Avalanche forecast

Northwest Mountains

Crested Butte Avalanche Center
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Avalanche danger

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

The combination of large storm totals and buried persistent weak layers requires a conservative mindset.  In steep terrain facing west, north, east, or high elevation south, you can trigger a surprisingly large avalanche that breaks several feet thick or more.  You also need to manage for storm instabilities in the new and drifted snow, especially in areas that saw more than 10" of new snow.  Last week featured numerous close calls in the Elk Mountains (see our weekly summary). The safest riding is on slope angles under 30 degrees or wind protected southerly slopes at lower elevations, where managing new snow instabilities is the primary concern.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1.5–2.5

    Slabs averaging 2 to 4 feet thick in sheltered terrain and larger in wind-loaded terrain can be triggered where they formed over well-developed facets and collapsible crusts on west, north, and east aspects. Additionally, wind-loaded slabs can fail above the stout crusts on high elevation, southerly aspects.  Relatively shallow slopes and terrain features where slabs get thinner are most problematic.  Slab behavior is challenging with variable and deceptive feedback, remote triggering potential, and wide propagation across terrain features.  You may not see obvious warning signs, such as collapses or cracking, before triggering an avalanche. You can reliably avoid the problem on slope angles less than 30 degrees without steeper slopes above you.  You can significantly reduce your risk on lower elevation southerly aspects.    

  • Storm Slabs

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

    Up to 18" of new snow, favoring the Kebler Pass area, will cause instabilities in the new and drifted snow to be a concern on slopes steeper than 35 degrees.  Slabs have the potential to be larger and more widespread in those favored areas, and elsewhere, most instabilities will be small and confined to windloaded terrain.  Look for cracking on small, steep features to gather feedback on how the storm snow is behaving today.  Storm slabs can pose a hazard by themselves, especially in terrain traps, or could step down and trigger a much larger and more difficult-to-manage persistent slab.  

    A thin storm slab triggered yesterday on Mt. Emmons.  Storm slabs will be about twice this size in favored areas near Kebler Pass.  

Avalanche Discussion

Storm totals range from 4" at CBMR to 10" in the Slate and 18" around Kebler Pass.   In areas where the storm over-delivered, observers reported sensitive sluffs, wind slabs, and storm slabs up to D1.5 in size (ex. Anthracites, Irwin).   Elsewhere, the new snow is only a problem where northwest to north winds are drifting it into more cohesive slabs.  Storm slab instabilities in low-density snow are typically short-lived after the loading period ends, but could remain sensitive today, especially at higher elevations where winds have stiffened the new snow.  Issues in the storm snow will be easy to recognize: look for cracking, use small test slopes, and know that a storm instability can gain enough mass to step down in terrain where persistent slabs are highlighted. 

The larger and more difficult problem to manage is slabs breaking on our weak layers deeper in the snowpack, which now range from about 4 feet deep around Kebler Pass to a foot deep in the shallowest parts of our forecast area near town.  In the Southeast Mountains and terrain near the boundary of the two zones, we continue to find evidence that the problem is reactive and easy to trigger, with abundant recent naturals and consistent reports of collapsing.  In much of the Northwest Mountains, the likelihood of the problem appears to be gradually decreasing (see this observation from the Slate).  It's important to recognize that the drop from "likely" to "possible" was not a dramatic and sudden change, but rather a gradual transition on a sliding scale:  "Possiblikely".   The seriousness of triggering a persistent remains the same: slabs will be large enough to injure or kill you in all but small terrain features.  

Events:  Our next Fireside Chat is this Thursday, 7 p.m., at the billy barr lounge, where Danielle Beamer will discuss Mental Health and Backcountry Decisions.  The event is free and open to all.  

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