Bollettino valanghe

Sawtooth & Western Smoky Mtns

Sawtooth Avalanche Center
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Thu
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3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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In sintesi

Several large avalanches were observed yesterday and are likely today, especially in shady (NW-N-NE-E facing) upper and middle elevation terrain. These slides can be triggered remotely—from flatter terrain below, above, and to the sides of steep slopes. Avoid slopes with stiff, windblown snow on the surface. Safer conditions exist in south-facing terrain sheltered from the wind.

The danger will increase tonight. Be careful if you're out after dark. 

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    2–3

    The snowpack provided some obvious clues yesterday:

    • SAC staff and public observers saw several large, powerful persistent slab avalanches (photo) that released naturally Monday night or early Tuesday.
    • While evaluating conditions on a reconnaissance mission, Sun Valley Heli Ski staff remotely triggered a persistent slab slide when the helicopter landed on a flatter slope about 1/4 mile away (photo to right/below).

    These avalanches ranged in size from "large enough to bury you" to "large enough to destroy your truck—and the garage it's parked inside." They occurred on shady, middle and upper elevation terrain where old, weak snow from October existed (see Aspect/Elevation diagram above, videos below). 

    We are not experiencing as much snowpack collapsing—an obvious sign of snowpack instability—as we were immediately after the big mid-December storm. Unfortunately, the decrease in observed snowpack collapsingdoes not mean slopes are safe. Today's snow and wind will not add a big load to the snowpack, but many slopes are still teetering on the edge of failure. You or your sled could be the proverbial straw that breaks the camel's back. 

    Do not trust any slope harboring old October snow. The old snow consists of sugary facets and crusts, and it appears as darker, crumbly layers near the ground in snowpits. Folks are finding safer conditions, and good riding and skiing, on low and middle elevation slopes that are more southerly-facing and hence lack the old October snow.   

    (12/21/2021) Sun Valley Heli Ski staff triggered this persistent slab avalanche remotely while evaluating snow stability and conditions - amazingly, this slide released when the helicopter landed on a gentle slope about 1/4 mile away. NE aspect near 8400' in the western Smoky Mountains.

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2

    Recent winds formed stiff and soft slabs near ridgelines and in exposed upper elevation terrain. Yesterday, I observed several small, natural wind slab avalanches on upper elevation slopes. Today's snowfall and wind will build existing and/or form a new generation of wind slabs. You're most likely to find windblown snow immediately below corniced ridgelines at upper elevations, but some slabs exist near prominent terrain features and in middle elevation terrain too.

    Some wind slabs may be large enough on their own to injure or bury you, but a bigger concern is the potential for weak faceted snow buried beneath (see Problem 1). Any wind slab avalanches you trigger could "step down", triggering a much thicker, wider persistent slab avalanche. 

    Look for drifts, dunes, or surface ripples and waves to help you identify wind-affected terrain. If you feel the snow stiffen under your skis or boards or see cracks shooting out in front of you, you've found a wind slab and it's time to move to gentle or more protected terrain. 

    (12/21/2021) Wind effects and critter tracks in the upper reaches of the Eagle Creek drainage north of Ketchum. Be on the lookout for wind slabs and pockets of stiff, windblown snow whenever you see patterns like this on the snow surface. 

Avalanche Discussion

DECEMBER GRADES FOR THE SNOWPACK

If you've read any of our Daily Forecasts this year, you're well aware of the weak, October snow at the base of the snowpack on many shady (NW-N-NE-E facing), middle and upper elevation slopes. The October layer(s) resoundingly failed their first test, the big mid-December storm. Since then, the Oct snow has been completing some little assignments—less snowpack collapsing and cracking—but continues to receive straight F's on exams: natural avalanche activity, remotely triggered avalanche activity, poor snowpack test scores. This snowpack obviously isn't going to make the honor roll in 2021; will it flunk out of school? The odds say yes. 

The extended period of snow and wind beginning today will present a huge challenge for the snowpack. Where the October snow exists, we expect to see large natural and human-triggered slides this week. Recently formed crusts and faceted snow at or near the current snow surface may become problematic as the snow piles up in the coming days; the "safer" south-facing middle and lower elevation terrain may not be so safe by tomorrow or Friday. 

I encourage you to embrace an "avoid all avalanche terrain where early season snow exists" mindset for the foreseeable future, and be ready to go into "avoid all avalanche terrain" mode. With feet of powder in the 3-7 day weather forecast, expect to find amazing skiing and riding conditions on slopes less than about 30*. 

    

 

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