Blues

Lawinenprobleme

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  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Hochgebirge
  • Waldgrenze
  • Unterhalb der Waldgrenze

  Wahrscheinlichkeit

  Likely

  Größe

  1–2

  Moderate to strong southwest, south, and southeast winds and associated wind slab formation  have been  observed and recorded by observers in the Wallowas, Elkhorns, and Blue mountains Friday and Saturday. While I was in the Spout Springs area in the Blues yesterday, I observed new windslab formation on north facing terrain near the ridge top. The new windslab at that location was 8" thick and resting upon a thin layer of very low density snow. Below that was a thick melt freeze crust. Windslab+weak layer+bed surface, all the ingredients needed to trigger a slab avalanche ! Forecasters and observers in the Wallowas were seeing new windslab formation on leeward terrain and associated red flag avalanche activity such as shooting cracks and small natural avalanches. Forecasted 3"-9" of new snow today accompanied by moderate to strong south, southwest, and west winds will ad increased weight to existing windslabs on northwest, north, northeast, and east facing leeward terrain throughout the day today, increasing the likelihood of triggering an avalanche. With plenty of new  snow available for transport, expect to see fresh, sensitive  windslabs on leeward facing terrain at all elevations and especially above treeline and near ridgelines throughout the forecast region today and tomorrow. Recent deposits of wind-drifted snow (windslabs) look like very large domelike pillows. Windslabs that are firm enough to support the weight of a human can have a hollow, drumlike sound and feel to them as well. Wind-drifted snow has a stiff, slabby feel to it that is noticeable while traveling through it on boards and snowshoes, not so noticeable if on a snowmobile. Other red flags that warn of windslab instability include "shooting cracks" projecting out in front of ski tips, snowboard tips, or snow shoes, and the observance of avalanches in your area of travel. If we observe these red flags that warn of snow instability, we need to avoid steep wind loaded terrain and do our recreating on terrain with a slope angle less than 30 degrees. Today, and especially this afternoon, expect to find dangerous windslab avalanche conditions on leeward facing terrain steeper than 30 degrees (avalanche terrain), particularly above treeline and  near ridgelines. It is recommended that recreationalists avoid avalanche terrain, especially in the alpine today. Careful snowpack evaluation, cautious route finding, and conservative decision making will be essential today and tomorrow if you choose to travel in and  through avalanche terrain.

  

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  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Hochgebirge
  • Waldgrenze
  • Unterhalb der Waldgrenze

  Wahrscheinlichkeit

  Possible

  Größe

  1–2

  We are still dealing with persistent weak layers (surface hoar and facets) buried at various depths in our snowpack in the Blues, Wallowas, and Elkhorns. I have been seeing these buried persistent weak layers at and below treeline on shaded and wind sheltered locations where the snow depth is 4' or less. These weak layers, once buried, persist in the snowpack for days or weeks creating poor bonds with the snow surfaces above and below. Most of the time, the only way to know if these persistent weak layers exist in areas where you might want to recreate is to dig a snowpit and look at the walls of your snowpit where these weak layers can be observed, and tested using various stability tests. Persistent slab avalanches can be triggered remotely and can propagate large distances. There are a lot of experienced avalanche practioners who will have nothing to do with steep slopes that contain buried persistent weak layers. Persistent slab avalanches are the cause of most avalanche fatalities in North America which is why we spend so much time monitoring the problem and talking about it. Please read Victor McNeil's recent observations about our persistent slab problem in the Blue mountains HERE.

  Also, please read the caption that accompanies the photo on the right. The caption is a description of what I observed in the Blues yesterday.