Lawinenlagebericht

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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Lawinengefahr

Thu
Hochgebirge
3 · Erheblich
Waldgrenze
2 · Mässig
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, especially where the wind has formed drifts or stiffened the snow near the surface. Large, destructive avalanches could fail on old, weak snow buried near the ground on shadier, NW-N-NE-E facing slopes. Avalanches could be triggered remotely—from flatter terrain below, above, and to the sides of steep slopes. Stability is better on sheltered, W-SW-S-SE facing, middle elevation slopes, and in all sheltered lower elevation terrain. 

Lawinenprobleme

  • Persistent Slabs

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

    Yesterday, I saw a fresh, persistent slab avalanche in shady, upper elevation terrain in the Murdoch Creek drainage (photo to right/below, Avalanche Observation here) just north of this zone. Sun Valley Heli Ski observed about a dozen fairly recent, similar avalanches near the crest of the Smoky Mtns (photo below), also just north of this zone. Monday, we observed recent natural avalanche activity in the Baker Creek drainage, and riders reported experiencing a large snowpack collapse that they could feel while sitting on their machines.

    In terrain where old, weak October snow exists at the bottom of the snowpack—many elevation slopes that face away from the sun (NW-N-NE-E)—evaluating stability is getting a little tricky:

    • Observations indicate triggering a destructive, 2-4 foot 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).

    You're most likely to trigger a persistent slab avalanche in relatively shady, exposed terrain where the wind has formed drifts or stiffened the snow in the upper part of the snowpack...but avalanches are also possible in the sheltered terrain that Ben describes in the video. Cornices falling onto slopes and smaller wind slab avalanches running downslope can also trigger thicker persistent slab avalanches. Avalanches may be triggered remotely—from flatter terrain below, above, and to the sides of steep 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
    Possible
    Größe
    1–2

    Expect to find wind slabs, large drifts, and fresh cornices in upper elevation and exposed middle elevation terrain. Today's snowfall and wind could help hide the large wind features that formed during the big storm in recent days as well as form a new generation of softer, smaller wind slabs. 

    Cracks shooting out in front of you are a clear sign you could trigger wind slab avalanches. Pay attention to how the snow feels beneath your boards or sled. If you feel the snow stiffen or notice you're staying closer to the surface instead of sinking deep into the snowpack, you've found wind-affected snow and should avoid steep, consequential slopes. 

    The wind slabs could be hazardous on their own and are also stressing the deeply buried weak snow creating the persistent slab problem described above. If you find yourself traveling where both of these problems exist, understand that you will be rolling the proverbial dice if you choose to ski or ride steep slopes.

Avalanche Discussion

To have an avalanche, we need a slope, a weak layer, and a slab. We have all three ingredients in steep terrain where the old October snow we've been talking about for weeks exists. Once we have all three ingredients, we can consider two steps in the process of how avalanches occur: initiation and propagation.

  • Initiation: The first step in triggering an avalanche is initiating a crack in the weak layer. This is easiest when the weak layer is buried less than about 3 feet deep, and the force from you/your sled can substantially impact the weak layer. The feet of soft storm snow from the past week are reducing skiers' and riders' impacts on the weak layer; this makes it more difficult to initiate cracks in the weak October snow on many slopes. 
  • Propagation: Once a crack initiates, complex properties of the weak layer/slab system (load, stiffness, creep rates, etc) help determine whether the small crack "peters out" or propagates throughout the slope and creates an avalanche. The added load from the feet of recent snow has made propagation more likely. 

Why does this matter? Initiation is becoming more difficult, so we won't observe as much snowpack collapsing or cracking. Stability tests are less useful because the soft snow at the surface attenuates the force from banging on your shovel. BUT,  the propagation side of the equation is as bad or worse than before the recent storm; the snowpack will take some time to adjust to the impressive load we just put on it. 

Evaluating stability is difficult when there's a lot of soft, low-density powder at the surface. Don't be fooled by "hard" or "stable" snowpack test scores, or lulled into complacency by a lack of obvious signs of snowpack instability (snowpack collapsing or cracking, natural avalanche activity). Mother nature may not have the flashing red neon "DANGER - STAY AWAY" signs up today, but conditions are still scary out there. 

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