside yesterday you probably saw some snow being blown around. This has been the theme for the past 4 days now and will continue to be a factor today. Where the wind is depositing this blowing snow, wind slabs are being built. A few recent observations indicate that these slabs can be touchy:

  • Yesterday, a group of riders in the Soldier Mountains triggered a 1-2' thick wind slab on a south-facing slope at 7,500'.
  • Chris was above Prairie Creek on Saturday and watched the wind rapidly loading a SE-facing slope. He remotely triggered a small(ish) wind slab avalanche from about 50 feet away (video at right/above).

  Wind slabs will be most common and dangerous along upper elevation ridgelines and in alpine terrain, and can be easily identified by surface texture and by their pillowed, rippled, or dune-like appearance (look for these in the video at right/above). From an avalanche danger perspective, the larger concern is still the additional loading on the persistent weak layers described above. Even small wind slabs can be great triggers for more deeply buried weak layers. Any avalanche that steps down to these persistent weak layers will be large and destructive.

  ADDITIONAL DISCUSSION: The ongoing wind event is moving plenty of snow and there has been some significant cornice formation in the past few days. Yesterday, I found large, fresh cornices in the Banner Summit Zone that were easy to trigger, often toppling off the ridge with a single kick (from a safe distance, of course). As the wind continues, these cornices will be failing naturally, tumbling down the slopes below. These dense blocks can penetrate deeply into the snow as the cartwheel downslope, providing a great trigger for both shallow and deeply buried weak layers.