Elkhorns

Problemi valanghivi

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  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

  Moderate to strong south, southwest, and west winds forecasted for today accompanied by a lot of new snow will create new and potentially reactive deposits of wind-drifted snow (windslabs) on leeward facing northwest, north, northeast, and east facing terrain above and below treeline today and especially at ridgelines. Windslabs look like smooth, rounded, large domelike terrain features.Sometimes described as large "pillows" , expect to find them  on leeward facing terrain features at all elevations. Also, windslabs  can sometimes sound hollow or drumlike when traveling across them. Another sign that you might be traveling on an unstable windslab is the observance of shooting cracks (major red flag of instability) projecting out in front of skis or board. Wind affected snow might also have a thick, heavy, or slabby feeling while traveling through it. If you start observing these red flags of snow instability, consider doing your recreating on terrain with a slope angle less than 30 degrees. To minimize or eliminate the risk of triggering a sensitive windslab avalanche today it would be best to avoid avalanche terrain with recent wind deposited snow, especially if weather goes according to the forecasted amounts of snow and wind. Avoid steep slopes today that you suspect are wind loaded !

  

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

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

  Probabilità

  Possible

  Dimensione

  1–2.5

  WAC forecasters continue to see persistent weak layers in the form of buried surface hoar, facets (weak sugar snow), and crust/ facet combos buried in the upper half of our snowpack, throughout the forecast region. When cohesive slabs sit on top of these persistent weak layers, the potential exists for large and destructive  persistent slab avalanches to be triggered by humans. Ways to recognize if you have a persistent slab problem include the observance of collapsing snow or whumpfing sounds on the snow you are traveling through. But lately we haven't been observing a lot of whumpfing or collapsing snow, and with persistent slabs there are no surface clues. Therefore, we have to dig down into the snow and look at the snow structure to find and observe the weak layers. We also have to perform stability tests to get an idea about the strength of the snow as well. As recently as Friday while traveling in the Western Wallowas, I dug a snowpit on a 35 degree NNW facing slope at 7700' . I performed an Extended Column Test at this location. My column failed and propagated using moderate force. The faceted (sugar snow) weak layer that failed was down about 2' from the snow surface. Not all our test columns are exhibiting propagation, but poor snowpack structure coupled with the observance of some propagation is enough to make us uncertain about the overall snowpack strength throughout the region, especially where the snowpack is shallow. These weak persistent layers located below the snow surface will get tested by the large amount of new snow forecasted to fall today and tonight. The newly added weight of snow will put a lot of stress on these buried weak layers. Add to that the weight of a human, that might be enough to make one or more of these weak layers fail resulting in a potentially large and consequential avalanche.Persistent  slab avalanches are the cause of most avalanche fatalities. The best way to eliminate the risk of triggering a large and dangerous persistent slab avalanche is to avoid slopes steeper than 30 degrees in steepness, especially today and tomorrow.  Now is the time to enjoy the good riding conditions that can be found on lower angle terrain.

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

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

  Probabilità

  Unlikely

  Dimensione

  1–3

  We are still seeing poor snowpack structure in our snowpack in the form of basal facets and crust facet combinations near the ground in the Elkhorn mountains and in the Northern Wallowas. These deeply buried persistent grain types are still failing in snowpit stability tests, so the deep persistent problem is still on the table for the Elkhorn Mountains and Northern Wallowas. These deep persistent weak layers that reside at the bottom of our snowpack will get tested today by the additional weight of  new snow that is forecasted to fall today and tonight. The likelihood of triggering one of these very dangerous avalanches is low, but today, the added weight of a human might be all it will take for one of these deeply buried weak layers to fail. The consequences of triggering a low likelihood high consequence deep persistent slab avalanche would be severe and most likely unsurvivable. A deep persistent slab  avalanche can be triggered on steep terrain where the snowpack is thin. Examples of where thin and weak areas are located would be lower elevations, windward side of wind swept ridges, and rocky cliff band areas. The best way to avoid triggering a persistent slab avalanche is to stay off of and out from under slopes steeper than 30 degrees while this problem persists, especially near shallow rocky areas and low elevation locations where the snowpack is relatively  thin.