Boletín de aludes

Banner Summit

Sawtooth Avalanche Center
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Peligro de aludes

Thu
Alta montaña
3 · Notable
Límite del bosque
2 · Moderado
Bajo el límite
1 · Débil
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Desactualizado: este boletín caducó 5 years ago. Estás viendo condiciones pasadas, no el boletín vigente.

Lo esencial

As you climb in elevation, you are likely to find dangerous, wind-loaded slopes. Avoid steep slopes with wind drifts or stiff snow on the surface to reduce the chances of triggering an avalanche. Although unlikely, some avalanches could fail on layers deep in the snowpack and be large enough to destroy houses. 

Problemas de aludes

  • Wind Slabs

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

    Expect to find wind slabs, large drifts, and cornices in upper elevation and exposed middle elevation terrain. The combination of snowfall and variable winds (different speeds and directions) added to existing slabs and is forming a new generation of wind slabs. Expect to find wind-affected snow in the usual locations—near ridgelines, beneath cornices, and on the sides of cross-loaded gullies—as well as near prominent terrain features lower on slopes and on aprons at the bottom of chutes. 

    Some of the wind slabs may have formed above weak snow in the upper part of the snowpack and could be more sensitive to triggering than you would expect. Evaluating stability could be tricky in wind-affected terrain; check for crusts and/or weak faceted snow below the drifts or slabs on any consequential slope you consider playing on today. 

    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 slopes. 

    The wind slabs are hazardous on their own and are also stressing the deeply buried weak snow creating the deep persistent slab problem in Problem #2. Although not likely, smaller wind slab avalanches have the potential to "step down", triggering much larger slides. Professionals are avoiding traveling on slopes where the wind slab and deep persistent slab problems overlap.

    (1/1/2022) Wind transport observed on New Year's morning.

  • Deep Persistent Slabs

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

    A mix of facets and crusts leftover from October snowfall make up the base of the snowpack on most shady middle and upper elevation slopes. Some sunnier, upper elevation aspects have this layer too. To the south of this zone, avalanches have failed on this layer following each major precipitation event.  Because this snow is now deeply buried, it has become much harder to impact it with our weight (see video to right).

    You are most likely to encounter this problem in alpine terrain where the snowpack is variable and thinner places in the slab give you "better access" to the weak layer. This same terrain is also getting loaded the most rapidly by ongoing snowfall and wind. While triggering one of these monsters is unlikely, it's nearly impossible to predict exactly which individual slopes are possible to trigger.

    How are local professionals managing this problem? They are generally taking it off the table by avoiding rocky, alpine terrain with variable snow depths, especially "prime time" slope angles over about 35 degrees. This travel strategy is pretty easy to swallow given the excellent riding and skiing conditions on other slopes.  

Avalanche Discussion

EVALUATING 'MIDDLE-AGED' FACETED WEAK LAYERS

The layer of faceted snow near the ground on many upper and middle elevation shady aspects (NW-N-NE-E) and some upper elevation sunnier aspects is not the young, eager whippersnapper it was during the first big storm in mid-December. How is it showing signs of age?

  • Less natural avalanche activity with each successive storm
  • No recent remote triggered avalanches
  • Reports of large snowpack collapses becoming less frequent
  • Inconclusive snowpack test scores
  • Getting physically "shorter" - mechanical compression is making the layer thinner where the slab above it is more than a few feet thick

As weak layers—and people—age, they tend to mellow. The faceted, old October snow appears to be "mellowing". What should we expect as this layer transitions to its golden years? 

  • Unpredictable: The steady load from snow and wind could cause it to have a "mid-life crisis" on some slopes, while on other slopes it will gracefully age and fade into the sunset. 
  • Less communicative: It is difficult to tell how the October layer is feeling. Snowpack collapsing and unstable snowpack test scores will become even less frequent; but, a lack of signs of instability IS NOT a green light signaling the layer has been laid to rest.
  • Bursts of anger: It will be hard to tell its coming, but I'd expect to see some isolated angry outbursts—very large avalanches—before this weak layer is done.

What's the moral of the story? Triggering a large, destructive avalanche is still on the table as snow piles up today and later this week. Unfortunately, it's a lot easier to predict how this type of layer will behave when it is younger (during the first couple of big storms). There's a good chance we'll be able to write the obituary for the October snow in the coming weeks, but we're not writing it today.  

 

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