Boletín de aludes

Northwest Mountains

Crested Butte Avalanche Center
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Peligro de aludes

Wed
Alta montaña
3 · Notable
Límite del bosque
3 · Notable
Bajo el límite
3 · Notable
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Desactualizado: este boletín caducó 3 years ago. Estás viendo condiciones pasadas, no el boletín vigente.

Lo esencial

Dangerous conditions persist after last weekend's storm and subsequent natural avalanche cycle.  You're likely to trigger avalanches in the upper snowpack as recent snow accumulated on fragile weak layers and there's a chance for some avalanches to break much deeper. Bring a conservative mindset into the mountains and expect the snowpack to behave in surprising ways.  Your safest bet is to choose low-angled terrain free from overhead hazards.  The snowpack is less problematic on lower elevation, wind-sheltered southerly slopes.  

Problemas de aludes

  • Persistent Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Likely
    Tamaño
    1–3

    The recent storm formed slabs 1 to 3 feet thick at all elevations over widespread weak layers of surface hoar and facets on most east, north, and west-facing slopes, and potentially faceted crusts on high-elevation southerly aspects.  These slabs are sensitive to human triggers, evidenced by numerous triggered and natural avalanches in the past two days.  They can be remotely triggered from flat terrain, or propagate widely across terrain features.  Shooting cracks or collapses are a clear sign of danger.  This problem is generally D1 to D2.5 in size.

    There are old faceted weak layers near the ground that are more stubborn to human triggers. These dense old slabs may not give warning signs before they release; they are most commonly triggered from terrain features with shallow or variable snowpacks, or if avalanches in the upper snowpack gain enough mass to step down.  This problem is generally up to D3 in size.  

    The most reliable strategy for avoiding these issues is by riding on slope angles consistently less than about 30 degrees.  Mid to low-elevation southerly slopes are less problematic.   

    2/4: This avalanche on Mt. Axtell exemplifies our newest generation of persistent slabs (recent storm snow breaking on facets or surface hoar), which then stepped down and triggered older hard slabs deeper in the snowpack on old weak layers near the ground.  

Avalanche Discussion

Storm totals from this past weekend varied greatly across both forecast zones.  The Northwest Mountains received 1 to 3 feet with areas around Irwin and the crest of the Ruby Range picking up the most and the Southeast Mountains snagged up to 14 inches.  Recent natural and human-triggered avalanches followed the snowfall pattern with most activity centered in the Northwest Mountains.

Areas that received more snowfall, the Northwest Mountains and potentially bordering areas in the Southeast Mountains, are the most dangerous.  Here you can find sensitive avalanche conditions in the upper snowpack where the recent snowfall accumulated above well-documented weak layers like surface hoar and facets (check out this video pre-storm describing the interface).  This weekend, backcountry travelers triggered avalanches on this interface in the Anthracites (1,2), Purple Ridge area, near Pittsburg, and Irwin.  The evidence points to easy-to-trigger conditions where you find slabs above the old, fragile surface.  Expect to commonly encounter these conditions in the Northwest Mountains.

Recent natural avalanche activity illustrates the potential for some avalanches to break deeply in the snowpack on weak layers at the bottom of the snowpack.  This appears to be more common on previously shallow terrain like this avalanche in Redwell or where avalanches in the upper snowpack can gather enough mass to step down as seen on Axtel and Bellview.  You are unlikely to receive warning signs, like cracking and collapsing, from the snowpack that this potential exists.  The recent natural activity is your warning.

Even though the Southeast Mountains received less snowfall, you can still trigger avalanches particularly where you find a foot or more of recent snow.  Recently drifted slopes are the most likely place to trigger an avalanche.  Weak layers at the bottom of the snowpack show little signs of improvement but may be less sensitive to triggers due to less recent loading.  Yesterday, forecasters experienced some collapsing and cracking on westerly-facing terrain in upper Cement Creek while snowmobiling.  This is a clear indication that basal weak layers can be impacted by backcountry travelers if you find a trigger point.

An incoming storm Tuesday night through Thursday is likely to worsen avalanche conditions in the coming days.  Consider the recent avalanche activity as a mere teaser of things to come if the storm delivers as forecasted.

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