Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Fri
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Out of date — this forecast expired 6 years ago.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

You're likely to trigger 2 foot thick avalanches on steep slopes—particularly in areas of recently wind-drifted snow. While unlikely, we can't rule out the chance of triggering a very large and destructive, 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

    A weak layer of facets and/or surface hoar exists on many slopes, buried under about 2' of snow. This layer was responsible for a slab avalanche on Copper Mountain near Banner Summit yesterday (photo) and continues to produce repeatable, poor snowpack test scores.

    Tuesday night's 6-10" of new snow has added stress to this weak interface, particularly in windier locations where fresh drifts formed near the snow surface. This layer will remain sensitive to your weight today and triggering an avalanche remains likely in steep terrain. 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 ski or ride.

    (2/3/2021) This avalanche failed 2' deep on buried surface hoar. This weak layer has presented itself as surface hoar in some areas and small facets in others. NE, 8400', Copper Mtn. Photo: Sawtooth Mountain Guides

  • 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. Chris looked at this layer on Friday and found that it was showing few signs of healing. Ongoing snowfall since the weekend is just enough to keep this monster from finally going to sleep.

    Avalanches occur when we are able to collapse a weak layer, initiating a fracture that propagates through 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 you're able to find a spot to collapse the weak layer, the resulting avalanche will be huge and probably unsurvivable. Triggering a deep slab is most likely in very steep, rocky, wind-affected terrain where "trigger points" exist. Trigger points are locations where the overlying slab is thinner and the weight of you and-or your sled 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 and avoiding uneven terrain that's peppered with rocks.  

     

    (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

If you haven't watched the video Ben captured yesterday, you should probably do so now (here). It's amazing how impactful a video like this is. Rather than "just a picture" you can imagine yourself moving with the avalanche, feel its power, and witness its speed. Quite spooky really. Now think again about the size and destructive potential of the other slides observed yesterday like this impressive avalanche in Eagle Creek, this fat slab off Boulder Peak, or this wide avalanche in Croy Canyon near Hailey. These slides paint a humbling picture, one that clearly indicates dangerous avalanche conditions.

There are two weak layers that are the driving force behind our unstable snowpack:

Weak snow underneath last week's storm snowThis weak snow formed in mid to late January, and the character of this layer varies by aspect (the direction a slope faces) and elevation. You might find crusts, crusts, and faceted snow, small facets, and some surface hoar (more widespread near Banner Summit). In field reports, you'll see this layer referred to by the date it was buried (1/27).

Weak snow near the base of the snowpackThis snow has been buried for 50 days and is not yet showing signs of going to bed. Up until last week, this layer was responsible for nearly all of the avalanches we've observed and recorded this season (approaching 500). This layer of facets has shown signs of improvement in the Banner Summit area and now has enough snow above it to begin the healing process in many parts of our forecast area. It won’t happen overnight, or even in a week, but there may be light at the end of the tunnel.

A bit of a wild card is the interface that was buried with Tuesday night's snowfall. Warm temperatures and sunshine formed crusts on many south and west-facing slopes. Surface hoar, observed in the Banner Summit, Galena Summit & Eastern Mtns, and Soldier & Wood River Valley Mtns zones, was also buried.

Tuesday's snowfall and another storm on tap for tonight and tomorrow aren't giving these weak layers a chance to improve. While the intricacies of our snowpack are ever-evolving we have a pile (literal piles) of data that point to dangerous avalanche conditions.

Regions covered