Continental

Avalanche problems

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  2–3

  We continue to be concerned about a layer of facets (weak snow) buried, on average, about 2 feet below the surface. The round of hurricane-force winds last week significantly changed the slab sitting above this weak layer and had a major effect on our snowpack.

  On many windward slopes, the persistent slab has been stripped away down to older layers in the snowpack, including rain crusts below 3000'. In these wind-scoured areas, the 1/26 persistent slab problem is no longer an issue.

  On leeward slopes, stout wind slabs have been deposited on top of the persistent slab. This has lowered the chance of triggering the 1/26 weak layer in many places because the hard wind slab at the surface is difficult to penetrate. However, the consequences have increased. What was a softer slab problem a couple of weeks ago is now a hard slab. If triggered, avalanches will likely be large enough to injure, bury, or kill a person.

  This is a difficult setup to assess because conditions may feel “locked up” even where weak snow still exists below the surface. Think of slopes as having isolated trigger points rather than being uniformly unstable. These trigger points are most likely where the slab is thinner—along slab edges, in cross-loaded terrain, near rocky areas, or anywhere the weak layer sits closer to the surface.

  Users can reduce risk by avoiding large connected alpine faces, especially those loaded by wind, cross-loaded features, and slopes that end in terrain traps.

  If you find terrain that is not wind affected, expect a higher likelihood of triggering an avalanche.

   

  

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Unlikely

  Size

  2.5–3.5

  Little change is expected with our deep, persistent weak layer until additional stress is added to our snowpack. Buried faceted snow and depth hoar near the ground is increasingly common as you move North through our forecast zones, with the Continental zone being the most concerning. 

  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 the 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.