Bollettino valanghe

Sawtooth & Western Smoky Mtns

Sawtooth Avalanche Center
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In sintesi

Large human-triggered avalanches failing on layers of weak snow are possible in both sheltered and wind-affected terrain. Yesterday's storm built wind drifts and slabs that will be reactive to your weight today. Small loose snow avalanches are possible in steep, rocky terrain, and will occur naturally as the sun warms the snow. The consequences of being caught in even small slides can be high 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
    1.5–2.5

    Several weak layers exist in the upper few feet of the snowpack. A prominent layer of surface hoar buried 1.5-2 feet deep has been responsible for a handful of skier and rider-triggered avalanches in the past few weeks (photo at right/above). This layer is at its worst on sheltered, shady (W-N-E) slopes. On slopes that face the sun, you'll find a few layers of crusts with facets that were buried in the second half of January. Unstable snowpack test scores involving both of these layers were reported this week in the Sawtooths (yesterday's observation here). Yesterday's storm added some weight on top of these weak layers and triggering a large avalanche involving them remains possible in both sheltered and wind-affected terrain.

    Slides that fail on persistent weak layers are less predictable than simple wind slabs. They may break wider than you expect and may wrap around terrain features that confine wind slabs. Persistent slabs can break well above you on the slope you are skiing or riding on.  Triggered wind slabs can "step-down" to these deeper weak layers, resulting in larger, more destructive avalanches. Keep these layers in mind when choosing your terrain today.

    ADDITIONAL DISCUSSION: Older weak layers (facets, crusts+facets, and 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. The resulting slide would be very large.

    A skier triggered this 1-2 foot thick avalanche on Sunday Jan 29th on a N-NE aspect at 9100' near Titus Ridge. It failed on a layer of buried surface hoar.

  • Wind Slabs

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

    Yesterday's storm brought 4-10" of new snow and a healthy dose of wind to this zone. Wind direction shifted through the storm, building wind slabs on many exposed slopes.  Wind loading and slab growth will continue this morning until wind speeds ease off later in the day. Be on the lookout for fresh slabs near ridgelines and on the sides of mid-slope features like gullies and ribs. In the steep terrain of the Sawtooths, slabs also commonly form below large cliff bands and on the aprons below couloirs during storms. 

    Though relatively small, these fresh slabs will be sensitive to your weight today. They are sitting on a variety of weak snow surfaces developed in the time since our last snowfall. As you travel through the mountains, look for signs that the wind has been at work. Dunes, pillows, and ripples on the snow surface indicate that the wind has been transporting snow. If the snow you are moving through is becoming deeper and denser, you're likely moving through wind-deposited snow. Cracks shooting out from your skis or machine are a clear sign of an unstable slab.

    In areas sheltered from the wind, small loose snow avalanches (sluffs) are likely today. These may be triggered by skiers or riders or may occur naturally as the sun heats up the new snow for the first time. Consider the consequences of being knocked off your feet and sent downslope in the terrain you are in.

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

Avalanche Discussion

In the past month, we've seen the character of the snowpacks in our region diverge. Snowfall in January favored our northern and western mountains, including the Sawtooth and W Smoky Mtn Zone. Here, the snowpack is 5-6 feet deep in most locations. Older weak layers have become deeply buried and triggering avalanches on them is unlikely, though not impossible. If my goal was to trigger one of these beasts I'd head for steep, rocky, wind-affected terrain where snowpack depths are variable. Our primary concern is with the weak layers in the upper 1.5-2 feet of the snowpack and the new snow at the surface. Read on. 

On sheltered slopes that face away from the sun, you'll find a layer of surface hoar buried about 2 feet deep. This layer produced a handful of human-triggered avalanches in January. A hefty slab rests on top of this layer and triggering slides on it is possible in sheltered terrain. On slopes that face the sun, you'll find a few sets of crusts with associated facets that were buried in January. Recent observations from the Sawtooths point to a problematic crust+facet+crust sandwich. These layers are a bit shallower than the surface hoar layer and so the overlying slab is thinner. 

At the surface of the snowpack, you'll find yesterday's 4-8" of storm snow. In exposed terrain, the wind has transported this snow into slabs that are likely to be reactive to your weight today. Look for them near ridgelines and in mid-slope terrain features like gullies and ribs. The wind has been blowing hard enough in this zone that you may find slabs down into middle elevation terrain. In sheltered terrain, you are likely to find some excellent powder skiing and riding. Unfortuately, you'll also encounter the persistent slab problem described above. Yesterday's storm started warm and ended cold which means you'll find low density snow sitting at the surface. Triggering small loose snow avalanches involving the new snow will be easy on slopes steeper than about 35 degrees today. On slopes that face the sun, these small sluffs will occur naturally as the sun warms the snow surface. These slides will start small but will pick up speed and snow quickly as they travel downslope, thanks to the stiff crusts the new snow is sitting on. The terrain in the Sawtooths is perfect for creating these types of slides. 

Go into the mountains today ready to deal with a range of avalanche problems. Spend a bit of time thinking about the consequences of triggering and being caught in an avalanche in the terrain you are in. Even small slides can have bad outcomes on slopes with cliffs, trees, and terrain traps. 

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