Sawtooth & Western Smoky Mtns

Problemi valanghivi

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alta quota
  • Limite del bosco
  • Sotto il limite del bosco

  Probabilità

  Likely

  Dimensione

  2–3

  Yesterday, snowmobilers remotely triggered a Persistent Slab avalanche north of this zone near Bull Trout Lake (photo), and riders remotely triggered cornices and slabs beneath them on Osberg Ridge (details).

  You're most likely to trigger a wide, 1.5-4 foot thick slab avalanche on steep slopes that tilt away from the afternoon sun (NW-N-NE-E-SE) where you find wind drifts, slabs, or wind-stiffened snow at the surface. Avalanches could fail on multiple weak layers (facets, surface hoar, crusts + facets or surface hoar) buried by the series of February storms. While wind-loaded terrain is most widespread at upper elevations, Wednesday's strong W-NW winds formed slabs or stiff snow surfaces on more middle and lower elevation slopes than usual.

  Persistent Slab avalanches can release in surprising ways:

  •  They can be much wider than expected (view many recent examples here).
  • They can break above you and pull into relatively gentle terrain.
  • You can trigger them remotely—from flat terrain below, above, and to the sides of steep slopes.  

  Watch for dunes, waves, and textured surfaces created by wind. Wind-driven snow and slabs feel stiff. Snowpack cracking and collapsing are clear signs of unstable conditions.

  Unfortunately, the best way to avoid trouble is to give avalanche terrain a wide berth. This includes gentler terrain below and above steep slopes. 

  

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alta quota
  • Limite del bosco
  • Sotto il limite del bosco

  Probabilità

  Likely

  Dimensione

  2

  Up to 2 feet of recent snowfall and strong wind built 2-4 foot thick drifts and slabs, predominantly at upper elevations. You could trigger avalanches where obvious weak layers do not exist beneath the slabs. Expect these drifts to be the largest and most sensitive on slopes facing N-NE-E-SE. 

  Look for them near or below ridgelines, along mid-slope ribs, in cirques below large cliff faces, and on aprons beneath chutes. Fresh cornice growth, dunes, and drifts are all signs that the wind has been at work. If you suddenly feel the snow surface getting stiffer, you've found a wind slab. 

  These slides are formidable on its own and may also serve to trigger the larger and more dangerous Persistent Slab avalanches described above.