Avalanche forecast

Northwest Mountains

Crested Butte Avalanche Center
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Avalanche danger

Mon
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Out of date — this forecast expired 3 years ago.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

Occasional collapses and recent avalanche activity demonstrate that our persistent slab problem is...persistent.    Slopes with recent or previous drifting at higher elevations are more dangerous.  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 summary of recent persistent slab activity:  Northerly winds on Thursday night spurred a few large natural persistent slabs in the high terrain of the Southeast Mountains on west to southwest aspects, and skiers remotely triggered a similar avalanche (video).  Around that same timeframe, a large natural ran in the deeper snowpack of the Raggeds Range, on a high north-facing slope.  This D2.5 near Redstone may have also occurred in the past few days.   Yesterday, skiers on Schuylkill Ridge triggered several shallow persistent slabs on our most recently buried surface hoar layer, about a foot deep.  In just this small sample of recent activity, persistent slab avalanches occurred on SW, W, NW, N, NE, and E aspects.  Our concerns for the problem span most aspects above treeline and the northern half of the compass below treeline.  

If you want to really nerd out on historical data, this Colorado study looks at the timing of nearly 2000 persistent slab avalanches relative to snow and wind-loading events.  The takeaway is that most persistent slab avalanche activity occurs within the first few days of loading events, then avalanche activity tapers but continues for 10 days or more after the last snowfall.  Of course, each winter and snowpack is unique.  This winter, our largest loading event ended two weeks ago, followed by several smaller but still notable loading events ending on December 8th and December 14th.  So as we move further into our current dry pattern, the likelihood of triggering a persistent slab will continue to gradually decrease but won't go away for some time.  Unfortunately, the avalanche size remains the same.  There are many slopes where you can ride in steep terrain today, but there are still some rotten eggs out there.  Identifying the problematic slopes can be difficult, so give yourself a wide margin for error with your snowpack assessments and terrain choices.  

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