Bollettino valanghe

Northwest Mountains

Crested Butte Avalanche Center
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Pericolo valanghe

Tue
Alta quota
3 · Marcato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
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Non aggiornato — questo bollettino è scaduto 6 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Wind transport continues and we observed more large naturals yesterday that broke on weak layers near the ground.   Avoid traveling on or below steep slopes where winds are loading snow.   Avalanches can break wider and larger than you might expect.  On or from below terrain that is steeper than 30 degrees, it is possible to trigger a deadly avalanche, especially in terrain where the snowpack is thinner and more variable in depth.  

Problemi valanghivi

  • Persistent Slabs

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

    The month-long string of natural persistent slab avalanches continued yesterday, driven by wind-loading.  Human triggered or natural avalanches have the potential to break 2 to 6+ feet thick - across entire start zones.  Avalanches that release from high elevation, leeward aspects will be deep and unsurvivable.   Even in wind-sheltered or windward terrain, you can trigger sizeable slabs that break on weak snow near the ground. The most reliable way to manage this problem is by traveling on slope angles less than 30 degrees that don't connect to steeper terrain above.   If you choose to ride in avalanche terrain, seek out planar or concave slopes with deep, uniform snow coverage, free of shallow spots, rock bands, and recent windloading.  

    A pair of very large persistent slabs that ran last week on Treasury Mountain.

  • Wind Slabs

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

    Gusty northwest winds over the past two days have formed shallow slabs of drifted snow on the snow surface.    They'll be harder to trigger today, but also stiffer and more likely to break above you.  Wind slabs triggered in the upper snowpack have the potential to step down and trigger much larger avalanches.  Avoid the lense-shaped lobes of drifted snow below cornices, in crossloaded gullies, and below terrain features where drifting snow accumulates.  

    Wind transport observed yesterday (2/21). Drifting snow forms slabs below ridges, cornices, and in gullies.

Avalanche Discussion

Natural persistent slab avalanche activity continued yesterday - the result of strong northwest winds adding stress to the poor structures that have been produced hundreds of dangerous avalanches this month.  The bulk of observed activity yesterday was in the Southeast Mountains (Example A, Example B).  Visibility was limited in the Northwest Mountains, but Irwin triggered a large persistent slab that snapped trees.   Avalanche conditions remain dangerous today where winds are actively or have recently drifted snow because large to very large avalanches can release naturally.  Our media gallery below highlights recent examples of persistent slab avalanche activity.

Pronounced weak layers from our December and January dry spells have plagued us all season.  They are now buried 2 feet deep in the shallowest parts of our forecast area, and 6 feet deep or more in the deepest parts.  The snowpack structure is ugly in the Southeast Mountains, and scary to look at in the Northwest Mountains.  Despite these poor structures, we are seeing diminishing signs of instability underfoot as we distance ourselves from the last major snowfall event and widespread cycle about a week ago.   Weak layers are deeper and harder to impact, and some slopes are starting to adjust (see this video).   As the danger starts to trend down, it is important to remember that downtrend is because of the decreasing odds of an avalanche, not because the consequences have decreased.   In the game of Russian Roulette, there were 4 or 5 bullets in the chamber in early to mid February. Now there are 1 or 2.  All bullets are equally deadly.   If you choose to ride avalanche terrain, careful snowpack assessments and terrain selection will reduce the odds of an avalanche even further.   Slopes that have recently shed their slabs offer safer options. Slopes with deep and uniform snow coverage, out of the wind, offer lower-risk options.  However, trigger points and weaknesses in the slab aren't always easy to identify.  The most tried and true way to remove all of the bullets from the chamber is on slopes less than 30 degrees that don't connect to steeper terrain above.    

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