Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

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Out of date — this forecast expired 6 years ago.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

Highlights

New snow today will add stress to the weak snow buried in this zone. Human-triggered avalanches breaking 1-2 feet below the snow surface are likely on steep slopes, both in sheltered and wind-affected terrain. The likelihood will increase as today's storm adds stress to these buried weak layers. And while it would be unlikely, we can't rule out the potential of triggering a very large, 4-foot thick avalanche—especially on very steep, rocky, wind-affected slopes.

Avalanche problems

  • Persistent Slabs

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

    Observations over the past week point to a problematic layer of weak snow buried 1.5-2' down. Today's storm will be adding additional stress to this buried weak layer. Chris was touring in this zone yesterday and found a layer of surface hoar at the interface between the old and the new snow that was reactive in his snowpack tests. He noted a large, natural avalanche that appeared to have failed on this layer at the tail end of last week's storm. The video at right/above shows how this layer was behaving during the storm last week.

    As the snow stacks up over the course of the day, the likelihood that you will be able to trigger a slide on this weak snow will increase, particularly in areas where the wind is building slabs. If the storm is more intense than forecast you may be able to trigger slides that fail at the interface below today's snow. These slides are most likely in upper elevation terrain where the wind is blowing. Any such slide you trigger will likely step down to the weak snow described above. Conditions will be dangerous today—keep your slope angles in check, be alert to the terrain above you, and pay attention to the consequences of the slopes you plan to travel on.

    You can read more about weak layers in the Forecast Discussion section below.

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    3

    Our snowpack is deepest in this zone, but there is a monster sleeping in the basement. A dense, 4+ foot thick slab of snow is resting on a weak layer of sugary depth hoar near the ground. The only cure for this type of problem is to bury it deeply and give it plenty of time to heal. Unfortunately, the weak snow doesn't seem to be in any hurry to do so. Chris looked at this layer on Friday and found that it was showing few signs of healing. The new snow that fell last week and the snow that will fall today will add more stress to this layer, keeping this monster from finally going to sleep.

    Avalanches occur when we are able to collapse a weak layer, initiating a fracture that propagates along the weak snow. Because the slab over this weak snow is so thick and dense, it is difficult for us to impact the weak snow. However, if we are able to collapse this weak layer we will be creating a very large avalanche that is unlikely to be survivable. Triggering a deep slab is most likely in very steep, rocky, and wind-affected terrain where "trigger points" exist. Trigger points are locations where the slab is thinner and where your weight can more easily impact the weak layer.  You can reduce the chances of triggering one of these beasts by sticking to slopes less than 35 degrees with a more uniformly deep snowpack.  

    (1/18/2021) This very large avalanche occurred in our Banner Summit Zone. Based on the depth and appearance of the crown it probably failed on facets buried deep in the snowpack.

Avalanche Discussion

THE EVOLUTION OF A WEAK LAYER

Weak layers in the snowpack don't just magically appear. Generally, they develop at the snow surface and are then buried by subsequent snowfall. After burial, different types of weak layers behave differently. Some are transient, causing stability problems that last for short periods of time, perhaps just a day or two. Others can stick around for weeks or months, causing avalanches every time they are loaded. Right now, we have two main layers of weak snow in our area and we are about to bury a third:

  1. Weak snow near the base of the snowpack: This snow has been buried for almost 2 months and it still keeps me up at night. Up until last week, this layer was responsible for nearly all of the avalanches this season (400 or so). It produced numerous avalanches during the storm and will continue to do so. It is now buried 3-4' down in this zone and any avalanche that fails on this layer is likely to be unsurvivable.
  2. Weak snow underneath the recent storm snow: The character of this layer varies by aspect (the direction a slope faces). On slopes that face the sun, a crust with some weak facets exists (the culprit weak layer in this large avalanche above Baker Lake). Other slopes are holding weak snow without a crust underneath and still others are harboring feathery crystals of surface hoar here.
  3. Weak snow currently at the snow surface: As avalanche forecasters, part of our job is to look into the future and predict how the weather will impact the snow that already exists on the ground. One way we do this is to look at snow surfaces before storms to see what is about to be buried. Yesterday, near Baldy and Sunday in Baker Creek, I observed layers of feathery surface hoar covering the snow surface.

It will take time to sort out which of the new weak layers in the snowpack are most unstable and how long they might be sticking around for. Some of the new weak layers may heal themselves over the course of days while others may be hanging with us for much longer. It takes time for us (and for you) to determine this. Even with days of fieldwork, there is a lot of uncertainty around these things. As we work to resolve some of this uncertainty we will be traveling cautiously and conservatively in the mountains. Alert to the terrain that we are in and the terrain that we are exposed to from above. I strongly urge you to do the same, it is a complex world out there and the likelihood of avalanches is increasing today.

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