Boletín de aludes

Sawtooth & Western Smoky Mtns

Sawtooth Avalanche Center
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Peligro de aludes

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

Lo esencial

You can trigger large avalanches that fail near the ground and involve the entire snowpack. You aren't likely to observe obvious signs of instability before triggering such an avalanche, and slides can be triggered after many skiers and riders have crossed a slope. Freshly deposited slabs of wind-blown snow will be reactive to your weight and add stress to the weak layer near the ground. Smaller natural and triggered avalanches can be effective triggers for slides failing on the weak snow near the ground.

Problemas de aludes

  • Persistent Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    1.5–2.5

    A layer of weak snow forms the base of the snowpack on many slopes. This weak layer is at its worst on upper elevation slopes and on shaded middle elevation slopes (those facing the northern half of the compass). In this zone, this weak snow is buried underneath a 1.5-2.5 foot thick slab of snow. If you dig down on shaded slopes you'll find an obvious layer of very weak snow near the ground. On slopes that face the sun, you'll find a less obvious layer of small facets on top of a stout melt-freeze crust.

    You are most likely to trigger large avalanches on slopes that have been recently wind-loaded. Slopes with variable slab thickness due to rockiness or wind thickening and thinning of the snowpack provide access points for triggering this weak layer. You aren't likely to experience signs of instability like collapsing of the snowpack as you travel through the mountains. The first indication of instability you see may be the large avalanche you trigger.

    Recent snowfall and wind slabs are adding stress to the weak snow in the snowpack. Small natural and triggered wind slab avalanches can "step down" to this weak layer, producing a larger and more dangerous avalanche. 

  • Wind Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Likely
    Tamaño
    1–1.5

    The combination of moderate winds and 3-5 inches of new snow have built wind slabs in exposed terrain. Expect most of these slabs to be relatively small, but easy to trigger, thanks to the very weak snow surfaces they are resting on. The size and threat posed by these slabs are dependent on the amount of new snow. In areas with more snow, slabs will be larger, easier to trigger, and more dangerous. Cracks shooting out from your skis or machine are a clear indication that you've found an unstable slab. Look for these slabs in exposed upper elevation terrain, where the wind is slowed down and forced to drop the snow that it is carrying. In the steep, rocky terrain of the Sawtooths, slabs also commonly form below large cliff bands and on the aprons below couloirs. 

    Fresh wind slabs add stress to the weak snow at the base of the snowpack described in the persistent slab problem above. These smaller slabs can be effective triggers for avalanches deeper in the snowpack

Avalanche Discussion

NEW SNOW, OLD SNOW, and the OVERLAP between

The snowpack in our mountains is becoming more complicated and more dangerous. Yesterday's storm brought 1-6 inches of low-density new snow, accompanied by moderate to strong, shifting winds. While this isn't a huge load, it is enough to begin to change the snowpack picture. Today, you'll need to manage new snow problems, old snow problems, and the murky overlap between the two. Read on:

NEW: We are on the heels of a 3-week-long split-flow drought. The weather pattern has shifted, but the drought left its signature on our snowpack. A variety of weak snow formed at the snow surface during this dry period. Widespread surface hoar developed and was preserved on many slopes, as did a range of flavors of facets with or without accompanying crusts. The pattern of these weak layers is complex, but the theme of weakness carries across most slopes. The combination of new snow and wind has constructed slabs on top of these layers of weak snow. Expect these fresh slabs to be small but reactive. Smaller slides can knock you off your feet and send you downslope, and can be effective triggers for avalanches failing deeper in the snowpack. 

OLD: At upper elevations, and on shaded middle elevation slopes (those facing away from the sun), a layer of weak snow makes up the base of the snowpack. This layer has been the source of our primary avalanche problems all winter long. The long drought gave the slabs sitting on top of this weak snow some time to get comfortable. However, with renewed loading by yesterday's snow and wind slabs, the pendulum is swinging back in the other direction. 

OVERLAP: The snowpack is the most worrisome in areas where these new snow and old snow problems overlap. I would not be surprised to see a small, natural wind slab that failed and kicked off a larger persistent slab as it traveled downslope. In fact, I will be traveling with this possibility in mind today and in the coming days.

As conditions change and complexity increases, we find ourselves stepping back from the terrain we've been playing in recently. This process is started yesterday and will continue over the coming days as active, wintery weather continues to impact our region. We are all hungry for powder, but I encourage you to take the time to clear your head and think about the avalanche problems that exist in the terrain you are planning on visiting today. 

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