Avalanche forecast

Northwest Mountains

Crested Butte Avalanche Center
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Avalanche danger

Sun
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Out of date — this forecast expired 3 years ago.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 natural and triggered persistent slabs yesterday highlight the potential for triggering an avalanche that breaks several feet thick on weak snow near the ground.  Slopes with recent or previous drifting at higher elevations are most concerning.  Carefully assess the snowpack for dense slabs over weak snow, and consider the potential consequences before riding in avalanche terrain.

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 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 this month, and has also recently been active on westerly facing terrain. Persistent slabs have generally become difficult to trigger and signs of instability are infrequent, 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 very large natural avalanche on an east-facing slope of Treasury Mtn that likely ran on 12/10.  

Avalanche Discussion

A few inches of snow on Wednesday followed by moderate northerly winds isn't an alarming load, but it was enough to trigger a couple of large persistent slabs in the alpine of the Southeast Mountains.  Yesterday, our forecast team remotely triggered a persistent slab (see video), and explosive work at Irwin triggered another persistent slab.  With new evidence to demonstrate the problem has become active on southwest aspects above treeline, we've expanded the persistent slab rose around to include high southerlies.  The last few storms have finished with extended periods of northerly wind events and have been building thicker slabs onto some slopes that are often windward, shallow, and weak (typically west to southwest facing terrain).  Slopes with productive fetches for northerly winds are most worrisome for this scenario.  That's the new kid on the block, but the usual gang is still lurking around:  northerly and easterly slopes, which have been the most troublesome for persistent slabs this month.  

In the big picture, we've seen persistent slab activity decline significantly since our major storm at the start of the month. You can explore the patterns of avalanche activity here.  Slopes subject to wind drifting have seen more stress from our two most recent and smaller storms, and we've observed a handful of natural and triggered persistent slabs at higher elevations following each of those systems.  Wind-sheltered, below treeline slopes (below about 10,800') have been quieter in the past ten days.  Although the structure remains poor on the northern half of the compass below treeline, slabs appear to be adjusting to the early December storm better.  At any elevation, the likelihood of triggering an avalanche isn't overly disconcerting right now, but the consequences of doing so are serious in most areas.   Many of the recent persistent slabs have been large enough to bury or kill you;  low elevations in the Southeast Mountains are the exception, where slabs are typically smaller.  Carefully consider the consequences of triggering an avalanche involving most of the season's snowpack before riding in steeper terrain today. 

With a few inches of snow to drift around, yesterday's northerly winds formed small wind slabs on some leeward features at high elevations.  Watch for these fresh drifts like these if you are riding in high-consequence terrain.  

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