Elkhorns

Lawinenprobleme

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Hochgebirge
  • Waldgrenze
  • Unterhalb der Waldgrenze

  Wahrscheinlichkeit

  Possible

  Größe

  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 and the observance of avalanches. When we are dealing  with persistent slabs there are can scenarios where there are no surface clues telling us there are persistent weak layers lurking below. 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. Not all our test columns are exhibiting propagation, but the observance of  poor snowpack structure coupled with the observance of a fair amount of propagation in column stability tests is enough to make us uncertain about the overall snowpack strength throughout the region, especially where the snowpack is less than 3.5' deep. These persistent weak  layers located below the snow surface are getting tested by the added weight of recent new snow that has fallen since Saturday night, and then redistributed onto leeward facing slopes by the wind. The newly added weight of snow has, and continues, to put a lot of stress on these buried weak layers. Add to that the weight of a human, and especially the weight of a snowmobile and rider, that might be enough stress on the snow to make one or more of these weak layers fail resulting in a potentially large and consequential avalanche. There have been several recent avalances that have occurred in our forecast area since February 7th. I will mention 3 of them in this discussion. The first avalanche was triggered remotely in the Blue mountains. You can read about it HERE. The other two recent avalanches were remotely triggered in the Southern Wallowas Wednesday and Thursday. One avalanche was triggered on a northeast facing slope at 7600' . The crown was estimated to be 1'-2' deep and the weak layer was buried surface hoar. The second avalanche was remotely triggered as well on a southwest facing slope at about 8000' . Nobody was injured in any of these snowmobile triggered  avalanches. See the photo attached to this discussion. 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 stay off of, out from under, and away from slopes connected to avalanche terrain, especially today.  Now is the time to enjoy the good riding conditions that can be found on lower angle terrain.

  

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Hochgebirge
  • Waldgrenze
  • Unterhalb der Waldgrenze

  Wahrscheinlichkeit

  Likely

  Größe

  1–2

  Moderate to strong south and southwest winds, overnight, and forecasted for today,  accompanied by a lot of recent snow available for transport , will create new and potentially reactive deposits of wind-drifted snow (windslabs) on leeward facing northwest, north, northeast, and east facing terrain at and above treeline today, especially near 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 upper 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 while traveling through wind deposited snow. Wind affected snow might also have a thick, heavy, or slabby feeling while traveling through it. If you start observing signs of recent avalanche activity in your area of travel and the other aforementioned red flag signs 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 the wind speeds and wind direction unfolds according to the forecast. Avoid steep slopes today that you suspect are wind loaded ! Careful snowpack evaluation, cautious route finding, and conservative decision making will be essential today while traveling on and near slopes steeper than 30 degrees.

  

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Hochgebirge
  • Waldgrenze
  • Unterhalb der Waldgrenze

  Wahrscheinlichkeit

  Unlikely

  Größe

  1–3

  We are still observing  poor 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 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  recent wind transported snow that is moved to leeward facing terrain by moderate, and in some locations, strong southerly winds forecasted  to affect our forecast region last night and today. 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 shady 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.