Continental

Avalanche problems

• 

  Storm Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1–2

  Storm slabs are slowly building over a weak upper snowpack that contains near-surface facets. The first wave of snow this past weekend brought only 2–4 inches along the road corridor, though higher amounts likely fell in high glaciated terrain. Snowfall totals over the last 24 hours remain somewhat unclear, but another 2–4 inches likely fell yesterday, with an additional 2–4 inches expected today above 1,000 feet. The highest totals are expected closer to the coast.

  This gradual loading is not expected to produce a sharp rise in avalanche danger today, but new snow totals will be one of the most important things for users to pay attention to. New snow is not expected to bond well to the faceted snow at the old surface. We have kept the hazard at Moderate because there is still likely not enough new snow in most areas to form a widespread, consequential slab. However, if you find areas with more than about 10 inches of accumulation, the hazard will be more in line with Considerable.

  Hand shears and small test slopes are useful tools for assessing how well the new snow is bonding to older layers. Shooting cracks, recent avalanche activity, and collapsing would all be signs that storm snow instability is increasing. Keep in mind these clues are useful for the upper snowpack, but they will not help you identify persistent weak layers buried deeper in the snowpack (see Problem 2).

  Wet Loose

  Rain on snow at low elevations will increase the potential for wet loose avalanches below 1,000 feet near the coast. Cloud cover should limit wet loose activity at upper elevations today. However, if the sun does break out, expect south aspects to become active first. New snow may shed quickly on solar aspects, especially where it has fallen on top of old, hard, wind-affected surfaces.

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1.5–3

  Facets exist throughout much of the snowpack, especially in the Continental zone. In some areas, the snowpack is right-side-up and has a more favorable structure despite the weak grain type. In other areas, more concerning structure remains, with stiff wind slabs overlying weaker faceted snow. Distinguishing between these two setups has become more difficult as new snow has masked many old surfaces.

  New snow is beginning to add stress to these older weak layers, but it does not yet appear deep enough to create a widespread storm slab problem today (see Problem 1). That said, it remains possible for a person to trigger an avalanche 1–3 feet deep failing on older weak layers, especially in areas where a stiff slab rests over weak facets.

  Snowpack conditions remain highly variable, so it is important to look for places where hard snow overlies weaker snow. Pole probing and digging down to identify stiff-over-soft structure are effective ways to look for this setup. Rising temperatures also have the potential to increase the sensitivity of these older weak layers and should be treated as a warning sign during periods of warming.

  

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Unlikely

  Size

  2–3

  Weak snow near the ground is increasingly common as you move North through our forecast zones, with the Continental zone being the most concerning. Although it still remains unlikely to trigger this layer, areas that have been scoured from strong northeast winds now have a shallower snowpack, making this layer closer to the surface and potentially easier for a rider to trigger.

  This is a low-likelihood but high-consequence problem most likely to be triggered in steep, rocky terrain and areas where the snowpack is thin or scoured. 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.