Bollettino valanghe

Sawtooth & Western Smoky Mtns

Sawtooth Avalanche Center
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Pericolo valanghe

Wed
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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In sintesi

Skiers and riders have triggered 4 large avalanches in recent days, and similar 1.5-3 foot thick avalanches remain possible in shady, middle and upper elevation terrain. These slides can be triggered remotely—from a distance above, below, or to the side of steep slopes. This problem appears to be more widespread in the western Smoky Mtns than in the Sawtooths closer to Stanley. Wet avalanches could release naturally on steep slopes that directly face the sun and on all steep slopes at lower elevations. Avoid areas with wet, soggy snow.

Problemi valanghivi

  • Persistent Slabs

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

    Skiers and snowmobilers have triggered 4 large avalanches since last week's storm, many of them remotely (from a distance above, below, or to the side of a steeper slope). On Thursday, Chris investigated an avalanche near Galena Summit that was remotely triggered by skiers from flatter terrain above (video at right/above). The 5-12" of snow that fell near Galena Summit and Smiley Creek since Sunday night is enough to keep the snowpack on edge, especially in the western Smoky Mtns and southern part of this zone. 

    The culprit weak layers in all of these slides have been sugary facets buried 1.5-3 feet deep. The distribution and sensitivity of these layers are somewhat irregular.  You are most likely to find them on slopes that face E-NE-N-NW above about 8000' in the western Smoky and southern Sawtooth Mtns. In this terrain, it's best to assume these layers exist and choose lower angled slopes not threatened by steeper slopes above. You're more likely to trigger persistent slab avalanches in areas where the wind has stiffened the snow surface or formed drifts, but they are possible in sheltered terrain as well.

    Dealing with persistent weak layers is tricky business, particularly as they become more deeply buried. Snowpack test results become less reliable, and obvious signs of instability are typically absent. The first clear sign of an unstable snowpack may occur when you trigger a large avalanche. These types of slides can be triggered remotely—from a distance above, below, or to the side of a steeper slope.

    ADDITIONAL DISCUSSION: Thin, soft, fresh wind slabs may exist in exposed, upper elevation terrain. Be alert to the consequences of triggering such a small to medium-sized slide in steeper, rocky terrain. These slabs are also adding stress to the weak layers of snow that are described above. Triggering even a small slide could help to trigger a thicker slab as snow flows down the slope.

  • Loose Wet

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

    Today's intense sunshine and warm temperatures will heat and melt the surface snow, making wet loose avalanches likely. These slides can occur naturally, without a human trigger. Where the snowpack is thin (less than about 2 feet thick), some of the wet loose avalanches could gouge down to the ground or rocks. Where is this problem most likely?

    1. On steep, middle and upper elevation slopes that received more than a few inches of snow yesterday and directly face the sun: NE-E-SE aspects in the morning, S-SW aspects near noon, and SW-W-NW facing slopes later in the afternoon.
    2. On lower elevation slopes that still have a foot or more of snow, regardless of the direction they face.

    Keep an eye on the condition of the snow surface while you're traveling. When it gets wet or when you see wet snow rolling downhill, it is time to move to gentle terrain or slopes where the snow surface is colder and drier. Wet loose instabilities can develop rapidly; think minutes rather than hours. Minimize the time you spend below all large, steep slopes that directly face the sun. 

Avalanche Discussion

The upper 3 feet of the snowpack is officially a mess in much of the forecast area. Choosing which weak layer or avalanche problem to prioritize varies dramatically over space (both weak layer distribution and recent snowfall amounts change as you move up or down in elevation and to slopes that face different directions) and time (how is the intense, April sun changing the stability equation). It's like picking a winner in a crowded, wide-open Kentucky Derby field. With that in mind, here's today's "Daily Avalanche Form" to handicap the avalanche field:

  • 1:1 (even money) Surficial wet loose avalanches: The hands-down favorite today. These relatively small avalanches are most likely on slopes directly facing the sun that received more than a few inches of snow since Sunday night. The mountains near Ketchum, Fairfield, Galena Summit, and Smiley Creek fit this description. While these avalanches will not be huge, they have the finishing kick to knock you down and carry you places you don't want to go. In the wrong terrain, they could be large enough to bury you.  
  • 3:1 Persistent slab avalanches: People triggered at least 4 large avalanches since the earthquake and the storm ended, and most or all failed on weak faceted snow that was near the surface before last week's storm. They are big enough to bury you and can be triggered remotely. They have been living in the Smoky Mtns and Galena Summit area this week, and possibly in the Soldiers as well. You could also find a stray in the mountains closer to Ketchum.  
  • 4:1 Full-depth wet loose avalanches: In areas with a thin snowpack and in rocky terrain, today's intense April sunshine will make larger, deeper wet loose avalanches a contender. Give this choice serious consideration if you're sinking into a thin, wet snowpack or are traveling in or below steep, rocky terrain. 
  • 7:1 Wet slab avalanches: These will have a better chance of finishing first later this week as the temperatures heat up, but they should not be discounted today. These beasts are most likely in the Soldiers and Wood River Valley Mountains where the snowpack is thin (less than 3 feet thick) and weak (lots of sugary facets beneath the snow from March storms). 
  • 7:1 Wind slab avalanches: Although the southern and western part of the forecast area picked up 5-12" of snow the past 2 days, winds were light. In alpine terrain, you could find isolated, soft wind slabs.

Daily Double options: Watch out where these problems overlap. Wind-drifting and persistent weak layers/slab avalanches form a potent one-two punch. Also, small wet loose slides could trigger slab avalanches lower on slopes as they move downhill. 

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