Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Tue
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Out of date — this forecast expired 3 years ago.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

Avalanches involving recent snowfall are likely on many slopes today. Slides running at the bottom of the new snow will move 1-2 foot thick soft panels of snow downhill. More destructive larger avalanches failing on deeper weak layers that involve 3-4 foot thick, dense slabs are much less likely but would be unsurvivable.

Avalanche problems

  • Wind Slabs

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

    Today's 1-2 inches of fresh snow will add to the 20-23 inches that fell at the end of the week. You can expect to find a wide range of problems involving this recent. These broadly fall into two categories:

    SLABS: Periods of shifting winds have built several generations of wind slabs at the surface. Look for drifts, dunes, and rippled snow surfaces as an indication that the wind has been at work, but expect much of this evidence to be masked by today's snowfall.  Cracks shooting out from your skis or machine are a clear sign that you've found an unstable slab that could be triggered on steeper slopes. Even in terrain that is apparently sheltered from the wind, you can trigger slabs that fail at the base of the February snow (see photo at right/above). The variable layers of weak snow that are generating this problem appear to be at their worst where either sun crusts or rain crusts exist at this interface. Read more about the budding persistent slab problem in the Forecast Discussion below. 

    LOOSE SNOW: In terrain sheltered from the wind, expect small loose snow avalanches to be easy to trigger. If you're seeking out cold, crust-free snow on northerly aspects these sluffs will have a dry character. On slopes where the sun is warming up and melting the snow surface this afternoon, these loose snow slides will have a wet character. If you're finding moist/wet snow at the surface or if you are seeing wet balls of snow rolling down the slope this type of slide is coming onto the menu. Loose snow slides will start out small, but can pick up steam and run quickly thanks to the stiff and crusty surfaces underneath the recent snow. 

    Avalanches involving the new snow will range from small to large enough to bury you. Avalanches failing near the surface can be effective triggers for avalanches failing on the deeper weak layer described below. 

    This large avalanche was remotely triggered by skiers near Dollarhide Summit on Friday. It failed on an E/SE-facing slope at 9,100'. Based on the appearance of the slide, it likely failed on a weak layer that was buried at the beginning of February. 

    Photo: Sun Valley Heli Ski

  • Deep Persistent Slabs

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

    A widespread layer of weak snow rests underneath very dense, 3-4 foot thick slabs of snow here. In general, triggering these slides is not very likely because the weak layer is well-armored by the dense slab in many locations. However, if you find the trigger point for one of these monsters, the resulting slide will break wide and deep, producing a very destructive avalanche. You're most likely to trigger this type of slide in steep, rocky, wind-swept alpine terrain, where thinner areas of the snowpack give you better access to the weak layer beneath. 

    You won't see obvious signs of instability before triggering this type of slide. Assessing slope-scale stability with deeply buried weak layers is next to impossible. If you're entering avalanche terrain, this layer is likely underneath your feet. If you head into areas with thin, variable snowpacks in alpine terrain, you are playing where triggering one of these slides is most likely. Spend a little time thinking about what the terrain and snowpack look like where you are riding, and the consequences of triggering and being caught in a very large avalanche there.

Avalanche Discussion

Throughout the season, you've returned to this place to see a similar vein in a lot of our messages about the snowpack: buried persistent weak layers can fail underneath newer snow to cause avalanches. While this remains true today, things are changing and becoming more complex, as we have a budding Persistent Slab problem. February storms have buried weak layers across our zones that are taking many forms on different slopes underneath 1-2 foot thick soft slabs, and taking on hard-to-predict characteristics. Primarily we've been managing a progressively more deeply buried weak layer that is creating our widespread Persistent Slab problem. Difficult to manage due to the ease of remote triggering avalanches on this layer, the potential size of these slides, as well as being entrenched in the same mindset for much of the season since the December drought ended, this weak layer has been mostly uniform throughout our zones. Our newly buried February weak layers are weak enough to produce large slides from a distance on some slopes (remote trigger), while bonding quite well on others. 

This budding Persistent Slab problem seems to be at its worst on slopes with thin crust with facets on either side (a crust + facet combo). Avalanche activity involving these weak layers has been focused on slopes that get more direct morning and evening sun ("solar margins", or slopes facing E-SE and WSW-W). A widespread rain crust that extends to above 10,000' exists here as well, producing more signs of instability like collapsing and unstable snowpack test scores. A 1-2 foot thick slab overlies these weak layers, and is growing with each storm.

While it's hard to say with certainty where these weak layers are heading, here are some data points nudging us to pay heed to them going forward:

Dollarhide remotely triggered avalanche - A large avalanche was remotely triggered by the professionals at Sun Valley Heli Ski on Friday (photo here). It broke several hundred feet wide on an E/SE facing slope at 9,100'.Profile drainage natural avalanche - A large natural avalanche was observed in the Profile drainage yesterday (photo here). It broke several hundred feet wide on an E/SE facing slope at 8,500'.Parks Peak natural avalanche  - A very large natural avalanche was observed on Parks Peak that likely failed at the tail end of the storm on Thursday on a SE-facing slope at 10,000' (photo here).Banner Summit area -  View my observation from Friday of collapsing and unstable snowpack tests involving this weak layer here.Durrance area natural avalanches - Very wide crowns involving this weak layer were observed adjacent to avalanches that failed on deeper weak layers following storms earlier in February.  

And here is a quick video from yesterday discussing this developing issue.

Complexity, uncertainty, and unpredictability have all been increasing in the snowpack over the last two weeks. The only proper way to address each of these is through giving yourself wider margins, taking attentive observations, and to continually remind yourself of the big picture. Yes, the snowpack has easy to trigger near-surface snow instabilities that may come as loose snow or wind slabs, a budding persistent slab problem that is likely to trigger 1-2 feet down, and a persistent slab probelm that is harder to trigger 3-4 feet down. But the big picture? It is a hard to predict world out in the hills right now. Adjust your travel plans accordingly, and come home to tell us about the wild slides you saw, from a safe distance, that you never would have expected.

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