Sawtooth & Western Smoky Mtns

Lawinenprobleme

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Hochgebirge
  • Waldgrenze
  • Unterhalb der Waldgrenze

  Wahrscheinlichkeit

  Possible

  Größe

  1.5–2.5

  Sunday, a skier triggered a 250 foot wide slab avalanche on a north facing, wind-loaded alpine slope in the Pioneer Mtns (photo 1, photo 2). You're most likely to trigger a similar, 1-3 foot thick avalanche where a dense slab rests above weak, faceted snow and crusts. Last week's extensive observed avalanche activity focused on N-NE-E-SE facing slopes at middle and upper elevations (near and above about 8500' in this zone). Many slopes in these areas have both thick slabs and suspect crust + facet combinations.

  You can use your boots, hands, or poles to quickly check for "old snow"—crusts and/or weak, grey, sugary facets—beneath a dense slab. Steep slopes with a > 1 foot thick slab above "old snow" are the most likely places to find trouble. While slabs are larger and more widespread in obviously wind-loaded terrain, triggering an avalanche is also possible on more sheltered slopes. 

  Cracks shooting out from you and "whumpfing" sounds as the snowpack collapses are red flags that indicate unstable conditions. If you experience these obvious signs of instability, avoid steep slopes with similar appearances, aspects, elevations, and snowpack depths. Unfortunately, you may or may not observe obvious signs of instability before triggering an avalanche. Avalanches could be wider than you'd expect, and you may be able to trigger them remotely—from gentler terrain above, below, or to the sides of steep slopes.

  Additional Discussion : Yesterday and last night's snowfall and wind did not signifcantly change the avalanche equation, but you may find fresh, thin wind slabs in exposed upper elevation terrain. The fresh slabs may have buried a new weak layer—facets, crusts and facets, or possibly surface hoar—and could be easy to trigger. The biggest threat these thin slabs will pose is triggering larger Persistent Slab avalanches as the small slides move downhill.