Continental

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1.5–2.5

  North winds are on the increase across all three forecast zones.  With significant snow available for transport expect to find wind slabs to exist and be building in depth on the lee side of high elevation ridge lines, terrain features, and cross loaded gullies.  Areas close to Thompson Pass `or other wind channeled terrain wind slabs will be older and may already be becoming stubborn to triggers.  As you move into more protected zones expect wind slabs to just now be forming and be softer and more sensitive to triggers.

  Today wind slab sensitivity will vary on their age.  The older they are the more stubborn to triggers they will be.  Older slabs will be very hard and will likely exist in areas near Thompson Pass or other wind channeled areas where the north wind has been blowing for multiple days.  Today, North wind is expected to affect more areas that were previously protected.  This will be forming fresh wind slabs that will be softer and more susceptible to triggers.  Stiffer wind slabs may be more difficult to trigger, however, they will be able to propagate longer distances once initiated.

  Pay attention to the degree of wind loading in the area you choose to travel as well as the age, depth, and sensitivity.  Small test slopes are a great way to test this.  Watch out for signs of instability such as shooting cracks, collapsing and active wind loading.  

  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).

  

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1.5–3

  On 2/22 a widespread natural avalnche cycle was triggered by intense snowfall rates and strong southerly winds at the tail end of the storm.  After 5 days of settlement,  the storm snow from 2/20-22 has been observed and reported to be well bonded and without signs of instability in most locations.  A layer of surface hoar that was buried on 2/17 exists in isolated wind protected locations and remains a concern.  Where the 2/17 BSH layer exists human triggered avalanches will remain possible ~ 2 feet in depth.

  This weak layer will exist in wind protected basins above 2500'.  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).

  

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Unlikely

  Size

  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.