Lawinenlagebericht

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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Lawinengefahr

Mon
Hochgebirge
3 · Erheblich
Waldgrenze
2 · Mässig
Unterhalb
1 · Gering
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Veraltet — dieser Lagebericht ist 7 years ago abgelaufen. Du siehst vergangene Verhältnisse, nicht den aktuellen Bericht.

Kernpunkte

Triggering an avalanche is most likely on upper elevation slopes with wind-drifted snow. Over the past several days, obvious signs of instability—triggered avalanches and snowpack collapsing—have occurred where recent wind drifts have formed over buried weak layers. Avoid steep slopes with signs of windblown snow, and be aware that slides can be triggered from gentler terrain adjacent to steep slopes.

Lawinenprobleme

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    2–3

    The snowpack in this zone is thinner than in the mountains to the north. You can find well-defined layers of surface hoar and depth hoar on some of the highest and shadiest slopes, but a thin, weak, faceted snowpack is more the norm. In general, the snowpack is lacking a slab except where the wind has created drifts over the weak facets—and that's a significant exception given the wind events since the start of the New Year.

    On Thursday a skier remotely triggered an avalanche in Croy Canyon, and on Friday snowmobilers triggered a slide near Couch Summit in the Soldiers. In both of these cases, the snowpack consisted of recent or past wind transported snow atop a weak, faceted snowpack. Watch for and avoid steep slopes with tell-tale signs of wind drifting: firm or dense snow, surface patterns and texture, and/or a deeper snowpack than on nearby slopes. Remember that many slides this year have been triggered from gentler terrain above, below, or to the sides of steep slopes.

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1–2

    Yesterday's storm brought strong, gusty winds, and an observer in the Wood River Valley yesterday reported significant wind loading. You may find some touchy wind slabs today, but the bigger issue is where the wind events since New Year's have created stiff wind drifts over shallow, faceted snow. The same observer reported a number of large collapses where this condition existed, some with cracks shooting out for over 20 feet. Is this a wind slab problem or a persistent slab problem? It's hair-splitting really. Avoid steep slopes with signs of past or recent wind drifting—these will be most common up high, but keep in mind that many of the middle elevation open slopes in this zone can behave like upper elevations.

Avalanche Discussion

For a while, it seemed so simple. In areas where there was enough snow to recreate, there were well-defined weak layers consisting of surface hoar buried in early December, and well-developed facets and depth hoar in areas that held the old October snow. But there is a simple rule to snowpack evolution: the longer a shallow snowpack sits around, the weaker and more variable it gets. When the snowpack is 1-3 feet deep, a few inches can mean big differences in the layering of the snowpack.

If we were forced to simplify our complex snowpack, we could split it into two general buckets:

  1. Areas with a "deeper" snowpack, say more than 2.5-3 feet. Depths like these can be found on shady slopes at mid to upper elevations in our northern mountains, and at upper elevations of our southern mountains. In these areas, the snowpack layering remains fairly well-defined. You have the December 7th surface hoar buried 2-3 feet deep, as well as well-developed facets and depth hoar near the ground.  These layers have been the primary players so far this year. Here are videos from the Western Smokys and Banner Summit showing that it remains possible to trigger a dangerously large avalanche in these deeper snowpacks.
  2. Areas with a "thinner" snowpack, less than about 2.5 feet. Prior to the New Year's storm, these places had a thin snowpack consisting mostly of weak, faceted snow with some intermingled crusts. A shallow snowpack is common in windy, alpine terrain, on sunny slopes, and in much of our southern mountains. In some places, the recent storm added thick layers of wind slab on top of this weak snow, effectively doubling the snowpack depth in places. These wind-loaded areas have displayed the most signs of instability, including cracking, collapsing, and triggered slides.

While obviously a simplification of the situation, it may help make sense of a highly variable snowpack. The common theme is that we have plenty of weak layers, and slopes that have new, recent, or even old wind drifted snow remain the most dangerous.

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