Bollettino valanghe

Northwest Mountains

Crested Butte Avalanche Center
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Pericolo valanghe

Sun
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
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Non aggiornato — questo bollettino è scaduto last year. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

You can trigger a slab avalanche in last week's storm snow.  The problem is more commonly found at upper elevations where the most snow accumulated and on drifted terrain features.  You can reduce the risk by favoring lower elevation, wind-sheltered terrain where slabs are thinner and softer, and by avoiding drifted alpine terrain.  Sunshine will warm and wet the recent snow on sunnier aspects and spur loose avalanche activity.  Look for roller balls and pinwheels to indicate the developing problem.  Head for colder aspects as the snow surface becomes wet. 

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2.5

    New and windblown snow this week has formed slabs 1 to 2 feet thick, especially on terrain features favored by westerly winds.  These slabs have bonded well except where they formed over a thin layer of facets.  This layer is hit-or-miss in distribution, but we have found it on various aspects.  Carefully assess slab size and terrain consequences before riding in steep terrain. Expect spatial variability in how slabs bond; use small steep slopes to gather stability feedback.  Cracking or collapsing are obvious signs of unstable snow.  Slabs are thinner and isolated at lower elevations.     

    There's a rare chance you can still trigger a deep, persistent slab on high, northerly aspects where the snowpack remains winter-like. Terrain where the snowpack is unusually shallow due to wind effects or previous avalanche activity is most concerning.  Choose deeper, more uniform snowpacks over thin, rocky areas to reduce your risk.   

    An example of a problematic snowpack structure at higher elevations in the Northwest Mountains. 

  • Loose Wet

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2

    After a week of cool and snowy weather, the sun will start to wet the recent storm snow.  Expect this problem to typically cover the southern portion of the compass but possibly extend into some east and west aspects.  Loose avalanches will be the largest in the snow-favored Northwest Mountains as more recent snowfall is available for entrainment.  Look for roller balls and pinwheels to help indicate a developing problem.  If you find wetter-than-expected conditions, move to a cooler aspect or lower-angled terrain. 

    Wet Loose avalanche for last week.  Expect similar-sized activity on Saturday as the recent storm snow becomes wet on sunny terrain.

    Photo: Evan Ross

Avalanche Discussion

A mix of dry slab and wet loose avalanche issues face backcountry travelers on Saturday.  Slab instabilities involving the recent snow could linger where well-developed slabs formed over a layer of thin facets above the meltdown crust.   These facets formed from melt-layer recrystallization - a process that requires a specific set of ingredients that weren't widespread at the start of this week's snowfall.   In other words, the layer is spotty in distribution. We have found it on various aspects around the compass (ex. Baxter Basin and Elkton) and have also found it absent on those same aspects.  We have lower confidence describing where the layer is most likely to be found, but higher confidence describing where in the terrain slabs above this layer have formed.    In the Ruby Range, storm totals this week exceed 2" of Snow Water Equivalent, and slabs are fairly common around the compass as you gain elevation into higher terrain.  As you move away from the favored Snowbelt, slabs have only formed on well-drifted slopes favored by westerly winds.  Carefully assess for the presence/absence of a slab and how the slab is bonding before riding in steep, consequential terrain.

High northerlies are the only slopes that didn't see extensive meltwater into the snowpack last week. On these coldest aspects, a more winter-like snowpack structure exists. This requires consideration for weak layers near the ground that have plagued us this winter, especially on terrain with unusually shallow snow depths. The most recent human-triggered examples of the problem include a snowmobile-triggered slide on Axtell and a skier-triggered slide on Gothic.   In the San Juans on Wednesday, a skier was surprised when they triggered a shallow wind slab that stepped down and produced a very large avalanche. 

Ample sunshine likely wets the snow surface today.  Temperatures remain cool, but solar radiation should be able to overcome this and create a Wet Loose avalanche problem.  Areas in the Northwest Mountains, which received the most snowfall last week, will be the most problematic, as the avalanche size could grow large (D2).  Some natural avalanche activity is expected today, particularly on steep, rocky, southerly terrain.  Keep a close eye on surface wetting, and use caution if traveling directly up steep, sunny slopes, as you could be surprised by a natural release. 

Events: CBAC's Party for the People is at the Talk of the Town next Thursday.  Come celebrate the end of the season with us!  (Don't worry, we'll be forecasting for at least a few more weeks.)

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