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

While not likely, you could trigger a very large, 3-foot thick avalanche. The resulting slide would likely be unsurvivable. Reducing your odds of triggering one of these beasts by avoiding very steep, rocky, wind-affected terrain. 

Lawinenprobleme

  • Deep Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    3

    Our area continues to be plagued by a layer of faceted snow—in this zone, it's buried beneath a dense slab more than 3' thick. Since it was first buried in December, we've recorded nearly 400 avalanches—the vast majority broke on this persistent weak layer. Around 20 of these were human-caused. 

    The good news: triggering an avalanche in this zone is less likely than in other portions of the forecast area. Bad news: if you were unlucky enough to trigger a deep slab avalanche, it's unlikely you'd survive the ride. It's the classic low-probability, high-consequence scenario.

    Trigger points are an important consideration with deep slab avalanches. You can reduce your chances of tickling the beast by avoiding slopes with shallow areas where you can more easily impact the weak layer. Trigger points are most common in alpine (or alpine-like) terrain:  very steep (over about 35 degrees), rocky, and wind-affected. Pay attention to what's below you—look for trees, rocks, gullies, and other terrain traps. 

    (1/18/2021) This very large avalanche occurred in our Banner Summit Zone, likely during last week's storm. Based on the depth and appearance of the crown it probably failed on facets buried deep in the snowpack.

Avalanche Discussion

FAQ's regarding the current snowpack, continued from yesterday...

Q: How do persistent weak layers like this one heal?A1: Through mechanical compaction or compression. When a very thick, dense slab overlies a weak layer, the sheer weight of the slab begins to "squash" the weak layer. Like a group of introverts stuck in a crowded room, at some point the faceted snow grains have no choice but to begin to connect. We're just starting to see this mechanism in action in the Banner Summit zone, where the weak layer is showing signs of relenting under the weight of the 3+' thick slab.

A2: Through rounding and bonding. When the snowpack is deep and air temperatures are warm, faceted snow grains will start to form bonds with one another through a process called rounding. At the same time, the sharp edges of the facets will soften and become rounder. In other words, the introverts slowly become extroverts.  Unfortunately, the snowpack in much of the forecast area doesn't qualify as deep (more than 3-4' deep), and the temperatures haven't been particularly warm. 

Q: Is there a way to tell when the snowpack has stabilized?A1: Unfortunately, there is no one piece of information that indicates a stable snowpack. Only through a cumulative lack of evidence pointing towards instability does a picture of improved stability emerge. So in other words, only once we stop hearing reports of avalanches (triggered or natural), collapsing/cracking of the snowpack, and poor snowpack test scores can we start to feel better about the snowpack. In much of the forecast area, folks are still reporting signs of instability. 

A2: Decreasing evidence of instability just tells us that the odds of triggering a slide are diminishing. As long as the slab over weak layer snowpack structure exists—and it will for some time, see above—the odds don't go to zero. There will always be "early adopters" who are willing to play the odds, and more conservative folks who like their chances of triggering an avalanche to be very, very low. Risk tolerance is a personal thing—I just encourage you to look at the "long game" and not just the odds of skiing or riding one steep slope.

Q: What happened this past week?A: Funny you should ask. Be sure to check out our latest Weekend Update where I attempt to answer just that.

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