Avalanche forecast

Northwest Mountains

Crested Butte Avalanche Center
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Avalanche danger

Sat
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
3 · Considerable
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Out of date — this forecast expired 6 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

Signs of instability like large collapses and fresh natural avalanches, in alpine terrain, were reported yesterday.  An extended period of northwesterly winds transported snow onto the eastern half of the compass near and above treeline.  Drifted slopes at upper elevations present the most concerning setup as the potential avalanche size is dangerously large and the wind-loading period just ended yesterday evening.  Sheltered slopes that face west through north through east continue to show signs of instability like collapsing and cracking.  The simple solution is to choose mellower slope angles and place a healthy margin between your group and nearby steep terrain.  Remotely triggering an avalanche from immediately below a slope remains a genuine concern.     

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    2

    To avoid getting caught in a dangerous persistent slab avalanche today, choose low slope angles or seek out sunnier aspects.   In the past week, there has been a constant stream of natural and human-triggered slab avalanches breaking on faceted weak layers near the ground.  The problem is serious: recent incidents include a helicopter evacuation and a fatality.   Be wary of traveling below steep, connected slopes because many of these avalanches have been triggered remotely from adjacent low angle terrain.  Persistent slabs have been most active on slopes steeper than 35 degrees that face west through north through east at all elevations, and in a few cases, high southeast aspects.  If you are traveling on a slope that fits that description, expect to trigger an avalanche easily if it hasn't already avalanched earlier this week.  In the Northwest Mountains, slabs are 1 to 2 feet thick in sheltered locations, and significantly deeper on drifted terrain features.  

    12/23 - A remotely-triggered avalanche below treeline at the head of the Slate River Valley.

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1

    Strong northwesterly winds transported snow for nearly two days and only subsided on Thursday evening just after sunset.  These wind will have removed snow from windward terrain features and build wind slabs on the lees sides of terrain features. Below treeline elevations are even in the mix, with down-valley winds building fresh slabs on cross-loaded terrain. This avalanche problem may also exist in unusual locations given the character of this wind event. These slabs may release up to about 12" thick, or further step down into the Persistent Slab avalanche problem where the two problems overlap. 

    Example of a small Wind Slab (not a recent photo)

Avalanche Discussion

Our current snowpack structure remains touchy and a bit concerning.  Reports of instability like cracking, collapsing, natural avalanches on drifted terrain and snowpack tests with moderate propagating results suggest human-triggered avalanches remain likely on slopes near or greater than 35 degrees in steepness.  Small storms and several periods of wind-loading over the past week have not allowed upper elevation terrain to get more than a day or so without increased burden.  Now that the wind-loading has ended, it is reasonable to expect that the natural avalanche activity will diminish, but many drifted alpine slopes will be prime for human-triggered dangerous Persistent Slab avalanches.  As mentioned in the bottom line, if you are traveling near or underneath large, drifted alpine slopes you should consider the possibility of getting a collapse in lower angled terrain that produces an avalanche on the steep slope immediately above or adjacent to you.  The weather will be warm and beautiful above the inversion zone but do not let that lull you into complacency.  The best way to manage the avalanche hazard on drifted slopes at upper elevations is to simply give them a wide berth and expect the potential for unusual avalanche behavior. 

Terrain that has not been drifted recently, much of the terrain below treeline, is not as concerning however this is more a product of the potential avalanche size and less about the likelihood of triggering an avalanche.  Avalanche size below treeline is smaller, yet plenty big to injure or bury you.  It is hard to ignore the collapsing and shooting cracks that observers are reporting each day.  The frequency of these collapses has been slowly diminishing, but the distance the failures have been moving appears to be increasing.  Consider this as you travel through the mountains today; the feedback from the snowpack might appear quieter, but that is simply a product of slabs stiffening.  The weak layer underneath the slab is showing no sign of improvement.  Cold temperatures and a generally shallow snowpack, less than 1 meter, is preventing any strengthening in the weak layer.  Prolonged periods of elevated avalanche danger can be challenging and frustrating.  The key is to stay the course of simple, low-angled terrain until the evidence strongly suggests otherwise.  Our current weak layer is as ugly as I can remember in the past 10 years.

 

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