Lawinenlagebericht

Salt River and Wyoming Ranges

Bridger-Teton Avalanche Center
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Lawinengefahr

Wed
Hochgebirge
2 · Mässig
Waldgrenze
2 · Mässig
Unterhalb
1 · Gering
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Kernpunkte

Triggering an avalanche large enough to bury a person is possible on many slopes. You’re most likely to trigger an avalanche on recently formed drifts of wind-blown snow that may be sensitive to the weight of a skier or rider. While less likely, it is also possible to trigger an avalanche on older weak layers buried 2-3 feet deep in isolated terrain features in shallower areas. These layers are not widespread, however, the resulting slide will be larger. Avoiding thinner areas in the snowpack at middle and upper elevations is one way to reduce your risk. 

Lawinenprobleme

  • Persistent Slabs

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

    Our most concerning persistent weak layers sit near the middle and base of the snowpack. An avalanche failing on one of these layers has the potential to be very large, particularly in open and well-connected terrain features. Persistent weak layers are difficult to deal with. They can produce avalanches many weeks after they are buried and they provide fewer and fewer obvious signs of instability as the days without a significant loading event pass. They can break wider than you might expect and may be triggered remotely, from gentle terrain adjacent to steeper slopes

    Trigger points are an important consideration with these avalanches. You can reduce your chances of tickling the beast by avoiding slopes with shallow, more southerly 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. 

    (1/5/2023)

    This avalanche was triggered by a rider at Jack Hollow on a southeast aspect at 9,500 feet.  It failed on weak snow in the lower half of the snowpack and was approx. 3 feet thick. 

    Photo: Antonioli

  • Wind Slabs

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

    New snow amounts varied across the zone in the last 48 hours with up to 10” of new snow favoring the southern portion of the Greys. Wind slabs previously formed before this new snow may still be sensitive to triggering 1-2 feet in depth. The weight of new snow is adding stress to a variety of weak layers in the snowpack.  You are also more likely to trigger a larger, deeper avalanche where the wind has affected the snow.

     

    (1/16/2023)

    This avalanche was triggered on a west aspect off of Albright Peak around 10,000 feet.

    Photo: C. Martin

Avalanche Discussion

The interplay between slabs and weak layers is phenomenally complex. Generally, the weight and the characteristics of the overlying slab affect the development of a weak layer after burial. In the Tetons, weak snow and crust+facets have been buried beneath a 3+ foot thick slab of snow for many weeks now. During this time, the weak snow has been mechanically compacted by the weight of the overlying slab. In addition, deeper snowpacks tend to create conditions where faceted grains in the snowpack can round and gain strength. Because of this, we saw signs of healing and dormancy. The snowpack in the Greys River and at Togwotee pass has remained thinner and weaker throughout the season and has shown consistent activity with larger loading events. With the addition of continued snowfall, the complex relationship is starting to show a different balance in the instability-stability dance, and this is worth noting as you travel in the backcountry today.

If you travel to regions of the Greys River or Togwotee, the slab overlying the weak layer is thinner and the weight has not been enough to significantly change the properties of the weak snow. Since the snowpack has remained thin, strong gradients in the snowpack have helped weak layers remain weak. from complications due to the nature of the loading. Instead of putting one big load of snow on at once and giving the weak layer time to adjust, we have gradually built a slab in small doses.

Wind-loading: A foot plus of recent snowfall over several days and the light to moderate winds have probably loaded some exposed slopes. Slopes with stiffer snow near the surface—whether from wind-loading or "spin drifting" below large cliff faces—are more likely to produce big avalanches than areas where the surface is loose powder.

Likely trigger points: You are most likely to impact deeper weak layers on certain aspects and on parts of a slope where the snow cover is thin, like near rocky outcrops. Heavily wind-affected areas where the wind created a complex pattern of scoured and loaded slopes are also more likely to be problematic.

Slope angle: Avalanches are most likely on slopes around 35-45 degrees in steepness. However, remember that persistent slab avalanches can be triggered remotely: from gentler terrain below or to the sides of steeper slopes.

As an aside, we are grateful as a community that all parties escaped relatively unscathed from human-triggered events yesterday. 

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