Sawtooth & Western Smoky Mtns

Problemi valanghivi

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

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

  Probabilità

  Likely

  Dimensione

  1–2

  What is it?   You are likely to trigger sensitive 2-3 foot wind slabs in exposed middle and upper elevations. Yesterday a group of snowmobilers remotely triggered the avalanche pictured to the right/above (report). Over the past week, multiple large natural and human-triggered avalanches released in wind-loaded terrain in the Sawtooths and Smokys. On Saturday, a rider in the Pole Creek drainage triggered a large wind slab that broke above them on the slope they were riding (observation here). Importantly, this slide occurred before the added load from recent snow and wind.

  Where is it? You'll likely find slabs in upper-elevation terrain, near ridgelines, and mid-slope terrain features like ribs, chutes, and couloirs. The rocky nature of the Sawtooth Mountains allows new snow to run down the sides of cliffs and build up additional weight in the aprons (similar to the picture). In these locations, it is also likely to trigger slab avalanches today.

  How can you recognize it? Yesterday I experienced shooting cracks extending out from my skis, which is an obvious sign of instability. Watch for drifts, dunes, and ripples on the snow surface. Wind slabs can have a smooth, chalky appearance and feel and sound hollow, like a drum. Wind-deposited snow will be stiffer and keep your skis or snowmobile on or closer to the surface. Some of these slabs might be masked by recent snow and be more hidden.     

  How can you manage it? You can significantly reduce your risk today by steering clear of wind-loaded terrain. Today's slabs might not behave like your "typical" wind slab. Various weak layers exist in the snowpack's upper 1-2 feet (watch the video at right/above). Because of this weak snow, today's slabs may break wider and deeper than expected from a "simple" wind slab.

  ADDITIONAL DISCUSSION: The layers of weak snow in the snowpack's upper 2-2.5 feet are being pushed toward their breaking points. Up to this point, we've needed the additional weight and stiffness from wind loading to make avalanches on these layers. Wind slabs and loose avalanches (sluffs) can step down into these deeper layers, releasing a larger avalanche than expected (see the photo above/right for an example). You are more likely to trigger these avalanches on slopes greater than 35 degrees in sheltered slopes.  

  

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

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

  Probabilità

  Possible

  Dimensione

  1–2

  What is it?  1-2 feet of recent snowfall is overloading layers of weak snow in the upper 3 feet of the snowpack—sugary facets, crusts, or crust + facet combinations—on some slopes. You may be able to trigger slab avalanches in protected terrain. You could trigger these avalanches remotely, from gentle terrain above, below, or to the sides of steep slopes.   

  Where is it?  This problem is most likely in middle and upper elevation terrain because more snow fell there in the past two days. The weak layers’ appearance and “behavior” will change over short distances; minor differences in aspect, elevation, and exposure to the wind created variability in the crusts and facets underneath the newer snow. You’re most likely to find trouble on slopes steeper than 35 degrees.  

  How can you recognize it?  Assume this problem exists until proven otherwise. You may experience snowpack collapsing and/or cracking while traveling. These red flags are obvious signs of instability. Snowpits and snow stability tests are of limited use due to the weak layer variability over short distances.        

  How can you manage it?  You can reduce your risk by avoiding slopes steeper than about 35 degrees—especially large avalanche paths and those with ugly consequences if you are caught in an avalanche. These slides may break wider than expected or higher on the slope above you. The first skier or rider may not trigger an avalanche on an unstable slope. Do not assume tracks mean a slope is safe.