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

  Warm temperatures and intense sunshine are causing significant settling in the upper portion of the snowpack. As we move into a period of climbing temperatures we focus our attention on our buried persistent weak layers. There are two broad categories:

  1. Weak layers that formed at the end of January and during the month of February are now buried 1-3' 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, but they have also produced slides on shaded aspects (photo at right).
  2. A pile of large-grained, weak snow (depth hoar) exists near the ground in this zone. 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.

  If I were to go and try to make an avalanche today I'd be looking in steep, rocky terrain where the snowpack is thinner and where it is being heated by the sun. Just the thought of that makes me cringe. 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 continue to 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 loose snow avalanches. These sluffs can be dangerous on their own and may trigger larger slab avalanches as they travel downhill. Scott observed this exact combination yesterday in the Boulder Mountains (observation here, photo here). The heat is also affecting the large cornices that have formed throughout the season. These beasts sag during the heat of the day, eventually tumbling off the ridges they are attached to. This makes for a "great" trigger for the persistent weak layers described above.

  

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1–2

  Over the weekend, the combination of new snow and wind built slabs in terrain where the wind blows. Continued wind transport was observed yesterday. These wind slabs are becoming less sensitive to the weight of a skier or rider, but some are sitting atop weak snow surfaces (facets and/or crusts) and remain touchy.  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. You are most likely to trigger a wind slab in steep, rocky terrain and below cliff bands (photo at right). Even small slabs can have high consequences in this type of terrain.