Sawtooth & Western Smoky Mtns

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1–2

  Yesterday, a skier triggered a wind slab avalanche in the southern Sawtooths while entering steep, exposed terrain. Today, triggering similar avalanches remains likely.

  What and where is it?  In alpine terrain, slabs could release naturally, are larger, and are most widespread. Recent wind and snowfall formed slabs and drifts up to 3 feet thick in upper and some middle elevation locations, and they will continue to grow today. Some slabs overlie crusts and/or thin layers of weak, faceted snow; the wind slabs could be easy to trigger and break wider than expected on some slopes. 

  How can we recognize and manage this problem? Fortunately, the fresh wind slabs are fairly easy to spot and avoid. Watch for smooth pillows below ridgelines and cornices as well as on the sides of cross-loaded gullies. In the craggy Sawtooth Mtns, you may find slabs where "spindrifted" snow has accumulated below large cliff faces and couloirs. Dunes and ripples on the surface are clues the wind has been at work transporting snow. If you feel the snow surface getting stiffer, it's time to pay attention. Cracks shooting out from your skis or sled are an obvious sign that you've found a wind slab. Hard wind slabs sound hollow, like a drum, and behave unpredictably; they can break above you and wider than expected. 

  Additional discussion: It is possible trigger 1.5-2’ thick avalanches in terrain sheltered from the wind. These avalanches could fail on a variety of old snow surfaces, such as crust, facets, or a combination of both. Additionally, loose snow avalanches (sluffs) are possible in steep terrain. Sluffs could trigger larger slab avalanches lower on aprons and in cirques.

  

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Unlikely

  Size

  1.5–2.5

  While unlikely, we can't write off triggering large avalanches in middle and upper elevation terrain while the snow keeps piling up. Unfortunately, you probably won't receive any obvious signs of instability before triggering one of these lingering monsters. We are most concerned with a layer of weak facets found near the ground on middle and upper elevation slopes, especially those facing away from the sun.

  Triggering a slide on this layer is most likely in rocky, heavily wind-affected areas where the snowpack depth is variable, and in more sheltered terrain where the snowpack is around 3 feet deep (read the Forecast Discussion for details). Smaller wind slab avalanches or sluffs (Problem 1 above) could "step-down" as they travel down slope, triggering much larger Persistent Slab avalanches.