Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Sat
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
2 · Moderate
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Out of date — this forecast expired 3 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

Conditions are dangerous. Expect to trigger a large, 1-3 foot thick avalanche on wind-affected slopes. Slides are also possible on some wind-sheltered slopes. Avalanches may be triggered remotely—from gentler terrain above, below, and to the sides of steep slopes.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1.5–2

    You are likely to trigger a wind slab avalanche in upper elevation, wind-affected terrain. While the winds have eased somewhat over the past 24 hours, stormy weather during the first half of the week produced a number of sensitive wind slabs.

    Many wind slabs have formed atop persistent weak layers (see Problem #2 below). As a result, they could break wider than expected. On Tuesday, Ben observed a large avalanche on the north side of Copper Mtn near Banner Summit (report). The slide was triggered remotely from low-angle terrain and released on a wind-loaded slope just below the ridge. 

    Wind slabs will be thicker, wider, and most dangerous at upper elevations, but you may also find them in wind-exposed middle-elevation terrain. Look for wind slabs near ridgelines and on mid-slope terrain features like ribs and gullies. Multiple rounds of snowfall and wind this week may mask visual clues that indicate wind loading. Even subtle wind effect may be enough to stiffen the snow and increase your odds of triggering a slide. In addition to finding stiffer drifted snow, collapsing or shooting cracks are obvious signs of instability. 

     

    (2/28/2023) This large avalanche was observed on a N aspect of Copper Mountain near Banner Summit. The crown extends into terrain that is mostly sheltered from the wind.

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1.5–2.5

    You could trigger 1-3' thick avalanches on buried weak layers (sugary facets or crust + facet combinations).  The weight of recent storm snow is bringing the weak layers to the tipping point on some slopes. Wind-loaded slopes have a thicker, more cohesive slab and are the most dangerous (see Problem #1). But recent storms have built enough of a slab atop persistent weak layers that some wind-sheltered slopes could also produce avalanches (photo above/right).

    The distribution, character, and sensitivity of the weak layers vary from slope to slope—your best bet is to assume this problem exists until proven otherwise. Given the nature of the weak layers, you could remotely trigger avalanches from gentle terrain above, below, or to the sides of steep slopes. Avalanches could be triggered by the second, third, or fourth rider on a slope.

    You can reduce your risk by avoiding slopes steeper than about 35 degrees—especially large avalanche paths and those with ugly consequences if you are caught in a slide. 

    (2/28/2023) NE face of Copper Mountain at 8,400'. This avalanche appeared to be remotely triggered by a skier from above. This crown in sheltered terrain points to the potential for encountering avalanche problems outside of areas that are obviously affected by the wind.

Avalanche Discussion

The early winter is notorious for forming a thick layer of sugary snow near the base of the snowpack. But as the season progresses, warm, sunny days and cold, clear nights readily form weak, faceted snow at the top of the snowpack, often sandwiched between frozen crust layers on sunny slopes. On colder slopes, the faceted snow surfaces are often referred to as “recycled powder”. This is just what happened during dry weather in the first half of February.

Sometimes weak layers are overloaded by a large storm, leading to an obvious spike in avalanche danger. Our recent loading pattern has been more insidious, with a number of 4-8" storms incrementally adding weight to layers of sugary facets and facet+crust combinations. With each small loading event, we watch for the “canaries in the coal mine”—indications that the snowpack is reaching its breaking point. During the second half of February, the canaries began growing in number. Early indicators of worsening conditions—such as natural and triggered avalanches—occurred on slopes that received additional loading from windblown snow. 

With the storms earlier this week, the canaries are beginning to form a flock. We’ve finally begun to see natural and triggered slides even on wind-sheltered slopes. In other words, a problem that’s been limited to wind-loaded areas has blossomed into a more widespread one that encompasses both wind-exposed slopes as well as sheltered terrain. As the slab atop the weak layers becomes thicker, avalanches are growing in size and are plenty large enough to bury or injure a person.

The transition in conditions requires a shift in mindset. A dry-ish February brought relatively stable conditions, but now we need to take a big step back. This can be challenging as spring approaches and folks often expect a more stable snowpack. But if you look back over the past 20 years, our area has experienced seven avalanche deaths in March and April. So by all means, get out there and enjoy the great powder conditions! Just remember that conditions are more dangerous than they have been, and take a conservative mindset.

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