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

  There are two persistent weak layers of concern in our snowpack:

  1. A thin layer of facets and sporadic surface hoar buried about 2' deep. This layer formed during a week of cold, dry weather, and was quickly loaded by the late-January snowpocalypse. It was the likely culprit in a remotely-triggered slide in Beaver Creek last weekend. Observations indicate that the weak layer is becoming less problematic, but as persistent weak layers heal, it's not an on/off switch. You could still trigger a slide—it just becomes less likely, and it often takes a prime-time slope angle to trigger one (over 35 degrees).
  2. Our well-known layer of December facets buried 3-4' deep. In contrast to our newer weak layer, this layer formed over a long period (3+ weeks) and was very slowly loaded. Because the slab over this weak snow is so thick and dense, it is difficult for us to impact the weak snow.  It would take a dose of bad luck to trigger a deep slab, but it could happen in very steep, rocky, wind-affected terrain where more "trigger points" exist.

  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 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.

  

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1–2

  Welcome to day 3 of our ongoing NW wind event. Yesterday, observers again reported large quantities of snow getting transported. I was above Prairie Creek in the Galena Summit & Eastern Mtns zone and watched the wind rapidly loading a SE-facing slope. Further up the ridge, I remotely triggered a small(ish) wind slab avalanche on this same slope from about 50 feet away. Check out the video (above/right)—it provides an excellent example of how a slope loads, wind slab surface clues, and the end result. In some areas, the wind is running out of "ammo" as windward slopes become devoid of transportable snow. But some portions of this zone received an inch or two of new snow, and this can quickly turn into a foot or more of fresh wind slab.

  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. From an avalanche danger perspective, the larger concern is still the additional loading on the persistent weak layers described above. If a wind-loaded slope fails on one of these layers, the resulting avalanche will be much larger.