Bollettino valanghe

Banner Summit

Sawtooth Avalanche Center
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Pericolo valanghe

Wed
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2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
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Non aggiornato — questo bollettino è scaduto 3 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

You can trigger avalanches that will move 1-2 foot thick slabs of snow downhill today. These slides have the potential to break hundreds of feet wide and be triggered from a distance away. 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. Wet snow avalanches may become possible as the sun comes out in the afternoon.

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2

    February storms buried a series of new weak layers under 1-2 feet of snow. The variable layers of weak snow responsible for these slides appear to be at their worst where either sun crusts or rain crusts exist, but still are behaving unpredictably differently on each slope. They have produced a large, remotely triggered slide in addition to other large natural avalanches. spring weak layers are variable in places but also seem to be bonding well on other slopes.

    While you won't trigger these slides on all slopes, if you find the right slope it is possible to trigger large, destructive avalanches on this layer from a distance away. If you do trigger a slide on this layer it will be 1-2 feet deep, have the potential to break hundreds of feet wide, and can be large enough to bury you. Due to the uncertainty around the exact presentation of this layer from slope to slope, it will be hard to evaluate exactly what slope this problem lives on at upper and mid-elevations.

    ADDITIONAL DISCUSSION - WET SNOW ISSUES: On slopes where the sun is warming up and melting the snow surface this afternoon, loose snow slides are possible and 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. 

    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

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    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

You may have noticed that the danger dropped from CONSIDERABLE to MODERATE in this zone today. While the danger may have decreased, there are still real avalanche concerns — including a new Persistent Slab problem overlying our Deep Persistent Slab. While you won't trigger avalanches on all slopes, there is a list of evidence below pointing towards the new Persistent Slab problem being capable of producing large, destructive avalanches that would have significant consequences.  Want to know more? Read on. 

Complexity, uncertainty, and unpredictability have all been increasing in the snowpack over the last two weeks. Throughout the season, you've returned to the Forecast Discussion 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 new Persistent Slab problem. February storms have buried new weak layers (the 2/1) 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. These have produced a large, remotely triggered slide in addition to multiple other large natural avalanches in the Sawtooths, collapsing and unstable test results near Banner Summit, and very wide natural avalanches near Durrance, but also seem to be bonding well on other slopes. 

This complex new Persistent Slab problem seems to be at its worst on slopes with thin crusts 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) but still has a lot of uncertainty in its exact distribution. 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. Here is a quick video from a couple of days ago discussing this issue as it developed.

The 2/1 Persistent Slab issue increases our snowpack's complexity, uncertainty, and unpredictability. It also adds each of these to our decision-making when traveling in the backcounty as well. The only proper way to address each of these is through giving yourself wider margins, taking attentive observations, and continually reminding yourself of the big picture. Yes, the snowpack has some wind-drifted snow near the surface, a new Persistent Slab problem that is possible to trigger 1-2 feet down, and a Deep Persistent Slab problem 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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