Bollettino valanghe

Northwest Mountains

Crested Butte Avalanche Center
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Pericolo valanghe

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

In sintesi

Rider and explosive triggered persistent slabs yesterday highlight our ongoing concerns for triggering a large and deadly avalanche that breaks several feet thick on old, weak snow near the ground.  The problem exists at all elevations, most commonly on northerly and easterly-facing slopes.   Safer riding is on wind-sheltered, southerly aspects or low-angle terrain less than 30 degrees.  

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–1.5

    Yesterday's northwest winds formed relatively shallow but dense drifts onto high-elevation, southeasterly aspects.  Although these slabs are stubborn to triggers, they are hard enough to break above you or sweep you off your feet in consequential terrain.  Look for stiff, smooth-looking pillows of drifted snow below cornices, ridgelines, or crossloaded gullies to identify the problem.   Seek out wind-protected terrain to avoid the problem.  

    An example of a terrain feature below a leeward ridgeline where wind slabs formed yesterday.  

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2.5

    Slabs 1 to 4 feet thick have formed over several persistent weak layers in the snowpack:  Facets and depth hoar near the ground and surface hoar layers in the upper snowpack. The problem has been most active on northerly and easterly facing slopes, with some avalanches on westerly facing terrain as well. Persistent slabs are gradually becoming more difficult to trigger and signs of instability are becoming less frequent, but if you cross the wrong slope or the wrong spot on a slope, a single collapse can result in a surprisingly large avalanche involving most of the snowpack.  Persistent slabs could break wider than you expect, and they can be triggered from flat slopes connected to steeper terrain above you. You can reduce your risk by traveling on slopes where the problem isn't highlighted above, or by traveling on small terrain features without terrain traps.  You can avoid the problem on slopes consistently less than 30 degrees.    

    12/8: The flank of a persistent slab avalanche that was remotely triggered from lower on the slope.  

Avalanche Discussion

Notable avalanche highlights from yesterday include a large persistent slab remotely triggered on Schuylkill Ridge, an explosive-triggered persistent slab 2 feet thick at Irwin, and a large natural persistent slab in the Raggeds Range that ran sometime on Friday night.  These slides continue to fit into the pattern of persistent slab concerns demonstrated in last week's video.   Wind-protected southerly aspects are a reasonable place to travel with a low risk of encountering today's avalanche problems.  

Since the avalanche danger spiked to High on December 3rd, we've been in a period of gradually decreasing danger, interrupted briefly by Friday's storm.  With mostly dry weather ahead, that trend looks to continue.  It's important to recognize that today's change from orange to yellow doesn't signal a drastic decrease in avalanche danger.  Rather, it's a subtle boundary crossing (think yellow-ish orange) as the likelihood slider for our persistent slab problem continues to inch downward, while the destructive size remains the same.  When choosing terrain, it is important to focus on the potential consequences right now, rather than the likelihood.  

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