Avalanche forecast

Northwest Mountains

Crested Butte Avalanche Center
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Avalanche danger

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

Storm totals from this weekend's storm range from 8 to 20+ inches around the Northwest Mountains.  Moderate northwesterly winds transported the new low-density snow to leeward features.  Expect to find slabs sensitive to the weight of a person or machine on terrain that is currently or previously loaded by the wind.  Look for and avoid smooth, rounded snow textures below ridgetops or near cornices.  You can also feel snow surfaces to identity the stiffer texture of drifted slabs.  Sheltered terrain offers a smaller slab size, but persistent weak layers exist just below the recent storm snow on slopes that face northwest through north through east and require an assessment before committing to avalanche terrain.

Avalanche problems

  • Wind Slabs

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

    Northwesterly winds transported the new snow from Saturday and Sunday onto leeward features creating cohesive slabs.  You are likely to find fresh drifts below ridgetops, underneath cornices, and behind ribs.  You may also find fresh drifts on more windward aspects cross-loaded into gullies and behind tree fences.   You can identify these features by a smooth rounded pillowy appearance and stiffer feeling textures.  Look for scoured areas to help locate the adjacent freshly drifted slopes.  Active transport is a clear Wind Slab formation. 

    Example from earlier this winter of active transport and Wind Slab formation. Terrain like this beneath a ridgetop and cornice are highly suspect places to trigger a Wind Slab today.

  • Persistent Slabs

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

    Hard slabs that break 2 to 6 feet deep on weak layers near the ground remain a low likelihood concern.  The problem has become increasingly stubborn and isolated, and it has been several weeks since we have observed a persistent slab avalanche.  There is lingering uncertainty that skiers or riders can trigger a large avalanche from the margins of the slab where it thins out,  especially at higher elevations on northerly and easterly aspects.   Cornice falls can also trigger large avalanches.  You can reduce your risk by traveling where the snowpack is deeper and more uniform.  Regroup in safe zones that are clear of unusually wide and long-running avalanches.  

    An example of a thick, hard persistent slab that released naturally in mid-February.

Avalanche Discussion

"Doughnut hole" is a good way to describe the snowfall totals from the recent storm around Crested Butte.  Totals range from 6 to 20+ inches.  Areas further west like Irwin accumulated 20+ inches and places in the far east near the top of Cement and Brush creeks squeezed out totals close to a foot.  Areas very near town only produced around 6 inches.  Two important factors are driving the hazard today; the wind and snow totals.  The first key factor will be the gusty northwesterly winds from Sunday and the anticipated westerly winds today.  Expect to find fresh slabs on leeward terrain features particularly at upper elevations.  Triggered avalanches on drifted terrain features have easily grown large enough to bury or injure you.  You should plan to identify and avoid slopes with suspect wind-loading.  Both the Northwest and Southeast Mountains will offer dangerous avalanche conditions on leeward terrain features today.  One important difference between the Northwest and Southeast Mountains will be the size of recent wind drifts due to the difference in snow totals.

For areas in the Northwest Mountains near Irwin and the Ruby Range, the winds had more fuel to build slabs.  Snow totals are commonly double in the Northwest Mountains which leads to more available volume for the wind to transport and redistribute.  Anticipate that some leeward slopes in the Northwest Mountains could have drifted slabs 3 feet thick.  Triggering a slab 3 feet thick is an obvious recipe for disaster.  Areas in the Southeast Mountains did not receive that same volume of new snow, so slabs on leeward features will generally be smaller in size yet large enough to pose a real threat.

Below treeline terrain generally offers safer avalanche conditions as slabs are thinner and there will be dramatically less wind transport.  It is worth your consideration and assessment to check how the new snow is bonding to old surfaces particularly in the Northwest Mountains where you can find soft slabs up to 18" thick below treeline.  Find small, consequence-free test slopes to gauge the new snow's reactivity.  Here are two observations with small skier-triggered avalanches from yesterday at the Anthracites (1,2).  Much of the storm-related instabilities are settling out,  BUT there are facets and collapsible crusts beneath the storm snow that may continue to produce avalanches today particularly on slopes that face northwest through north through east. You can further hedge your bets by choosing slopes that are free from terrain traps if you do venture into avalanche terrain.  One last important consideration below treeline is previous avalanche activity.  Slopes that avalanched back in February have crusty bed surfaces with weak facets above that have now been buried by a healthy storm.  This observation from Saturday, prior to the storm, helps to illustrate the poor structural setup on slopes that avalanched in February.

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