Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Wed
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
3 · Considerable
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Out of date — this forecast expired 4 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

You're likely to trigger a 1-3 foot thick avalanche on many slopes. Wind-drifted terrain is most dangerous, but you could also trigger a slide in sheltered areas. Choose lower-angled terrain to reduce your risk. Avalanches may be triggered remotely—from gentler terrain above, below, and to the sides of steep slopes.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    2–3

    Triggering a dangerous persistent slab avalanche remains likely on many slopes. Weak layers (sugary facets, surface hoar, and/or crusts) are buried beneath a 2-3' thick slab. This poor snowpack structure continues to produce clear evidence of being unstable. Yesterday in this zone, my partner and I received widespread collapsing from the valley floor to the mountain top. We did a "simul-stomp" in low-angle terrain near the top of a steep slope. Upon weighing the snowpack, we collapsed the weak layer and triggered a large, 3'- thick avalanche (photo, video).

    The best/only way to avoid triggering a persistent slab avalanche is by sticking to lower slope angles. You could trigger an avalanche in both wind-affected and wind-sheltered terrain, although wind-loaded slopes are most dangerous and likely to produce a larger slide. Remote triggering (explanatory video here) is also possible—from gentler terrain above, below, and to the sides of steep slopes. 

    ADDITIONAL DISCUSSIONThis zone received 6-8" of snow on Sunday along with moderate to strong winds. Wind slabs that formed during this storm have likely stabilized, but a few may remain sensitive to your weight.  Any wind slab you trigger could "step down" into the more deeply buried weak layers described above, producing a much larger avalanche. 

Avalanche Discussion

Storms at the end of November buried persistent weak layers—sugary facets, surface hoar, and/or crusts—beneath a 1.5-3' thick slab of snow.  The exact configuration of this weak layer varies across our forecast area, but the presence of weak snow is widespread. Generalizing a bit, here are some examples of what you might encounter:

  • Upper elevation, shady slopes—especially in our northern mountains: These areas held on to the snow that fell in October, and have the deepest overall snowpack. The weakest layer in the snowpack is often a thinner layer of very weak facets or surface hoar immediately beneath the recent storm snow.
  • Lower and middle elevation shady slopes: These areas held on to less October snow and had a very shallow snowpack prior to the recent storms. You're likely to find a thick layer of well-developed facets between the slab and the ground.
  • Sunny slopes: On slopes that face the sun that didn't burn off during November, you're likely to find a crust just beneath the recent storm snow. This wouldn't be so bad, except that in most places this crust is capping a thick layer of rotten facets just above the ground.

While this may seem complicated, the result has been quite simple. These weak layers have proven to be very touchy, producing widespread avalanche activity from the start of the storm (small slide on Banner after only 8" of new snow) to the end (very large avalanche near Smiley Creek). There is plenty of evidence that these persistent weak layers—regardless of their configuration—can still produce dangerous avalanches. Yesterday, observers reported triggered avalanches, widespread collapsing and cracking, and poor snowpack test scores. So when will the stability improve? While none of us know the answer to that question, we can be certain of one thing—we're not there yet.

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