Lawinenlagebericht

Banner Summit

Sawtooth Avalanche Center
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Lawinengefahr

Mon
Hochgebirge
2 · Mässig
Waldgrenze
2 · Mässig
Unterhalb
2 · Mässig
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Veraltet — dieser Lagebericht ist 6 years ago abgelaufen. Du siehst vergangene Verhältnisse, nicht den aktuellen Bericht.

Kernpunkte

Recent snowfall and wind are loading some slopes, and you could trigger a 1-2 foot thick avalanche in steep, upper elevation terrain. Avoid steep slopes where you find evidence of drifted snow. While unlikely, we can't rule out triggering a huge, 4-5 foot thick avalanche—especially on very steep, rocky, wind-affected slopes. 

Lawinenprobleme

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1

    Around 12" of snowfall has accumulated in this zone since Thursday. This snowfall has been accompanied by periods of moderate W to NW winds, and you're likely to find fresh wind drifts and slabs along upper elevation ridgelines. Yesterday, I found wind slabs to be small and isolated, but an uptick in NW winds overnight produced fresh drifting overnight.

    These fresh wind slabs are likely to be soft, but still slightly denser than snow on more sheltered slopes. Watch for these drifts in wind-exposed, upper elevation terrain. Avoid steep and/or consequential slopes where you find evidence of wind drifted snow.

    ADDITIONAL DISCUSSION:There are two weak layers in the snowpack that are unlikely to produce avalanches but that you should keep on your radar nonetheless:

    1. A layer of surface hoar buried 2-3 feet deep seems to have healed, but a quick stability test can help you determine if you've found an isolated slope where it remains an issue.
    2. The December facets still lurk near the base of the snowpack buried beneath a very dense, 4-5'+ slab. It would take being in the wrong terrain, as well as a dose of bad luck, to trigger this layer. So what's the wrong sort of terrain? Slopes that are very steep, rocky, and wind-affected. 

    I found both of these layers in the Cape Horn area yesterday—watch the video.

    The ongoing snowfall has been accompanied by periods of moderate westerly winds, and you're likely to find fresh wind drifts and slabs along upper elevation ridgelines.

Avalanche Discussion

ON THE IMPORTANCE OF SNOWPACK DEPTHOne very simple, yet powerful, observation you can make in the backcountry is snow depth. It turns out you're already carrying the measurement device—your avalanche probe—and it takes no time at all to pull it out and measure the total depth of the snowpack. You're also carrying another important tool—your eyeballs. Visual clues tell you about the snow depth: Are you in a deep area that is generally covering rocks, small trees, and other surface features? Or do you see a lot of these poking through the snow or giving it a "lumpy" appearance? Is the snow depth variable or relatively uniform?

Why is the depth of the snowpack important? I'm glad you asked:

  • Deeper snowpacks are warmer and more insulated from cold temps at the surface. I don't think you want me to dive into temperature gradients, but processes that strengthen the snowpack are more pronounced when the snowpack is deep. What's deep? In the avalanche world, rules of thumb are problematic at best, but let's say "deep" means a total depth greater than around 4'. Snowpacks less than 4' deep tend to have more weak snow, and the weak snow improves slowly.
  • It's not uncommon to see a divergence in our snowpack conditions in the middle of winter when our deeper areas surpass this threshold. For example, the snowpacks in our deeper zones—the Sawtooths & Western Smokys and Banner Summit—are quickly becoming very different from our southern zones where the snowpack is less deep. In these areas, triggering the December weak layer has become unlikely, and surface problems due to new snow and wind are becoming the primary concern. In our southern zones where the snowpack is closer to the 4' depth, there is still a real threat of triggering an avalanche on a persistent weak layer.
  • On a smaller scale, how snow depth varies across a slope plays a critical role once a weak layer is buried deeper than about 3'. Once a weak layer becomes deeply buried, it's more difficult to impact with our weight. But on a slope with variable snow depth—which is common in steep, rocky, and especially alpine terrain—shallow areas exist where the weak layer is closer to the surface. We call these areas potential trigger points.

Next time you go into the backcountry, try putting more observational focus on snow depth, both on a large scale (how does the average depth compare to other forecast zones) and on a small scale (how much depth variation do I see on a particular slope).

Be sure to catch up on the happenings this past week in Ethan's latest Weekend Update.

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