Avalanche forecast

Southeast Mountains

Crested Butte Avalanche Center
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Avalanche danger

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

Skiers and riders yesterday easily triggered slabs breaking in the drifted snow.  More snow and wind transport today will continue to fuel dangerous avalanche conditions on wind-loaded slopes, where the threat of large avalanches breaking in new and old snow is most severe.  In wind-sheltered terrain, monitor for shallow storm instabilities and assess the snowpack for the low likelihood chance of a large avalanche breaking deeper into the snowpack on buried weak layers.   

Avalanche problems

  • Storm Slabs

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

    Up to a foot of snow and strong winds yesterday formed reactive slabs on drifted slopes.  Expect storm instabilities to continue today with more snowfall and wind transport in the forecast.  Slabs will be largest and most reactive on leeward terrain near and above treeline.  Cracking underfoot, blowing snow, or intense snowfall rates signal potential danger.   Although slabs are thinner and less common in wind-protected terrain, they could cause trouble above terrain traps.  

    2/28:   A shooting crack in a wind drifted slope.  This is an obvious sign of unstable snow.  

  • Persistent Slabs

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

    Over the past few weeks, we have observed an infrequent but continuous pattern of large avalanches or collapses failing on weak, faceted layers deeper in the snowpack.  These have been most common during loading events like today's. Persistent slab activity has been most problematic on wind-loaded terrain this month, though issues are developing in some areas below treeline.  Carefully assess the snowpack structure and give yourself a margin for error in handling this problem because there is a high degree of variability across the region and on different aspects.  Some recent examples of persistent slab activity include naturals on Express Creek and Carbonate Hill, and a remotely triggered avalanche on Mt. Emmons. 

    2/23:  A large collapse and shooting cracks failed on weak, faceted snow about 3 feet deep on Mount Emmons.  Don't expect to get feedback like this before triggering an avalanche, but it's a clear sign of danger if you do.  

Avalanche Discussion

Today's storm is tracking a bit further south than yesterday's forecast, and storm totals have been downgraded.  We're keeping the danger at Moderate below treeline, but if storm totals exceed the forecast, expect a higher danger.  Avalanche conditions are most dangerous near and above treeline, where intense wind loading occurred throughout yesterday.  Today's pulse will reignite snow and wind transport, thickening yesterday's wind-drifted slabs and forming a fresh generation of drifted slabs.  Stay away from leeward terrain to avoid the most dangerous slopes.  

In wind-sheltered terrain, there is potential to trigger sluffs or soft slabs in the new snow.  Irwin reported storm slabs starting to develop yesterday afternoon.  Periods of heavy snowfall this afternoon could form thin and reactive slabs in today's new snow, or could spur instabilities at the storm interface, which is about 6" to 12" deep this morning.   The hazard posed by these relatively shallow instabilities will depend on what type of terrain you are riding in: the more consequential, the more hazardous.  You can monitor for storm slab formation on small, steep test slopes and by keeping an eye on new snow accumulations. 

Unfortunately, we have the nagging threat of avalanches breaking deeper than the storm snow.   Persistent weak layers in the upper and lower snowpack have been getting stressed during the loading events of the past few weeks.  The persistent slab problem has recently produced avalanches and obvious signs of instability at all elevations in the Southeast Mountains.  One layer of concern is small-grained facets or faceted crusts up to a few feet deep, which has produced collapses and avalanche activity on Mount Emmons (Example A, Example B, Example C).  We have yet to observe collapses or avalanches on these layers in the deeper snowpack of the Northwest Mountains.  However, we have found a few concerning pit results in recent weeks, such as this propagating test this week in the Anthracite Range.  Given the significant loading this week, we recommend carefully assessing the snowpack and/or choosing more conservative terrain to manage for potential surprises of avalanches breaking deeper than expected.  Most concerns are in the upper few feet of the snowpack, and a well-placed extended column test can help in identifying potential issues.  In the shallower parts of the Southeast Mountains, there is also the isolated threat of avalanches breaking on weak layers near the ground, exemplified by this unstable test result near Mt. Whetstone.   Wind-loaded slopes are most worrisome for these deeper persistent slab issues.

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