Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Wed
Alpine
2 · Moderate
Treeline
1 · Low
Below
1 · Low
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Out of date — this forecast expired 6 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

Small slabs exist in upper elevation terrain exposed to the wind. The chance of loose snow avalanches will increase today as the sun melts the snow surface. Both types of slides will be small, but encountering them in very steep terrain could have high consequences. Avalanches in terrain sheltered from the wind and sun are unlikely. 

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1

    Sunday night's storm brought 2-3" of fresh snow to this zone. Winds blowing during and after the storm transported some of this snow into thin, isolated drifts in upper elevation terrain. Today, these drifts will be most reactive where they rest on a layer of facets on top of a stiffer surface or crust underneath. I spent the past two days traveling in the Sawtooths, just to the south of this zone, and described what I found in these videos from Sunday and Monday. Yesterday near Galena Summit, where less snow fell, a skier intentionally triggered one of these wind slabs.

    Look for obvious signs of wind loading such as dune-like drifts and ripples in the snow surface to help you identify wind-affected slopes. If the snow you are traveling through rapidly increases in depth and feels stiffer than loose powder, you've found yourself a wind slab. Cracking in fresh slabs are a clear sign that you've found a sensitive drift. These slabs are unlikely to be large, but encountering one of them in very steep, rocky terrain could have high consequences.

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

    ADDITIONAL DISCUSSIONToday will be the first sunny day after Sunday night's storm. On slopes on the southern half of the compass—those that directly face the sun—the snow surface will warm as the day progresses. This warming is more pronounced where rocks and trees are also being warmed by the sun and radiating out that energy into the adjacent snow. We will likely see some natural loose snow avalanches today, though slides should remain small. If you're seeing small balls of snow rolling downslope, or if the snow surface is starting to feel moist and stick to your skins, be aware of this problem and think about looking for a more shaded aspect to recreate on.

Avalanche Discussion

If you are going to run into a slab avalanche problem today, chances are it will be in an area where recent, thin wind drifts are resting on weak snow left behind in the wake of last week's cold, clear weather. In light of that, let's spend a minute thinking about crusts, facets, and the way the wind transports the snow. The following is a simplification of complex processes that take place in the snowpack, but I think it will help you to better understand the basic points:

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. Right now, the type of crust that we are dealing with is a sun crust. Sun crusts are distributed in a predictable pattern: slopes that directly face the sun will have the thickest crusts, slopes on the margins (E-SE and W) will have thinner crusts, and slopes on the northern half of the compass won't have any crust at all. 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 certain property over a 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 these faceted snow grains. Facets are problematic because they don’t bond well to the snow around them. They are frequently found in conjunction with crusts—the presence of the crust tends to increase the strength of the gradients in the immediate vicinity of the crust, which helps generate faceted snow grains. And just like that, you've got an ugly crust+facet combination that could make for a "great" weak layer. Now we just need a slab...

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 or slabs once they're deposited. The speed of the wind plays a critical role in this process. Steady, moderate winds are great at building widespread, thick slabs. 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 during the recent storm have been more on the gusty and variable end of this spectrum, leaving an irregular pattern of loaded and scoured areas. 

Keep these patterns in mind as you travel through the mountains 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. The wind slab that was triggered on Titus Ridge yesterday is a great example of this. 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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