Bollettino valanghe

Sawtooth & Western Smoky Mtns

Sawtooth Avalanche Center
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Pericolo valanghe

Sun
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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In sintesi

Triggering a large slab avalanche that fails on weak snow near the ground is today's primary avalanche concern. 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 forming wind slabs are likely to be relatively small and soft, but may be reactive thanks to the weak snow underneath. These smaller slides can knock you off your feet and send you downslope, and can be effective triggers for avalanches failing deeper in the snowpack. 

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    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 steeper than about 35 degrees 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.

    ADDITIONAL DISCUSSION: 2-4 inches of snow fell here in the past two days. By nightfall, an additional 1-3 inches of snow will accumulate. Light to moderate winds are pushing this snow around and building small drifts and slabs in exposed terrain. These are unlikely to be large enough to bury you, but may be able to knock you off your feet and send you downslope. Expect these small slabs to be reactive, thanks to the weak snow they are sitting on. These smaller slides can be effective triggers for avalanches failing deeper in the snowpack. 

Avalanche Discussion

NEW SNOW and OLD SNOW

A return of snowfall brings shifting avalanche conditions. The avalanche-related issues that you'll face today fall into two broad categories — old snow and new snow. Let's start near the surface:

NEW: After a 3-week drought, the broad weather pattern has begun to shift and snowfall has returned to our area. During this drought, a variety of weak snow formed at the snow surface. 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. Winds are currently re-distributing the few inches of snow that fell at the end of the week, and aother few inches will fall today, providing the wind with some additional ammo for building slabs. Expect these fresh slabs to be small but reactive, thanks to the weak snow they are sitting on. These smaller slides 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 avalanche problems all winter long. The likelihood of triggering a slide involving this weak snow has decreased significantly, but the possibility remains on the table. In areas where more new snow is falling and the wind is building fresh slabs, this load is adding new stress to the long-buried weak layer. There isn't enough new snow to throw our snowpack into revolt today, but the roots of unrest are building.

Go into the mountains thinking about both the old, weak snow near the ground and the layers of weak snow that we are actively burying. As conditions change and complexity increases, we'll find ourselves stepping back from the terrain we've been playing in recently. This process is starting today and will continue over the coming days as active, wintery weather returns to our region. 

Regioni coperte