Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Sat
Alpine
3 · Considerable
Treeline
2 · Moderate
Below
1 · Low
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Highlights

New snow and wind are creating dangerous avalanche conditions. You’re most likely to trigger a slab avalanche in freshly formed wind drifts. Underneath these drifts, weak snow buried near the ground continues to pose a threat. To reduce your risk, steer clear of slopes where the wind has stiffened the snow surface.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2

    The Banner Summit Zone picked up 8-13” of new snow since Wednesday. This snow, accompanied by moderate to strong NW wind, built wind slabs in exposed terrain. Yesterday, small slab avalanches were triggered in the central Sawtooths, near Galena Summit, in the Boulder Mountains and in the Wood River Valley. 

    Expect to find touchy wind slabs again today, especially as you climb into more wind-affected terrain at upper elevation. Temperatures warmed during this storm and shallow, top-heavy storm slabs may also be triggered in relatively sheltered terrain. 

    These slabs will be most sensitive where they are sitting on firm surfaces, like crusts or older hard slabs. Ripples, dunes, or wave-like patterns can help you identify freshly loaded slopes. Cracks in the snow surface are a sure sign that the slope you’re on is unstable. A shallower wind slab that begins near the surface has the potential to break into deeper weak layers of snow near the ground (see below).

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    2–3

    Ben visited an avalanche that released Sunday in the White Cloud Mtns across the highway from the Sawtooths (photo to right). It broke over 500 feet wide, 4-6 feet deep, and wiped out the majority of the season’s snowpack, covering snowmobile tracks from the previous day. The snowpack isn’t identical in this zone, but similar layers of weak snow exist near the ground. When we talk about unlikely events with high consequences, this is what we’re referring to. 

    What are the common threads in the recent persistent slab avalanche activity?

    • Steep: slopes in prime-time avalanche terrain around 35-45 degrees.
    • Wind-loaded: heavily wind-loaded by recent wind events.
    • Alpine flavor: complex terrain with rock ribs, cliff bands, or minor ridges near the avalanche crowns or flanks.

    The consequences of a triggering a destructive, 2-6 foot thick, persistent slab would be severe. Be aware that even if you aren’t in steep terrain yourself, the snow that you are traveling on may be connected to that sort of terrain above you.

Avalanche Discussion

There are two avalanche problems to contend with today: freshly forming wind slabs at the snow surface and persistent slabs that have the potential to break on weak layers in the lower half of the snowpack. 

We’ve topped a foot of new snow in areas around Banner Summit, the Sawtooths, the White Clouds and the western Smokys. The hazard posed by new snow and wind is relatively straight-forward. Fresh drifts will be sensitive to your weight, and if you venture into windier upper elevation terrain, the snow stinging your face will be a good reminder to keep wind slabs at the top of your mind. 

Less obvious are the hazards that lurk below. The weight of new-fallen and wind-blown snow will stress weak layers deeper in the snowpack. Over the weekend and earlier this week, these layers proved they weren’t quite ready to go away quietly. Persistent slab avalanches were observed in the White Clouds, the Boulders, and the Pioneer Mountains. Similar avalanches are possible again today, particularly in areas where storm snow overloads a shallow weak snowpack.

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