Continental

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1–2

  N/NE winds have been impacting our snowpack for much of the week, leaving limited snow available for transport and slab formation.  New windslab formation during the current wind event will be minimal; however, you can expect to find isolated pockets of fresh sensitive windslab on some lee slopes. These newer slabs will likely be a bit softer and much thinner than the ones created earlier this week. Avalanches involving the most recently formed windslabs will likely be on the small side but could bury a person in areas where small slopes end in a terrain trap, such as a gulley or ravine. Pay attention to the depth and sensitivity of freshly deposited snow and avoid terrain traps and slopes with exposure.  

  An additional concern includes older, deeper windslabs that sit on top of the 1/26 persistent weak layer in some areas. (see problem 2). Older windslabs up to 3 feet thick have a low likelihood of being triggered today, but could have devastating consequences. Use cautious terrain selection to avoid exposure to large wind-loaded slopes that could produce destructive avalanches. 

   

   

  

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  2.5–3

  Winds have been reshaping the top several feet of our snowpack over the last week or more. This has created a lot of variability with the depth of the 1/26 persistent weak layer that we have been talking about for a while now.  Exposed windward slopes, benches, and ridges have been stripped of the overlying slab, leaving the weak layer on the surface. On leeward slopes, you can expect to find windslabs up to 3 feet thick on top of the weak layer.  Specific leeward terrain features, steep gullies, and large cross-loaded slopes will be the areas where this problem exists.   Expect the slab in these areas to be hard and unlikely or stubborn to human triggers. Although sensitivity has decreased, there will be a lingering possibility of triggering a destructive avalanche where thick windslabs sit on top of old faceted snow.  Signs of instability will likely be absent with this problem; however, if you experience collapsing windslabs, you can expect conditions to be very hazardous.

  The main points we would like to convey to users is that:

  1- The problem has moved from widespread (exists everywhere) to specific (lee slopes or terrain that was protected from outflows)

  2- Sensitivity has decreased due to hard wind slabs capping the weak layer.

  3- Consequences have increased.  Triggering an avalanche at this layer would result in a dangerous hard slab.

  Users can reduce their risk by avoiding large connected alpine faces and slopes that have been previously loaded by wind.

   

   

   

   

  

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Unlikely

  Size

  2.5–3.5

  A deeply buried weak layer near the ground remains a low-likelihood, high-consequence problem—especially in steep, rocky terrain and areas where the snowpack is thin or highly variable. Triggering this layer directly is unlikely at this time, but avalanches that start in the upper snowpack could step down to the basal weak layer.  There is a high degree of uncertainty regarding poor structure at the base of the snowpack.  What we can be sure about is that this layer is less of a concern in the Maritime zone and more so as you move towards the Continental zone where the snowpack is thinner and the weak layer is more developed.

  Watch for thin spots near rock outcrops, scoured features, and along ridge tops. Wind loading adds stress to these shallow areas, and we don’t know where the tipping point is.