Bulletin d’avalanche

Northwest Mountains

Crested Butte Avalanche Center
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Danger d’avalanche

Mon
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 3 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

The threat of human-triggered avalanches exists on many slopes due to widespread slabs and weak layers.  You are most likely to trigger an avalanche where slabs become thinner.  Slabs are commonly thinner on windward aspects, rocky terrain, and portions of the forecast zone with less snowfall this season.  You can reduce your chance of avalanche by selecting terrain with uniform coverage and depth consistently more than 5 feet.  Avalanche size remains deadly so make careful terrain choices and defer to lower consequence or low-angled terrain if you're in doubt.

Recent northerly winds drifted snow at the highest elevations forming sensitive drifts on the south half of the compass.  Although small, observers reported the fresh drifts as easy to trigger on Saturday.  

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    2–3

    January storms formed slabs averaging 2 to 4 feet thick over widespread persistent weak layers lower in the snowpack.  These avalanches have the potential for wide propagation and remote triggering,  and may not give any warning signs. Avalanche activity has seen a notable downturn in the past week as we have moved further away from the last loading period under mild weather. That downturn is mainly due to slabs gaining strength, but the weak layers have seen only minor improvements in many areas. Those weak layers are still very capable of producing large and deadly avalanches.  The easiest and most reliable way to manage this problem is to ride on slopes consistently less than 30 degrees in steepness.  Other strategies for reducing your risk include riding on slopes where the problem has recently avalanched away, avoiding trigger points where the snowpack thins near rock outcrops or wind-eroded terrain features, or traveling where the snowpack is consistently deep (>150cm) and uniform. 

    1/24: Winds have created variable slab thicknesses on slopes like this.  Red circles are areas where I would expect the slab to thin out and be easier to impact buried weak layers.  

    Photo: Zach Guy

Avalanche Discussion

The avalanche hazard has slowly relaxed over the past week resulting in a danger rating of MODERATE (level 2 of 5).  However, MODERATE does not mean safe conditions.  Here are a few clear examples of an untrustworthy snowpack; explosive work continues to release large avalanches, reports of human-triggered avalanches continue to trickle in regionally like this very close call from Marble on Friday, and this shallow Wind Slab that stepped down into deeper weak layers on Mt. Bellview.  Slabs and weak layers continue to exist on most aspects and elevations (except for some slopes that avalanched at the end of the last storm cycle thus removing the slab component).

The suggestion in the bottom line of utilizing depth as a gauge is simply a general rule.  Snowpacks in the 3 to 4-foot range are the most suspect for human triggering, however, even places with 150 cm (about 5 feet) could have hidden or obscured trigger points laced throughout the terrain.  Choosing uniform depth slopes greater than 150 cm is not a silver bullet, just a way to reduce your chances of triggering an avalanche.  Cracking and collapsing are clear indications you have found a dangerous snowpack and make it easy to defer to lower-angled terrain, but these overt signs have become infrequent. 

Also, consider the consequences of an avalanche for the terrain you intend to ride.  Are there terrain traps like trees, rocks, or gulleys?  Selecting terrain with open runouts can increase your margin of safety in case you're surprised and trigger an avalanche.

Lastly, an increase in northerly winds formed fresh, thin slabs on southerly aspects on Saturday.  A few small, natural avalanches were observed including the one mentioned above on Mt. Bellview that stepped down into deeper weak layers as it ran.  Although a Wind Slab stepping down seems to be a less likely issue, it's worth taking note of and exemplifies the unpredictable nature of the Persistent Slab problem.

 

Régions couvertes