Continental

Lawinenprobleme

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  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Hochgebirge
  • Waldgrenze
  • Unterhalb der Waldgrenze

  Wahrscheinlichkeit

  Possible

  Größe

  1.5–2.5

  Strong North winds are forecasted to continue today.  A significant amount of snow was transported yesterday at all elevations.  The strength of winds is stripping a lot of surfaces with a lot of sublimation ( snow goes directly from solid to gas state) occurring at upper elevations.   Wind slabs were observed to be developing in specific areas such as cross loaded gullies and slopes that are directly top loaded.  On 2/28 snow surfaces were stripped down to the 2/20 rain crust below 2500' with spotty wind slab distribution that were mostly stubborn to triggers.  Rain crust was limiting further gouging of the snowpack.  Above 2500' stripping of the surface is more severe with the potential for deeper wind loads.

  On 2/28 some areas of wind slab were still showing signs of instability such as shooting cracks, although,  many areas were already very hard and becoming stubborn to triggers.  Wind slab sensitivity today will depend upon their age and stiffness.  Older wind slabs will be very hard and difficult to trigger.  More protected areas that are just starting to see wind slab formation will be softer and more sensitive.  These softer wind slabs will become less likely to encounter as strong winds are expected to be effecting a large majority of our terrain.  Triggering a wind slab avalanche 1-3 feet in depth will be possible today with stiffer slabs being able to produce wide propagations.

  Pay attention to the depth and distribution of wind slabs.   Test their sensitivity with small inconsequential test slopes. In areas where signs of instability exist such as shooting cracks or collapsing unstable snow exists.

  Wind slabs that are triggered will have the potential to step-down to the interface of the storm snow from 2/17 or deeper and produce  more destructive avalanches (see problem 2+3).

  

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  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Hochgebirge
  • Waldgrenze
  • Unterhalb der Waldgrenze

  Wahrscheinlichkeit

  Unlikely

  Größe

  1.5–3

  A layer of surface hoar was buried on 2/17 in isolated areas.  This layer became reactive on 2/22 with strong south winds and heavy snowfall.  Many areas have cycled naturally where this layer existed , however, some slopes could still harbor this instability.  As strong winds create very hard snow surfaces this layer will become unlikely for a person to effect.  There is a lot of uncertainty as to the exact location of where this layer is buried and its' sensitivity so caution is advised.

  This weak layer may exist in wind protected basins above 2500' ~ 2 feet below the surface.  Below 2500' rain on snow at the beginning of the 2/20 storm will have removed this layer.  

  Digging snowpits and having an understanding of what areas are typically wind protected are the best ways to identify this problem.  Realize that digging snowpits can often give confusing or misleading results.  Triggering an avalanche at the 2/17 BSH layer will have the potential to step down to deeper layers in the snowpack especially in steep, rocky south facing terrain (see problem 3).

  

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  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Hochgebirge
  • Waldgrenze
  • Unterhalb der Waldgrenze

  Wahrscheinlichkeit

  Unlikely

  Größe

  3–3.5

  A natural deep slab avalanche occurred on the south face of Tones Temple on 2/24.  Crown depth was variable but was estimated at 10 feet deep in the deepest spot with a track width of 1700'.  The weak layer is unknown but the significant depth of the crown indicates that the weak layer was near the ground.  Another deep slab was observed on the southwest face of Girls Mountain that released from the summit ridge and included the entire snowpack but was much narrower.  This was observed on 2/27 with the failure date unknown.

  The Tones avalanche is very concerning for multiple reasons.  First off was the depth and width.  Being involved in an avalanche of this magnitude is unsurvivable.  Secondly was how the avalanche initiated and the weather that tipped the scales.  When our area typically sees deep slab releases such as this one they generally occur during times of significant snowfall accumulation and often rising temperatures. The stress that is being applied to the snowpack during these times can not be replicated by a human, so they are generally not a human trigger problem.  This avalanche was triggered by a D1 windslab that released near the summit and initiated catastrophic failure to the slope below.  There was a wind loading event in progress that would be adding stress to the slope but the fact that a small slide could step down so drastically is very concerning as it is very conceivable that a person or machine could have applied the same stress.

  Unfortunately, there is no way to assess this problem.  Digging to identify and test this layer is unreasonable as the snowpack is over 4 meters deep in many places and spatial variability is likely.  This avalanche problem will not have any signs of instability.  The only travel strategy is avoidance, especially during and directly after times when the snowpack is being put under increasing stress.  Steep rocky terrain where thin spots in the snowpack are likely to exist are the areas of highest concern.