Avalanche forecast

Northwest Mountains

Crested Butte Avalanche Center
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Avalanche danger

Fri
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
2 · Moderate
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Highlights

You are likely to trigger an avalanche on leeward terrain.  Avalanches could break just in the drifted storm snow or release deeper in the snowpack producing a dangerous avalanche.  Extreme westerly winds may form fresh slabs in unusual places below treeline, well below ridgetops, or pressed into depressions.  An avalanche in the storm snow could serve as a trigger for a deeper failing avalanche.  Choose conservative terrain options and avoid traveling on or beneath wind-drifted slopes.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2–3

    Storms this month have built thick and hard persistent slabs over a widespread weak layer of facets and faceted crusts. These slabs are generally 2 to 4-feet thick. The vast majority of avalanche activity this month has occurred on west to north to southeast aspects, with only a few avalanches from alpine south to southwest facing alpine terrain. Persistent slabs are notorious for erratic feedback and unusual behavior, such as remote triggering and wide propagation. To reliably avoid the problem, choose routes that keep you off and out from under terrain steeper than about 30 degrees that hold old, weak snow near the ground. In steeper terrain, you can reduce your chance of triggering an avalanche by avoiding trigger points where the snowpack is thinner than average.  Thin spots are most common in wind-affected terrain or near rock bands or vegetation.  

    This persistent slab avalanche wrapped from shady to sunny aspects and broke on weak layers near the ground. This a good example of the potential size and widely propagating behavior of our current persistent slab problem. 

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2

    Extreme westerly winds have blown the new snow and old snow surfaces to leeward terrain features.  When winds are as strong as last night and today (consistent 30 to 40 mph with gusts to 80), the loading patterns can be atypical.  Look for loading in traditional areas below ridgetops, underneath cornices, and behind terrain breaks, but don't be surprised to find fresh drifts below treeline or lower in start zones than you are accustomed to.  An avalanche in recent drifting may trigger an avalanche deeper in the snowpack where it overlaps with the Persistent Slab problem.

    Shooting cracks like this are a sure sign you have found sensitive wind-drifted snow.

Avalanche Discussion

The wild winds are creating the headlines on Thursday.  Fresh drifts formed in the past 24 hours and will continue to increase in size during daylight hours.  You are likely to trigger an avalanche where you find wind-deposited snow.  Avalanches just in the storm snow are expected to remain small in size but some highly drifted slopes may produce large avalanches.  Human-triggered avalanches in the storm snow seem a straightforward proposition on drifted terrain, but there is uncertainty about the impact of wind loading on deeply buried weak layers that requires caution.  Avalanches breaking deeply into the snowpack are a deadly proposition.

The uncertainty creeps in due to the extreme wind speeds.  When winds are this strong loading patterns are difficult to predict and often from slabs in uncommon places.  Storm totals roughly sit at a modest 3 to 6 inches, but the winds may be able to move enough snow to overburden deeply buried weak layers making it more likely for you to trigger a deep failing avalanche.  Some of the dangerous slopes that approach their tipping point may be below treeline due to the strength of winds at low elevations.  These slopes are likely to be open areas where the wind can gather large amounts of snow. An avalanche in the surface snow could step down to deeper weak layers on highly drifted terrain.  Your best bet is to avoid steep slopes highlighted on the Persistent Slab rose that have signs of recent drifting.

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