Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

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Out of date — this forecast expired 6 years ago.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

Highlights

Avalanche conditions are still dangerous. You could trigger a 3' thick avalanche on some slopes—which would likely be unsurvivable. Avalanches may be triggered remotely—from below, above, and to the sides of steep slopes. 

Avalanche problems

  • Deep Persistent Slabs

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

    We have a well-established persistent slab problem across the entire forecast area. The problem has been slowly building since the weak facet layer was buried over a month ago. This load was enough to cause a widespread avalanche cycle in the adjacent Sawtooths (observation), but less natural avalanche activity was observed in this zone (observation) where the weak layer is the most deeply buried.

    The December facet layer is buried about 3+' deep on almost every slope. While there are early signs that the layer is starting to strengthen—we're definitely not out of the woods yet. Deep slab problems are very difficult to assess. Snowpack tests are less trustworthy, and classic signs of instability become less common or non-existent. It's important to come back to the simple facts—there is an extremely meaty slab overlying a known weak layer, and if you trigger a slide, it'll likely be unsurvivable. It's not a gamble worth taking. Further muddying the waters, observations indicate that some slopes that avalanched during the Solstice storm are capable of re-loading and running again. 

    The best—and often only—way to manage a deep persistent slab problem is by sticking to lower-angled terrain. Given the possibility of remote-triggering, you'll also want to avoid being below steep slopes. 

    (1/14/2021) Avalanche that released south of Copper Mountain. This slope avalanched during the Solstice storm and has become a "repeat offender". E, 8700'

  • Wind Slabs

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

    Strong winds this past week ravaged the snowpack. It formed touchy wind slabs on many exposed slopes, and the added weight of windblown snow contributed to many natural avalanches. Yesterday's few inches of new snow combined with moderate winds may have created a fresh batch of small wind drifts at upper elevations. 

    Although triggering a wind slab will be less likely today, it's tough to separate the wind slab problem from the persistent slab problem described above. Wind-transported snow has added stress to the buried weak layer making wind-loaded slopes more dangerous, and if you triggered a wind slab, it could "step down", failing on layers deeper in the snowpack and becoming much larger than the initial avalanche. 

    Wednesday's strong to extreme wind built fresh wind slabs on many slopes.

Avalanche Discussion

While the December facet layer exists nearly everywhere, the slab overlying the weak layer is anything but uniform. In fact, it's interesting to look at how the slab and weak layer vary across the forecast area as it illustrates nearly the full life cycle of a persistent slab problem.

Formative Years. The Wood River Valley has received the least amount of loading on the weak layer, and the slab is very soft and only about a foot thick. There isn't quite enough slab to produce widespread instability—the important exception being in wind-loaded terrain.

Troubled Teenager. In the western portion of the Solder & Wood River Valley zone, and the entire Galena Summit & E Mtns zone, it took nearly a month but the slab has matured into 2' of fairly dense snow. It has the growing pains to prove it—the storm last week pushed the snowpack into a tailspin of avalanche activity. This part of the forecast area is particularly dangerous—a 2' thick slab is the "perfect" combination of human-triggerability and dangerous consequences.

Fully Mature. In the Sawtooths & Western Smokys, the slab depth increases to 2-3' and becomes even denser. Last week's storm produced a number of large to very large avalanches—several could have destroyed vehicles or a house. As the slab becomes increasingly thick and dense, avalanches become harder to trigger. But if you do trigger one, it's likely to be unsurvivable.

A Few Gray Hairs. The Banner Summit zone has the meatiest slab of our forecast area and is 3+' thick in many places. This part of the forecast area had less natural avalanche activity following last week's storm, and the weak layer is starting to show early signs of getting "squashed" beneath the weight of the slab. But don't mistake being middle-aged for past its prime—this deep slab problem may have gained some tolerance with time, but getting on its bad side will have disastrous consequences. 

RIP. Sorry, this is likely a long ways off!

Be sure to check out Ethan's latest Weekend Update. Spoiler alert—it was yet ANOTHER exciting week!

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