Bulletin d’avalanche

Northwest Mountains

Crested Butte Avalanche Center
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Danger d’avalanche

Fri
Alpin
1 · Faible
Limite forestière
1 · Faible
Sous la limite
1 · Faible
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Points saillants

The prolonged dry spell is causing avalanche concerns to become increasingly rare.   Previously wind-loaded slopes in very steep or rocky terrain are the most likely locations to find isolated exceptions to a generally stable snowpack.  A triggered avalanche will still be large and several feet thick.  Terrain with clean runouts free of trees, cliffs, or gullies will be more survivable if you find an isolated instability.  

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely
    Taille
    1.5–2.5

    Slabs 1 to 4 feet thick exist over several persistent weak layers in the snowpack:  Weak facets and depth hoar near the ground and surface hoar layers in the upper snowpack. The problem has been most common on northerly and easterly facing slopes earlier this month, along with some wind-loaded westerly facing terrain.  Although many slopes are becoming dormant under our dry spell, there are a few terrain features where you may be able to trigger a surprisingly large avalanche involving most of the snowpack.  Persistent slabs can break wider than you expect, and they can be triggered from flat slopes adjacent to steeper terrain. You can reduce your risk by traveling on slopes where the problem isn't highlighted above, or by traveling on smaller terrain features without terrain traps.  You can avoid the problem on slopes consistently less than 30 degrees. 

    This large persistent slab was triggered by skiers descending the ridgeline on the Aspen side of the Elk Mountains last weekend, 12/17.  

Avalanche Discussion

The avalanche danger continues its gradual decline under our current dry spell.  Persistent slab instabilities have become increasingly isolated and stubborn under the relatively dry pattern over the past 12 days.  The Northwest Mountains saw a few small triggered avalanches last weekend, and a few large avalanches associated with the last significant storm that exited with strong winds on December 10th.  The Southeast Mountains are slower to see the danger drop because they have seen more action in the recent week: both from explosive-triggered slides at CBMR, and a few natural and triggered slides last Friday.    In both forecast areas, poor snowpack structures still exist; without the added stress of snow or wind loading, the slabs start to find a temporary balance with the weak layers.  That pattern will reverse if the upcoming holiday storm delivers as forecasted. 

In a typical Crested Butte winter, we see a mid-winter drought that causes explosive and skier-triggered avalanche activity to taper off despite an untrustworthy snowpack structure, and we timidly start lowering the danger. We look at timing since loading events, avalanche activity patterns, and trends with stability tests to make these assessments. In the words of long-time forecaster Drew Hardesty from the Utah Avalanche Center:  "How do we know when a buried persistent layer is dormant?  This has been a tough and troubling problem for avalanche forecasters and professionals for a long time.  I’m reminded of the old joke with God and Nietzsche.  After years of deep contemplation, the German philosopher Friedrich Nietzsche declared that God is dead.  Weeks later, God declared that Nietzsche was dead."    There's always uncertainty with a dry, winter snowpack, and of course, low danger does not mean no danger: we expect isolated instabilities to exist out there.    

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