Lawinenlagebericht

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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Lawinengefahr

Fri
Hochgebirge
3 · Erheblich
Waldgrenze
3 · Erheblich
Unterhalb
2 · Mässig
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Veraltet — dieser Lagebericht ist 5 years ago abgelaufen. Du siehst vergangene Verhältnisse, nicht den aktuellen Bericht.

Kernpunkte

Triggering avalanches is most likely in upper elevation terrain. Fresh wind drifts may be sensitive to your weight and are adding additional load to old, weak snow buried near the ground. Large, destructive avalanches could fail on this old snow, particularly on shadier, NW-N-NE-E facing slopes at middle and upper elevations. Avalanches could be triggered remotely—from flatter terrain below, above, and to the sides of steep slopes.

Lawinenprobleme

  • Persistent Slabs

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

    Tuesday, Sun Valley Heli Ski staff observed about a dozen fairly recent slab avalanches near the crest of the Smoky Mtns (report). The same day, Scott saw a fresh, persistent slab avalanche in shady, upper elevation terrain in the Murdoch Creek drainage just north of this zone (report). 

    In terrain where old, weak October snow exists at the bottom of the snowpack—most middle and upper elevation slopes that face away from the sun (NW-N-NE-E) and also some sunnier aspects at upper elevations—evaluating stability is getting a little tricky:

    • Observations indicate triggering a destructive, 2-4' thick slab avalanche is becoming more difficult
    • If you do trigger a slide on weak snow buried deep in the snowpack, the consequences could be severe—this recent slide on Galena Peak paints a clear picture

    Ben made a video Monday to help explain where you will find the weak October snow and why that is crucial to understand (watch it here). Today's snowfall and wind-loading are stressing this weak layer once again. Avalanches may be triggered remotely—from flatter terrain below, above, and to the sides of steep slopes. 

    Additional Discussion:Weak layers of facets, crusts, and surface hoar may exist in the upper half of the snowpack. These avalanches observed by Sun Valley Heli Ski on Tuesday, likely failed on one of these layers. We may see similar avalanches today, breaking wider and deeper than an average wind slab. This potential problem adds yet another reason to steer clear of steep, wind-affected slopes.

    (12/28/2021) This persistent slab avalanche failed near the end of the recent storm on a NW-W aspect at 9650' (upper elevation) in the Murdoch Creek drainage.

  • Wind Slabs

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

    Today's snow and wind will build wind slabs that are likely to be sensitive to your weight. Expect to find slabs and cornices in exposed, middle and upper elevation terrain. Cracks shooting from your boards or sled are a clear sign you could trigger a wind slab avalanche.

    There's a lot of soft snow out there today. If the snow stiffens or you notice that you're staying closer to the surface instead of sinking deep into the snowpack, you've found a wind-affected slope and should avoid steep, consequential terrain. These wind slabs could be hazardous on their own but are also stressing the deeply buried weak snow described in Problem 1 above. Steep slopes where these problems overlap are a dangerous place to be. Smaller wind slab avalanches or large cornice failures could provide the rapid force needed to "step-down" to larger avalanches failing on weak snow near the ground.

    (12/28/2021) This avalanche was observed on Carbonate, above the city of Hailey, on an east aspect at 6,500'. It failed in a wind loaded location, likely near the end of the stormon 12/27.

Avalanche Discussion

Similar Weak Layers, Differing Depths

A weak layer of facets and crusts near the ground (Problem 1 above) is an unfortunate constant across shadier, middle and upper elevation terrain.  This layer is responsible for the vast majority of the avalanches we've seen this season. An intense 5-day storm ended on Monday leaving 2-3'+ of snow and 2.5-3.5" of SWE. In some areas, this storm doubled the snowpack depth.

Quickly adding weight is a good way to push a fragile weak layer to its breaking point. The avalanches we observed during this prolonged storm are testament to that (report, report, report). However, as we move away from rapid loading events the odds of triggering an avalanche on this layer will begin to slowly diminish. Obvious signs of instability such as remotely triggered avalanches, snowpack collapses, and cracking in this layer become less frequent, and snowpack test scores can be misleading.

As Scott pointed out yesterday, initiating a fracture on a weak layer is easiest when the weak layer is buried less than about 3' deep. Much deeper and the force from you/your sled has a harder time impacting the weak layer. The depth of this layer in this zone is variable depending on your location, but you'll still find spots where this layer is less than 3' deep. Evaluating stability is difficult when there's a lot of soft, low-density powder at the surface. Performing snowpack tests by banging on a shovel at the snow surface won't always produce a result, and collapses are becoming less frequent. That said, the weak layer is still there, and it's still ugly. If you do initiate a fracture, it's likely to propagate.

Be patient. Don't get lulled into complacency by a lack of obvious signs of snowpack instability. A major storm event ended just 3 days ago and we've got a nasty-looking weak layer at the base of the snowpack that's shown it can produce massive, unsurvivable avalanches. Today's storm will add stress to this layer, and wind slab avalanches or large cornice failures could provide the rapid force needed to "step-down" to weak snow at the ground.

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