Avalanche forecast

Northwest Mountains

Crested Butte Avalanche Center
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Avalanche danger

Wed
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
2 · Moderate
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Out of date — this forecast expired 3 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

Several persistent slabs over the weekend exemplify our ongoing concerns for triggering a dangerous avalanche that breaks 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 south-facing terrain or small terrain features without terrain traps.  

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1.5–2.5

    Slabs 1 to 4 feet thick have formed over several persistent weak layers in the snowpack:  Weak facets 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.    

    A natural avalanche on an east-facing slope of Treasury Mtn into North Pole Basin. Debris spilled over and triggered another avalanche on the north-facing slope below the cliffs.  Likely ran 12/10.  

Avalanche Discussion

Wind slabs that formed during Sunday morning's northerly wind event have generally healed up and we've pulled the problem off the list.  If you can find southerly-facing terrain with adequate snow coverage, that's an appropriate place to step out into more aggressive avalanche terrain right now.  The exceptions to a generally safe snowpack on the south quadrant are on high, southeast aspects, which have buried facet/crust concerns on some slopes.  Irwin's explosive-triggered persistent slab on Sunday is a good example. 

In some areas, below treeline slopes are starting to recover from the early December loading period.  In the shallowest parts of our forecast area, the thinner slabs are adjusting and becoming less concerning now. In the deepest parts of our forecast area, weak layers are gaining some strength and slabs are harder to trigger.  With careful snowpack assessments and consideration for terrain consequences, there are options to step out into more challenging terrain below treeline.  There's probably a Goldilocks zone where slabs aren't too thick, aren't too thin, and are just right for human triggering, where snow depths are roughly 3 to 4 feet thick (90 to 120 cm).  

The most dangerous slopes are previously drifted near and above treeline, where weak layers have seen more stress and where slabs are larger and more variable in thickness.  An observation came in yesterday of a destructive natural avalanche that ran over the weekend on Treasury Mountain, with an estimated size somewhere in the D2.5-D3 range.  Our size distribution this month has generally been up to D2.5 in the Northwest Mountains, and up to D2 in the Southeast Mountains, with potential for outliers in the largest and/or most windloaded terrain features.  Although the likelihood of triggering an avalanche in the alpine is decreasing now, the consequences are serious.  Your assessments should include more than just "Can I trigger it?"   You need to also ask yourself, "Do I have uncertainty?" and "What are the consequences if my assessments are wrong?"  Persistent slabs are notorious for their deceptive behavior.  

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