Continental

Avalanche problems

• 

  Storm Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1–2

  There are multiple avalanche problems today, but the primary concern is storm slabs. Over the past 72 hours, 12–24 inches of new snow has accumulated, including around a foot at sea level in the last 24 hours. An additional 4–10 inches is possible today. Expect storm totals to be higher in the Maritime zone than farther inland. Southerly winds were increasing on Thompson Pass as of 5 AM, which will create pockets of deeper, more sensitive snow on leeward slopes and in terrain features where snow is drifting.

  The persistent slab problem (see Problem 2) remains a serious concern, but the nature of the problem has shifted. Recent storm snow is creating more sensitive, near-surface slabs that are easier to trigger, while the deeper persistent weak layer continues to be hard to evaluate. With continued loading, more slopes are capable of producing avalanches. Obvious red flags such as,recent avalanches, shooting cracks, and collapsing, confirm instability, but the absence of these signs does not mean conditions are safe today.

  Storm slabs will become thinner and less of a concern as you move away from the coast, but areas that have received less snowfall are also those that have a more defined buried weak layer and a higher risk of persistent slab.

  A cautious approach to terrain is recommended today.  If you decide to travel in the mountains, watch your slope angles and pay attention to not just the slopes you are on, but also the ones that are above you.

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  2–3

  Our persistent slab issue is a growing concern with additional snowfall over the last 72 hours. Low and mid-elevation areas in the Intermountain and Continental zones that did not have a sufficiently deep slab last week to be reactive may now be near the tipping point. The weak layer of concern is now buried under 1.5–3+ feet of snow and is capable of producing large, consequential avalanches.

  This weak layer is deep enough that signs of instability may not be present even when dangerous conditions exist. This avalanche problem is less widespread in the Maritime zone, but hazard remains elevated there as well due to storm snow and wind-drifted instabilities associated with higher snowfall totals (see Problem 1).

  A conservative approach to terrain is recommended not just today, but also in the upcoming days.

  

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Unlikely

  Size

  2–3

  A deeply buried weak layer near the ground remains a concern, especially in steep, rocky terrain and areas with a thinner snowpack. Triggering this layer is unlikely for most travelers right now, but if it does fail the result would be large, destructive avalanches with severe consequences.

  This problem is most likely to show itself where the snowpack is shallow or variable around rock outcrops, scoured features, convex rollovers, and ridge tops. The likelihood may increase over time with additional loading from new snow and wind, or with warming from the sun in the future.

  Travel advice: Avoid committing to big, consequential terrain where a deep release would be unsurvivable. Choose routes that minimize time spent under overhead start zones and avoid thin, rocky trigger points.