Bollettino valanghe

Tetons

Bridger-Teton Avalanche Center
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Non aggiornato — questo bollettino è scaduto 2 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

 As temperatures warm throughout the day, natural and human-triggered avalanches will become more likely. Upper-elevation shady slopes have lingering wind-affected snow that may react to your weight, and the sun and warm temperatures will rapidly change sunny slopes. Pay attention to the surface conditions—avoid slopes where you're either sinking into a wet, mushy snowpack or encountering stiff drifts and cracking. Middle and upper-elevation features near or under cornices should be given a wide buffer. 

Problemi valanghivi

  • Loose Wet

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

     As the day heats up, more slopes will become unstable. Yesterday, wet loose avalanches were reported on very steep sunny slopes. These avalanches are possible again today. Wet loose avalanches begin at a "point", usually the base of a warm rock or tree, and fan out to collect snow as they move. They are most problematic in confined terrain or on large slopes where they gather a lot of snow. Rollerballs, or small snowballs peeling off steep terrain, can be a helpful clue that the snow's surface is weakening and wet loose slides may occur. 

    Most natural wet avalanches have been relatively small lately. While we haven't quite reached the warming threshold for more activity, each day ticks closer. The cumulative effects of the prolonged warming can lead to an uptick in the size and frequency of avalanches. While wet slide conditions are fairly predictable, avalanches' expected nature (type, frequency, and size) can be hard to pin down. The forecast discussion below provides more information about wet snow. 

    (3/10/2024)

    Recent wet avalanche activity in the upper elevations of the Tetons.

     

  • Wind Slabs

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

    If you go poking around on enough very steep, wind-sculpted slopes, you may find isolated wind slabs that are sensitive to your weight. This problem can persist longer in areas where slabs rest on icy surfaces and small layers of facets that are hard to detect and predict. This is not a widespread problem, nor is it likely to produce large avalanches, but think about the consequence of taking a ride in a small avalanche before committing to a slope. If you find yourself in the wind slab's natural habitat today (i.e., the alpine), look for drifted pillows, cornices, and dunes. 

    This same wind-affected terrain harbors very large cornices. Extended periods of near or above-freezing temperatures may encourage these unpredictable, massive snow blocks to sag and fail. Be careful along ridgelines, as they can break further back than expected. Limit exposure under cornices, particularly in the afternoon, as natural cornice fall that triggers a very large avalanche on the problem listed below is possible, and many large avalanches have continued to be noted over the past several weeks. 

    (3/14/2024) Skier-triggered wind slab on No Name traverse.

  • Deep Persistent Slabs

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

    Hard slabs 3-5 feet thick overlay weak snow that formed earlier in the season. Shallow, rocky, & unsupported terrain on the north half of the compass is the terrain most likely to harbor this problem. Thin areas in the snowpack are more likely to act as trigger points and are most common in these locations. It is currently unlikely that a human would trigger one of these slabs. Larger triggers, like cornice fall or surface snow avalanches (i.e., wind slabs or wet loose avalanches), still have the potential to impact weak layers buried deeply in the snowpack and initiate a much larger avalanche.

    (3/5/2024)

    Very large persistent slab avalanche that failed during the early March storm cycle. This terrain characterizes where this problem is most relevant. Very steep, unsupported slopes above cliffs. Also note the large cornices and potential trigger. 

    Photo: Dan Nagy

Avalanche Discussion

With a warm weekend ahead, it's time to remind ourselves about W.U.T.A.N.G. snow conditions. What are these conditions, you ask? What was meant as a silly acronym two fellow forecasters and I made up for forecasting wet avalanche conditions rings quite true. It stands for Wet, Unconsolidated (re: mush), Terrain (capable of producing an avalanche), Aspect (solar), Non-refrozen, and Glide (not really an issue here, but a few pockets hang out in select terrain). It's easy to remember; these are all things to keep on your radar this weekend. Forecasting the transition from a dry to a wet snowpack is fascinating and unpredictable. Adding liquid water to a snowpack takes the complexity to the next level, as water is the only material on earth that can exist in all three phases (liquid, solid, and vapor). 

The sun adds energy to the snowpack surface, which melts the surface to produce water. This water can melt the bonds holding snow grains together and weaken the snow. If melt continues, particularly if the snowpack does not refreeze overnight (the N in WUTANG), things become saturated. Rocks, being darker in color, can absorb more solar energy and transfer that to the surrounding snowpack more rapidly. This is also why ridgelines and outcrops can be initiation points for wet loose and wet slab avalanches.

All in all, warming can do unpredictable things to a snowpack, even on shady aspects. Today, expect to find some isolated wind slabs still sensitive to your weight in upper-elevation terrain. And for all their complexity, wet snow avalanches are often easier to manage. Plan your days early and retreat from steep slopes as the sun rises in the sky and things warm throughout the day. 

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