Boletín de aludes

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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Peligro de aludes

Thu
Alta montaña
3 · Notable
Límite del bosque
3 · Notable
Bajo el límite
3 · Notable
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Desactualizado: este boletín caducó 6 years ago. Estás viendo condiciones pasadas, no el boletín vigente.

Lo esencial

Be careful—last night's snowfall and wind stressed an already unstable snowpack. Triggering large avalanches remains likely today. You could trigger slides in exposed terrain where the wind has drifted snow and in more sheltered areas with softer snow at the surface. Slides can be triggered remotely—from below, above, and to the sides of steeper terrain.

Problemas de aludes

  • Persistent Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Likely
    Tamaño
    2–3

    Last night's snowfall and wind-transported snow are stressing the weak layers responsible for the recent widespread cycle of large, natural avalanches. Many slides failed at the interface between the old and new snow (about 2 feet down), while others failed on weak snow buried near the ground (about 3-4 feet down). A few inches of new snow would not bother a typical snowpack that much, but our snowpack is not typical; it's still shouting "give me a break!" Some recent bullseye data to consider:

    • An experienced party reported a large avalanche that failed Monday or early Tuesday a few miles northwest of Galena Summit. The slide occurred just below the ridgeline on a NW aspect near 9500'.
    • Several groups continued to experience loud, rumbling collapses in recent days—even in areas that have seen a lot of ski or sled traffic this year.
    • On Sunday in the Beaver Creek drainage near Smiley Creek, a rider remotely triggered an avalanche that broke up to 3' deep and was large enough to snap a large, dead tree.
    • Also on Sunday, Soldier Mountain Ski Patrol and Cat Skiing staff triggered large avalanches with explosives during mitigation work

    The recent avalanche activity has involved nearly all aspects and elevations and both sheltered and wind-loaded terrain. One obvious pattern has emerged: avalanche terrain can't yet be trusted.  If you choose to ski or ride steep slopes today, you're almost guaranteed to have weak snow underneath your feet or track and are likely to trigger an avalanche.  The Beaver Creek avalanche and continued snowpack collapsing indicate that slides can be triggered remotely—from below, above, and to the sides of steeper terrain.

    (1/30/2021) Persistent slab avalanche near Triple Peaks in the Lake Ck drainage, 8100' NW. Note how this slide over ran the recent ski tracks (appeared to be from during the storm).

  • Wind Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Likely
    Tamaño
    1–2

    Last night's 2-5" of new snow and strong winds formed a fresh crop of thin wind slabs and drifts in exposed terrain. Given the gusty, erratic winds that blew from a variety of directions overnight, you may find fresh drifts in both standard and unusual locations: 

    • Near and below prominent ridgelines
    • Near large terrain features
    • On aprons lower in cirques and below cliff bands
    • Lower on large, exposed slopes

    Last night's storm also covered the thick drifts and wind pillows that formed during last week's storm and are still stabilizing. While the older wind slabs are becoming more difficult to trigger, the additional load will keep them on edge on some slopes. On more southerly aspects, the new wind slabs formed on slick crusts on some slopes and could be touchy.

    What's the moral of the story? Steer clear of the fresh wind slabs, and pay extra attention anywhere you feel thicker snow below the surface slowing down your sled or boards. Watch for cracks shooting out from your skis or sled. Recognize that any wind slab you trigger may "step down", causing a much larger persistent slab avalanche (Problem 1 above). 

    (1/30/2021) Avalanches on E-NE aspects in the Deer Ck drainage. Note the interesting patterns of wind loading

Avalanche Discussion

THE EVOLUTION OF WEAK LAYERS: PART 2

An update on Ben's discussion on weak layers: 

Weak snow near the base of the snowpack: This snow has been buried for 50 days and is not yet showing signs of going to bed. Up until last week, this layer was responsible for nearly all of the avalanches we've observed and recorded this season (approaching 500). It produced countless avalanches during last week's storm and will probably continue to misbehave. It is now buried 3-5' deep, and any avalanche that fails on this layer will be large enough to injure or kill people.

Weak snow underneath last week's storm snow: This weak snow formed in mid to late January, and the character of this layer varies by aspect (the direction a slope faces) and elevation. You might find crusts, crusts and faceted snow, small facets, and some isolated surface hoar (more widespread near Banner Summit). 

Weak snow buried last night: Before this storm, warm temperatures and sunshine formed crusts on many south and west-facing slopes. Also, some surface hoar just got buried on a variety of slopes (observed in the Banner Summit, Galena Summit & Eastern Mtns, and Soldier & Wood River Valley Mtns zones). Surface hoar is tricky and has a history of fooling seasoned avalanche professionals. Some surface hoar basics:

  • Most surface hoar avalanches are 1-3 feet thick.
  • Most surface hoar slides happen during or after the storm following the first snow event that buries the surface hoar layer. Near Banner Summit, a surface hoar layer was buried in last week's storm, so this zone now fits this description.
  • Most surface hoar slides occur within the first 3 weeks of burying the week layer (we're about a week out from burial near Banner Summit and starting the clock in other zones where surface hoar was buried intact last night).

Wasn’t it nice when the snowpack was just a simple, hot steaming mess, and all you had to worry about was the obvious weak snow relatively close to the ground? The deeper weak layer (December facets) has shown signs of improvement in the Banner Summit area and now has enough snow above it to begin the healing process throughout our forecast area. It won’t happen overnight, or even in a week, but there may be light at the end of the tunnel. Hopefully, that light isn't a freight train. If/when the December facets finally fade into the sunset, how will our snowpack behave? The magic 8 ball says “future uncertain.”

  1. One of the newer weak layers (now buried in the top 1-3 feet of the snowpack) could take the baton from the December facets.
  2. A “yet to be named” player could emerge.
  3. The older weak layers stabilize, we get frequent small storms, and no additional ugly weak layers form. This option gets my vote, but it’s the least likely of the 3.

The snowpack has become a lot more complex, and we’re going to have a difficult time deciding when it’s safe to play on those steep slopes we’ve been patiently waiting to ski and ride all season. While we wait to see how the multiple weak layers evolve in the coming weeks, we need to focus on the here and now: it’s dangerous today.

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