Bollettino valanghe

Sawtooth & Western Smoky Mtns

Sawtooth Avalanche Center
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Pericolo valanghe

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

In sintesi

Many natural large to very large wet avalanches occurred yesterday. Similar slides are likely today. Sustained sunshine and warm temperatures are weakening an already unstable snowpack. The snowpack is delicate and will produce dangerous slides today — build wide safety margins. 

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    2–3

    A hefty, 1.5-2.5 foot thick slab lays over a delicate weak layer. In wind-loaded terrain, the slab may be twice this thick. This slab + weak layer combination exists on most slopes and has produced countless avalanches in recent days, including numerous large, remotely triggered slides.  Digging into the snow you'll find a layer of snow that resembles an early-season weak layer: thick facet stacks on colder aspects, crusts with associated facets on sunnier slopes, and a good helping of surface hoar sprinkled in. 

    Avalanches that fail on this fragile layer will break widely and in surprising ways. These slides can be triggered from long distances away (remote triggering) and can extend back into lower angle terrain. Smaller avalanches involving snow near the surface like we are expecting today can step down to this weak snow to produce larger, more destructive slides. Today's direct sunshine and warm temperatures continue to add stress to this layer, melting the upper snowpack and forcing it to pull itself downhill. 

    ADDITIONAL DISCUSSION - WET SLAB: Water is making its way to the weak layers described above. A large wet slab avalanche cycle occurred yesterday involving these layers. With warm temperatures and sunshine, these slides will be likely today. Wet slabs will become increasingly likely throughout the day, and can run well past the heat of the day.

  • Loose Wet

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

    Expect wet avalanche activity driven by the sun today. These are most likely at lower and middle elevations and on upper elevation slopes that face the sun, where the snow is moist enough to make a snowball. These slides will start small and pick up speed and momentum as they move downslope, especially on sunnier slopes where they may run on slick crusts. These slides may "flank out" as wet slabs, producing even larger, more dangerous avalanches.

    Loose slides will start at a point and pick up snow as they travel downhill, becoming increasingly dangerous as they do. At lower and middle elevations these may gouge down and entrain much of the snowpack. As you climb higher into colder, windier terrain, look for small fresh cornices and wind slabs. Steep slopes that haven't been affected by the sun can produce dry loose avalanches. Surface-level wet and dry snow problems are likely to be on the small side today, but can be very effective triggers for much larger persistent slab avalanches mentioned above. 

Avalanche Discussion

Expect the Dangerously Unexpected

The Boulder and Pioneer Mountains experienced a remarkable avalanche cycle yesterday with dozens of slides that could destroy your house, and more slides than you can count that could bury a person (see video). While all zones experienced a wet avalanche cycle with the spring-like temperatures and sunny skies after a rain event the day prior, most of these slides were wet loose avalanches. The largest avalanches that ran in the Pioneers and Boulders were wet slab avalanches. Wet slabs are notoriously hard to predict the specifics of where and when they will run, and even more difficult to understand exactly what mechanism caused them. Given our complex snowpack setup prior to our warm up, this unpredictability is increased. Did the weak layer lose strength? Did the slab change? While these academic questions are interesting, they don't matter today. The implications are quite simple: large, dangerous avalanches are likely to run in unexpected ways today. Being on or near steep slopes is a dangerous decision.

We have the same weak, complex snowpack that we started the weekend with. Widespread natural avalanches and a cycle of large, remotely triggered slides showed that our thick, meaty slab on top of weak snow from the December drought is likely to break as large and destructive persistent slab avalanches. These large persistent weak layers haven't gone away. We changed the equation by adding rain, sky-rocketing temperatures, and sunshine. These inputs are enough to make a stable snowpack produce large slides. Coupled with our unstable setup this season, the only reasonable thing to expect is the dangerously unexpected. 

While the story of the snowpack is quite complex, the moral of the story is plain and simple: being anywhere in avalanche terrain is a dangerous decision today.  Danger will increase throughout the day with warm temperatures and sunshine, and avalanches can occur well past the heat of the day. The slides that fail are going to big. This combination is enough to keep me avoiding avalanche terrain and being aware of the terrain above me. I'd advise the same for you today. 

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