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

  As we move away from the loading event that brought improved skiing and riding conditions to our area, we focus our attention on the persistent weak layers that are buried in the snowpack. There are three broad categories:

  1. Last week's storm buried a variable snow surface. In most portions of this zone, the storm didn't bring enough snow to build much of a slab on top of these layers yet. In areas where the wind has built slabs, these layers may make those wind slabs easier to trigger (Problem 2 below).
  2. Weak layers that formed at the end of January and during the month of February are now buried 2-4' deep. These layers have produced avalanches across most of our forecast area in the past two weeks (Dollarhide, Galena Summit, Smiley Creek, Beaver Creek, Sawtooths, Banner Summit). The pattern of activity outlined by these avalanches points to these layers being most unstable on slopes that face the sun, where they are accompanied by crusts. The photo at right/above shows a slide that likely failed on one of these layers.
  3. A layer of large-grained weak snow (depth hoar) exists near the ground in this zone. Triggering a slide on this snow is a low probablility/high consequence type of problem. You are more likely to trigger slides on the weak layers higher up in the snowpack, but any slide triggered on this deeply buried weak snow is likely to be unsurvivable.

  As their name implies, persistent weak layers tend to stick around for quite a long time. As days pass without loading, the snowpack is getting some time to rest, which will gradually decrease the likelihood of triggering an avalanche on these weak layers. However, the possibility of triggering a slide here will be on the table for some time to come. If you choose to enter avalanche terrain, be alert that at least one of these weak layers almost certainly exists in the snowpack underneath you. Even as the danger decreases we still advise cautious and conservative terrain selection. Be alert to the consequence of being caught in a large avalanche in the terrain you are recreating in.

  ADDITIONAL DISCUSSION:Today's warm temperatures and bright sunshine are a perfect combination for generating loose snow avalanches. Expect to see sluffing of the new snow as the surface warms up. The rocky terrain of the Sawtooths can produce large avalanches in these conditions. These slides will be dangerous on their own and could trigger larger slab avalanches as they travel downhill.

  

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1–2

  On Saturday, Chris observed more than a half-dozen natural slab avalanches in the northern Sawtooths. Most of these had released on the aprons below cliff bands (photo above/right). Slabs like this are typical in the Sawtooths where the wind often blows spindrift down cliff faces, overloading the slopes below.

  When low-density snow is blown into wind slabs and drifts, the difference in texture may be subtle. Today, drifted areas may still feel like powder, but may have just enough added cohesion that you could trigger a slab avalanche. In addition, as the sun warms the snow for the first time, the low-density powder will transform into denser, more cohesive snow. Slabs may be forming atop weak old snow surfaces (facets and/or crusts) making them more reactive to your weight. Recognize that wind-loading also adds stress to the weak layers buried in the snowpack (described in Problem 1 above). Any wind slab you trigger could step down to these deeper layers, producing a much larger, more dangerous avalanche.