Bulletin d’avalanche

Banner Summit

Sawtooth Avalanche Center
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Danger d’avalanche

Tue
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Points saillants

Avalanches are most likely on slopes where the wind has built slabs on top of weak layers in the snowpack. Slabs failing on these weak layers in sheltered terrain are less likely but can't be ruled out entirely. Very large avalanches failing on weak snow at the base of the snowpack have become unlikely but would be unsurvivable events.

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–2

    The combination of new snow (over 3') and plenty of wind during last week's storm were a great recipe for building wind slabs. These slabs have had a bit of time to bond to the surfaces that they were deposited on but there are still places where you can trigger them. Yesterday, on an off-duty ski day in the Sawtooths, my partner and I intentionally triggered a few small wind slabs in steep, alpine terrain. This type of terrain is unforgiving and you would not want to be caught on the wrong side of one of these slabs. 

    In this zone, these wind slabs are most concerning where they overlap with the persistent slab problems described below. Stiff slabs on top of weak layers (those in the middle of the snowpack as well as at the base) are never a good combination. 

    ADDITIONAL DISCUSSIONS

    • A crust+facet combination exists underneath last week's storm snow on slopes that directly face the sun (southern half of the compass). This layer produced a number of large, natural avalanches during the storm. Our observations from this zone are limited, but I saw at least 3 avalanches in the Sawtooths yesterday (observation and photos here) that likely failed on this layer. One spanned nearly 1/4 mile of terrain. Time will tell how long this layer is problematic, but I'll be leery of slopes where this layer exists for the next bit.
    • An observer near Banner Summit on Saturday found weak snow beneath the recent storm snow at lower elevations. This weak interface likely formed during temperature inversions last weekend. 
    • Weak October snow is buried 5-6+' deep on most shady middle and upper elevation slopes—deep enough to be very difficult to impact with our weight. Where might it still be possible to trigger a deep slab? I'm still avoiding very steep (>35 degrees), open, rocky, wind-affected slopes at upper elevations. 

    (1/8/2022) This wind slab avalanche in the southern Sawtooths released near the end of the storm. 9400', SE.

Avalanche Discussion

LIFE CYCLE OF A WEAK LAYER Here at the Avalanche Center, we spend a lot of time thinking about weak layers. We live in a snow climate that is particularly good at generating layers of weak snow that linger for days and weeks. These layers don't just spontaneously appear in the snowpack, they are created when the snow is still at the surface, exposed to the environment, and then get buried by subsequent snowfall. Right now, we are dealing with a few different weak layers in the snowpack; one that has been buried for over a month and another that has been buried for just a few days. Across our region, there is a lot of variability in both the nature of these weak layers and the character of the slab overlying them. 

I'm sure that you are well aware of the layer of "October snow" that exists at the base of the snowpack. This layer has been responsible for dozens and dozens of avalanches that we've observed in the past month. We've had the benefit of a few storms worth of data to watch how this snow has behaved during loading events and dozens of field days looking at it up close. The new player in the weak layer game, which we started to bury on January 3rd, is a different story. We know this layer can produce avalanches because we've already seen a handful of them. But we don't know how it will behave going forward. 

In my personal backcountry travels, I like to take a step back when I have a high degree of uncertainty, and that is how I'm dealing with this weak layer for now. We have a good sense that it is most problematic on steep slopes that face the sun directly, where it exists as a layer of facets adjacent to a crust. Because of that, I have taken this type of terrain off my personal backcountry "menu". I'm also planning to focus much of my attention on this layer in the coming days in order to get a better sense of where it is at its worst and how it is changing over time. I'll head out the door with a hypothesis in my head, compare that to the observations that I make in the field, and update my mental model accordingly. Expectations, observations, repeat.

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