Bollettino valanghe

Sawtooth & Western Smoky Mtns

Sawtooth Avalanche Center
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Pericolo valanghe

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

You may be able to trigger avalanches in both sheltered and wind-affected terrain. In areas protected from the wind, 1.5-2 foot thick avalanches are possible on shady slopes facing E-N-W. In more exposed locations, you could trigger hard or soft drifts or slabs that formed over the past several days. Pay attention to the consequences of being caught in even small slides in terrain with rocks, cliffs, trees, and terrain traps.

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1.5–2

    What is it?  In the past two weeks, skiers and riders triggered several avalanches involving a layer of surface hoar buried 1.5-2 feet deep. It remains capable of producing avalanches on slopes steeper than about 35 degrees. 

    Where is it?  Thesurface hoar is most widespread and problematic in protected, middle-elevation terrain that faces E-N-W. Surface hoar is a tricky weak layer. It can be "spotty" in its distribution—it may exist on some slopes, but not others. 

    How can you recognize it?  Assume this problem exists until proven otherwise. You may or may not experience snowpack collapsing and/or cracking while traveling. These red flags are obvious signs of instability. The layer of surface hoar is subtle and may not "jump out at you" when searching for it in a snowpit.         

    How can you manage it?  You can reduce your risk by avoiding shady slopes (facing E-N-W) that are very steep (greater than about 35 degrees)—especially those with ugly consequences if you are caught in an avalanche. While the observed avalanche activity points to the layer being more common on sheltered slopes, don't rule out its existence in terrain exposed to the wind. These avalanches may break wider than expected or higher on the slope above you. 

    ADDITIONAL DISCUSSION: Older weak layers (often facets found near crusts, with some surface hoar) exist deeper in the snowpack. These layers are buried beneath dense, 3-4+ thick slabs. While triggering them is unlikely, it can't be ruled out in steep, rocky, wind-affected terrain. 

    (1/23/2023) This natural avalanche in the W Smokys likely occurred during the wind event on Jan 22. The depth and characteristics are consistent with the early-January surface hoar layer. It appears to have been triggered by a small wind slab. 8500', E. Sun Valley Heli Ski photo.

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–1.5

    What is it?  Last week's NW-N-NE winds formed 1-3 foot thick, hard and soft wind slabs in exposed terrain. This weekend, light snowfall and continued northerly winds formed a thinner, softer generation of wind slabs. Yesterday, we spotted a large, fairly recent wind slab avalanche in the Soldier Mtns (view photo here). Recent observations report widespread wind-affected snow surfaces throughout the forecast area. Reports of snowpack cracking have gradually decreased in the past couple of days, but professionals continue to steer clear of recently formed wind slabs.

    Where is it?  Wind slabs are largest and most widespread near exposed ridges. You may find them near ridgelines, on the sides of gullies and chutes, and near prominent terrain features. In the steep, rocky terrain of the Sawtooths, you'll also find slabs where snow has cascaded down and accumulated below steep cliffs.

    How can you recognize it?  Cracks shooting out from your skis or sled are an obvious sign of instability. Watch for drifts, dunes, and ripples on the snow surface. Wind slabs can have a smooth, chalky appearance and feel and sound hollow, like a drum. Wind-deposited snow will be stiffer and keep your skis or snowmobile closer to the surface.       

    How can you manage it?  The easy move is to avoid obviously wind-loaded avalanche terrain. Hard slab avalanches can behave unpredictably, breaking above skiers and riders. Any triggered wind slab could "step-down" to buried persistent weak layers (Problem 1) lower on slopes, causing a larger destructive avalanche.  

    (1/29/2023) This recent small wind slab released naturally on Titus Ridge near Galena Summit. It failed on a E/NE-facing slope at 9,600'.

Avalanche Discussion

Over the past month, we've seen a significant divergence in the character of the snowpack within and between forecast zones. In a broad sense, you can lump them into two categories: shallow and deep. The Sawtooth and W Smoky Mtns Forecast Zone falls into the deep category.

Snow depths in most of this zone exceed 6 feet. Weak layers that were buried in November and December lie beneath very dense, heavy slabs of snow. Impacting these weak layers is unlikely but not impossible. However, the persistent weak layer (facets or surface hoar) buried in early January lies beneath a 1.5-2 foot thick slab; impacting it remains a very real possibility. This layer is ugliest on wind-protected, shaded slopes (those on the northern half of the compass) where surface hoar formed. However, a pattern of unstable snowpack test scores on sunnier slopes—where facets and surface hoar lie above a crust—is worth keeping in mind as you travel through the mountains. Triggering an avalanche on this weak layer is possible in both sheltered and wind-affected terrain.

On top of these weak layers, a few generations of wind slabs formed in the past week. The most recently formed slabs are likely to be softer and more reactive to your weight. However, the large, recent slide in the Soldier Mtns probably involved rock-hard snow that drifted onto the slope last week; triggering beasts like that is a possibility. Triggered wind slabs can be effective triggers for deeper weak layers in the snowpack. 

The consequences of triggering avalanches can vary significantly. Triggering a small slide in terrain free of rocks, trees, and terrain traps is much less dangerous than triggering a slide in extreme, rocky, treed terrain with a terrain trap at the bottom. Spend some time thinking about the consequences of being caught in a slide as you are choosing your terrain for the day. 

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