Bulletin d’avalanche

Sawtooth & Western Smoky Mtns

Sawtooth Avalanche Center
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Danger d’avalanche

Mon
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 2 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Triggering 1-2 foot thick slab avalanches that break widely on buried weak layers is possible. It would take a dose of bad luck, but triggering a much larger slide that fails 3-5 feet below the snow surface remains on the table. Smaller, wet snow avalanches will occur on steep, rocky slopes where the snow is being melted by the sun. Dense, freshly formed wind drifts may be sensitive to your weight in steep, upper elevation terrain. 

Problèmes avalancheux

  • Loose Wet

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–1.5

    Sunny slopes start the day capped with a stout crust that will break down as the sun goes to work. As the snow surface becomes wet, these types of small slides will come on the menu. You're most likely to trigger this type of slide in steep terrain where rocks and trees are adding some extra heat to the snow. Wet snow issues should remain relatively small and predictable again today, but even small slides can knock you off your feet and send you down through unpleasant terrain.

    Natural and triggered slides will travel quickly downslope on buried crusts. These types of slides can also serve as triggers for the larger avalanches failing on layers of weak snow described above.

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1.5–3

    Several weak layers are buried within the upper 2 feet of the snowpack. Multiple human-triggered avalanches involving this snow were reported in the past 10 days (Profile drainage, Alturas Lake Creek, near Dollarhide Summit). This weak snow has produced avalanches on both shaded slopes and slopes facing the sun. On sunny slopes you'll find a sun crust with associated facets. On shaded slopes you'll find a few thin rain crusts with small facets sandwiched in between. 

    This weak layer is highly variable. On some slopes it is well bonded, while on others it is producing large avalanches. You may find good stability on one slope and then trigger an avalanche when you change to a nearby slope. Cracks shooting out from your skis or machine are a clear sign you've found an unstable slab. The human-triggered slides that have failed on this layer have been triggered from a distance (remotely). 

    ADDITIONAL DISCUSSION - DEEP PERSISTENT SLAB: The weak layer that produced widespread avalanche activity in January is now deeply buried here. If you dig down, you'll eventually find it buried underneath a very dense 3-5 foot thick slab of snow. Triggering one of these monsters is unlikely, but the result would be catastrophic. The most likely terrain for making one of these beasts is steep, rocky, wind-affected terrain where the snowpack has thinner spots that give you access to the weak layer. You can watch a video describing this problem here.

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–1.5

    Winds picked up significantly yesterday and continued overnight. Look for dense, relatively thin wind slabs that have been plastered into the terrain by the ongoing wind. While these aren't likely to be big, triggering a small hard slab can pull the rug out from under you and send you tumbling downslope.

Avalanche Discussion

50 Shades of YELLOW

Yesterday, the Avalanche Danger dropped to MODERATE for the first time since January 10th. Each level of the danger scale describes a broad range of conditions, and conditions vary widely across our Forecast area. In this zone, you'll encounter some obvious issues near the snow surface, and some less obvious issues related to buried weak layers. Here's a look at the world you'll walk out into:

A weak layer of sun crusts, rain crusts, and facets formed at the end of January. Around 2 feet of settled snow from storms in February rest on top of this layer. A smattering of natural and human-triggered avalanches involving this layer clearly indicate that it is capable of making avalanches, but stability varies significantly from slope to slope. On some slopes, you'll find a strong bond at this interface, while neighboring slopes may be harboring an unstable slab+weak layer combination. Avalanches that fail on this layer will break widely and will be plenty large to bury you. 

A few smaller issues are going on at the snow surface. Winds picked up significantly yesterday, and I could see large plumes of snow blowing off the top of the Sawtooths at sunset. Look for dense, relatively thin wind slabs that have been plastered into the terrain by the ongoing wind. While these aren't likely to be big, triggering a small hard slab can pull the rug out from under you and send you tumbling downslope. In steep, rocky, sunny terrain, wet loose avalanches will become possible again as the day warms. If the snow you are moving through is wet, these types of slides are on the menu. Pay attention to what is above you, these may release naturally and they can pick up a lot of steam in steep terrain.

Near the base of the snowpack, a deeply buried weak layer lies dormant. Triggering an avalanche involving this snow is unlikely, but surviving such an avalanche would also be unlikely. We can stack the odds in our favor by avoiding the type of terrain where triggering a deep persistent avalanche is most likely:  big, alpine terrain that is steep, wind-swept, and peppered with rocks. Here, the snowpack has a lot more trigger points where we can access the weak layers underneath the very dense slabs. 

As we choose to enter avalanche terrain with buried weak layers, we accept the outcome of those choices. If we're lucky, we can play in big, steep terrain, get tons of snow in our faces, and return home to tell our friends about it. If we are not so lucky, we trigger an avalanche that can wipe our house off its foundation. The goal of this forecast is to give you the information you need to travel how you want to travel in the mountains. What you do with that information is completely up to you. If you are entering avalanche terrain today, be aware that triggering avalanches large enough to bury you (or much worse) remains a worrisome possibility. 

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