Bollettino valanghe

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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Pericolo valanghe

Tue
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3 · Marcato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
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In sintesi

Numerous human-triggered avalanches were reported this weekend, most involving layers of weak snow buried 1-3' beneath the snow surface. Triggering a large avalanche is more likely on recently wind-loaded slopes, but is a distinct possibility in wind-sheltered terrain as well. Avalanches can be triggered from gentler terrain above, below, and to the sides of steeper slopes. 

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1.5–2.5

    A number of weak layers exist in the upper 1-3' of the snowpack. The most concerning of these is a widespread layer of surface hoar buried 1-2 feet deep. Yesterday, skiers in the Soldiers triggered two avalanches that failed on a crust with surface hoar on top. Further north, two separate groups in the Sawtooths remotely triggered large avalanches that appear to involve this surface hoar (observations here and here). Three other similar slides occurred on Saturday; in the Smiley Creek and Pole Creek drainages and near Galena Summit. Most of these slides occurred in middle elevation terrain on slopes where any signs of wind loading were not obvious. On slopes that face the sun, you may not find surface hoar here but you are likely to find crusts with weak facets on top.

    You are most likely to trigger slides on weak layers that are closer to the surface, like the surface hoar layer described above. However, deeper weak layers also exist in this zone. These layers are harder to impact with your weight, but the resulting slides will be even larger and more destructive. It's a safe bet that most slopes harbor at least one of these layers of weak snow. And while you're more likely to trigger avalanches on slopes where the wind has built slabs and stiffened the snow surface (example), you can certainly trigger these avalanches in sheltered terrain as well.

    Pay close attention to the snow you are moving through: cracking and collapsing are obvious signs of instability. But, don't expect to get direct feedback today.  Persistent slab avalanches behave in ways that catch experienced backcountry travelers off guard. The first sign of instability may be an avalanche that you trigger. These slides can break wider than you expect and can be triggered long distances away from steeper slopes.

    ADDITIONAL DISCUSSION: Weak snow exists near the base of the snowpack. Triggering slides on this snow is unlikely, but the possibility can't be ruled out in steep, rocky, wind-affected alpine terrain. Slopes like these have variable snow depths, with thinner areas of the snowpack that allow us to impact deeper weak layers.

    (1/14/2023) Surface hoar found in a pit at 7,900' on a NE aspect near Banner Summit.

  • Wind Slabs

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

    Saturday, Soldier Mountain Cat Ski triggered large avalanches involving fresh wind slabs using ski cuts and explosives. Moderate to strong winds blew first out of the S, and then out of the NW, transporting the new snow into thick drifts that will remain sensitive to your weight today. Look for these slabs near ridgelines and in mid-slope terrain features like gullies.

    Pay attention to how deep and how dense the snow you are traveling in is. If the snow depth and density increase you are likely traveling through a fresh slab. Cracks shooting out from your skis or machine clearly indicate that you've found an unstable slab. Triggered wind slabs can step down to the layers of weak snow described above, creating much larger, more dangerous avalanches. 

Avalanche Discussion

CONNECTING DOTS

Eight human-triggered avalanches were reported this weekend, many of which were large enough to bury a person. Avalanches occurred across our Forecast area: in the Soldiers, the Smokys, the Sawtooths, and the White Clouds. Most of these slides appear to have failed on a persistent weak layer that was buried 10 days ago, near the start of January. Widespread surface hoar has been found at this interface, along with crusts with facets. Importantly, most of these slides failed in sheltered, middle elevation terrain. The parties that triggered these slides were enjoying great powder and avoiding obviously wind-affected snow. This is an effective way to manage wind slab problems, but it will not eliminate your exposure to persistent slab problems.

Persistent weak layers are hard to deal with. Right now, we are dealing with multiple weak layers at the same time. The new weak layer on the stage is particularly concerning. We don't often deal with widespread surface hoar in our neck of the woods, but that's not a good excuse to ignore the obvious pattern that has emerged. Avalanches involving persistent weak layers are possible in both wind-affected and wind-sheltered terrain. Your risk of triggering a slide in sheltered terrain is somewhat lower, but not by a ton. 

CONSIDERABLE danger with persistent weak layers in the snowpack is very serious business. You can trigger avalanches large enough to bury you on most steep slopes today. Stop and read that again. You may not receive any direct feedback from the snowpack that indicates instability. The first sign of the weak snow underneath may be the large avalanche that you trigger. Today will be beautiful in the mountains. The snow will be soft, and deep and the smiles will be large. Don't be distracted, the layers of weak snow in our snowpack should be at the front of your mind today. Keep your slope angles low, pay attention to what is above you, and consider the consequences of being caught in a slide in the terrain you are in.

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