Continental

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1–2

  Strong north winds continue to redistribute faceted snow that has formed on the surface of old, rock-hard wind slabs. These fresh slabs have been poorly bonded to the underlying wind slab, with failures observed on test slopes in multiple locations. Fresh slabs are generally less than 1 foot deep, but could be deeper in isolated areas depending on specific terrain features. In some locations, faceted snow has also been observed beneath these fresh slabs, which may allow them to remain reactive longer than is typical.

  Discerning the fresher, more sensitive wind slabs from the older, stubborn ones can be tricky. Newer slabs tend to have a smoother, cleaner look, while older slabs often appear more striated and rough.

  Avoid deep gullies and cross-loaded terrain where fresh wind slabs are developing.

  

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Unlikely

  Size

  2–3

  The 1/26 weak layer continues to be found intact, and in some snow pits it remains reactive. It has now been nearly two weeks since the last major wind-loading event and about three weeks since the last meaningful snowfall. This lack of significant loading has allowed the snowpack to settle, which has lowered the likelihood of triggering a persistent slab avalanche. Even so, there remains a low-probability, high-consequence chance for a person to trigger a hard slab avalanche 2–4 feet deep.

  This avalanche problem is unlikely to produce obvious signs of instability such as shooting cracks or collapsing, even when a dangerous setup exists.

  Over the last two weeks, continued wind has redistributed the snow above this weak layer and created a high degree of spatial variability in slab depth. In many windward areas, the slab has been stripped away and the 1/26 layer is now at or near the surface and is not a current persistent slab concern. In these areas, weak snow near the base of the snowpack is now closer to the surface and may be easier to affect, especially in the Continental zone (see Problem 3).

  On lee slopes, knife-hard wind slabs have increased the depth and stiffness of the slab overlying the weak layer. This makes it less likely for a person to affect the layer, but consequences would be high if an avalanche is initiated.

  Users can reduce risk by avoiding steep, connected alpine faces that have been previously wind loaded.

  

• 

  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.