Blues

Problèmes avalancheux

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpin
  • Limite forestière
  • Sous la limite forestière

  Probabilité

  Likely

  Taille

  1–2

  Moderate to strong south, southwest, and west winds forecasted for today accompanied by a foot or more  of new snow will create large,  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  slopes steeper than 30 degrees  today that you suspect are wind loaded ! Also, lets ride one at a time today on steep slopes and keep our eyes on our partners.

  

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpin
  • Limite forestière
  • Sous la limite forestière

  Probabilité

  Possible

  Taille

  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.