Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Tue
Alpine
2 · Moderate
Treeline
1 · Low
Below
1 · Low
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Out of date — this forecast expired 7 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

The combination of new snow and wind is building isolated wind slabs in exposed terrain. These slabs are likely to be touchy, thanks to the faceted surfaces they are being deposited on. While small, encountering one of these slabs in very steep terrain could have high consequences. Avalanches in terrain sheltered from the wind are unlikely. 

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1

    Overnight, 2-3" of snow fell in this zone. In advance of this snowfall, wind speeds picked up for the first time in a week. On sunny aspects, the 5-10" of snow from last weekend's storm was locked in place by crusts, but shaded aspects were still holding a fair bit of unconsolidated snow. As the wind speed increased yesterday it was finding this unconsolidated snow and easily moving it around (video from the Sawtooths here). 

    Cold and clear weather last week left a mix of facets and crusts that new slabs will have bonding to. Because of this, freshly formed slabs are likely to be touchy today. I'd expect these slabs to be most reactive where a layer of facets is sitting on top of a stiffer surface underneath. After last weekend's storm, I spotted a few avalanches in the Sawtooths that failed in terrain where this combination existed, like the one pictured here. While the most recent wind slabs will be smaller than the ones I saw last week, encountering one of them in steeper terrain could have high consequences.

    Read a bit more about crusts, facets, and wind slabs in the Forecast Discussion section below.

Avalanche Discussion

Let's spend a minute thinking about crusts, facets, and the way the wind transports the snow:

CRUSTS:Crusts in the snowpack form when dry snow is melted and then frozen again. The most common ways these crusts form are with warm air temperatures, direct solar input, or rain. Crusts are often associated with faceted snow grains, which are described below. The combination of crusts and facets commonly form weak interfaces in the snowpack.

FACETS:Snow grains are in a constant state of change. Even before they hit the ground the structure of these crystals is rearranging in order to satisfy the whims of the laws of physics. Under weak gradients (the change in a given property over a certain distance), snow trends towards a more rounded state. Rounded grains tend to pack nicely with each other and form strong bonds. However, when placed under strong gradients, snow trends to a more angular state (facets). Cold, clear nights and days, like the ones we saw last week, create conditions that are conducive to the formation of faceted snow grains, or facets for short. We call them facets because they have sharp, angular edges on them, much like a cut gemstone. Facets are problematic because they don’t bond well to the snow around them, which makes them a “great” weak layer. This is a significant simplification of the processes involved here, but I hope it helps you grasp the basic points.

WIND:When the wind is sufficiently strong, it picks up snow and transports it to another location. While the snow particles blow through the air or bounce along the snow surface, they break up and become smaller. The smaller grains pack well into drifts once they're deposited. And just like that, you've got a wind slab. The speed of the wind plays a critical role in this process. Steady, moderate winds are great at building widespread, thick slabs. But shifting, variable, gusty winds don't do nearly as good of a job. If the wind doesn't blow hard enough it can't pick up much snow, but if it is blowing too hard it will throw the snow up into the air where it sublimates, never to rejoin the snowpack. The winds blowing yesterday and last night have been more on the gusty, and strong end of this spectrum. 

Keep these two patterns in mind — an overlapping patchwork of crusts, facets, and wind slabs. Today, triggering a wind slab is most likely where the presence of a crust+facet combination coincides with a fresh wind slab. Because of this, the problem will be isolated to specific terrain. In many areas, you won't find any touchy wind slabs. But in the right terrain, you will. Be alert to this as you travel through the backcountry today, and don't let the lack of touchy slabs in one location lead you to believe that this problem doesn't exist anywhere.

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