Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Thu
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
1 · Low
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Out of date — this forecast expired 4 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

You may be able to trigger avalanches up to 2 feet thick, especially where the wind has drifted snow. Expect small, loose snow "sluffs" to release naturally in very steep terrain. Large avalanches are unlikely on lower elevations slopes.

Avalanche problems

  • Loose Dry

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–1.5

    Expect natural and human-triggered loose snow avalanches (sluffs) in very steep terrain (approaching 40*) where more than a few inches of snow has fallen recently. Additional snowfall can trigger natural activity, as can even brief periods of direct sunlight on slopes sheltered from the wind.

    While these slides won't be very big, they will be "hauling butt" on the slick surfaces beneath them. The sluffs could easily pack enough of a punch to knock you over or off your machine, carrying you where you don't want to go. Also, there's a remote chance the small loose snow slides could trigger larger avalanches that fail on the weak layers described in Problem 1. 

    Today is a good day to pay attention to the terrain above you and limit your exposure to hazard from above. If visibility is limited, be careful in—or avoid—serious, complex terrain. 

  • Persistent Slabs

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

    The storm snow and wind-transported snow is adding weight to the buried weak layers responsible for the 1-2 foot thick avalanche cycle last week. Those avalanches failed on slopes on the northern portion of the compass (NW-N-NE-E) at middle and upper elevations. While stability was improving before this little storm, it's hard to predict whether the fresh snow will be enough of a load to "wake up" the weak layers that produced last week's avalanches. Slopes that fit the description above and where the wind has stiffened the snow surface or formed drifts are most likely to cause problems today. 

    Small changes in aspect or elevation can have a big impact on stability; be careful if you're playing in terrain that fits the description above, regardless of whether the wind has been blowing or not.  

    ADDITIONAL DISCUSSION: With today's expected snowfall and wind, fresh wind slabs could grow and form on more slopes as the day progresses, increasing the odds you'll find them. How will the wind slabs behave today?

    • They formed on a variety of surfaces and could be very sensitive (easy to trigger) on some slopes and stubborn (difficult to trigger) on others. 
    • Where the slabs buried stout crusts or weak snow, avalanches you trigger could be wider than you'd expect. 
    • You may find hard, stiff, and soft wind slabs. The harder/stiffer the slab, the higher the probability it could break above you rather than at your feet or sled.
    • Wind slabs could exist near ridgelines, on the sides of cross-loaded gullies, and in isolated areas lower on slopes. 
    • The relatively small slides could travel very quickly on slick surfaces beneath the new snow. 

    Pay attention to how the snow feels under your skis or boards. If the snow surface suddenly stiffens or becomes more supportable, you've found a wind slab. Cracks shooting out in front of your skis or boards are a red flag warning you to be cautious. Surface clues like dunes or ripples can tell you the wind has been at work.  

    While the wind slabs may or may not be big enough to bury people, they could be powerful enough to carry you downslope or trigger the persistent slabs described above. Think about the consequences of being caught and carried before committing to skiing or riding in consequential terrain (above cliffs, trees, creeks, etc). 

    (3/2/2022) This series of persistent slabs failed near the tail end of the warm, wet storm last week. These crowns span over 1,000' of NE/E-facing slopes around 8,400-8,700'. 

Avalanche Discussion

CHANGES IN ATTITUDES

After a couple of months of almost no snow, we started seeing some precipitation beginning a week ago. Last week’s warm storm brought enough snow and rain to set off several large slab avalanches near Banner Summit. The culprit was weak snow (facets, facets and crusts, and maybe some surface hoar shards on some slopes?) that formed in January. Additional weak layers formed in February and early March, making the upper snowpack messy and complex. The relative strength of the weak snow, the presence and thickness of crusts or surface hoar, and the characteristics of the slab above the weak snow change rapidly as you move up or down in elevation and around the compass to different aspects. Large slab avalanches are not likely to fail on the upper snowpack persistent weak layers today, but the weak layer and slab spatial variability make it nearly impossible to accurately forecast stability on a given slope.

How do we deal with complex situations like this? KISS: Keep it Simple Stupid/Scotty. The weakest snow in the upper 18” of the snowpack appears to exist on the northern half of the compass (NW-N-NE-E aspects) and spans the middle/upper elevation boundary. Strong winds have prevented widespread weak layers from forming in true alpine terrain, and rain and warm temperatures have “cooked” or melted most potential weak layers at lower elevations. In the “money zone” where the weak snow exists, we (the SAC forecasters) are dialing back our terrain choices. We’re beginning to avoid large or high-consequence starting zones over 35* where the weak layers might exist and a slab has formed in the upper 1-2 feet of the snowpack.  

We’ve seen a handful of interesting avalanches fail on the weak snow in the upper snowpack over the past month. The prudent move is to treat those events as foreshadowing rather than outliers. Each small storm, including today’s, bumps up the chances of avalanches failing on this weak snow and makes me less interested in playing in that terrain until stability/instability patterns emerge.  

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