Bulletin d’avalanche

Southeast Mountains

Crested Butte Avalanche Center
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Danger d’avalanche

Thu
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 4 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Warm temperatures and sunshine are creating two vastly different problems to deal with today.  On the north half of the compass, you can trigger a dangerous dry slab avalanche on steep slopes that did not avalanche naturally last week.  On the south half of the compass, you can trigger a wet snow avalanche on slopes that become wet and mushy during the day from warming.  Shooting cracks and collapsing are clear signs of unstable conditions.  Use caution today as the snowpack transitions on sunny slopes, and remain wary of shady slopes without signs of recent avalanche activity.

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–2.5

    You can trigger slabs 2 to 3 feet thick that fail on fragile facets below last week's storm snow.  Avalanches can be remotely triggered from below steep slopes and they can propagate surprisingly wide distances.  On slopes that didn't avalanche during the cycle, particularly at lower elevations, the chance of triggering an avalanche hover near 'likely' but given the vast number of slopes that avalanched last week, the reduced distribution of concerning slopes puts the overall likelihood at 'possible'.   To avoid an encounter with a large avalanche, choose slopes that are consistently less than about 30 to 35 degrees that don't connect to steeper terrain above you, or choose slopes with recent evidence of natural avalanche activity.

    This photo from Cement Creek illustrates how persistent slabs can be triggered from a long ways away from the start zone.  The avalanche was remotely triggered by a snowmobile on 2/27.

  • Wet Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–2

    Warming temperatures and sunshine could trigger Wet Slab avalanches today on slopes facing the south half of the compass.  Crust/facet structures that formed during the February dry spell are being infiltrated by surface meltwater.  Because this weak layer is so close to the surface it only takes a few days or so for the meltwater to reach the weak layer and cause it to lose most of its strength when saturated during the day.  Wet Slabs are notoriously hard to predict and commonly overlap with Wet Loose avalanche concerns.  Look for wet mushy surfaces and collapsing to help clue you in to the potential for Wet Slab avalanches.  If surface crusts have completely softened then expect meltwater to be diving towards buried weak layers.  Slopes where you find Wet Loose avalanches conditions are the same places you could trigger a Wet Slab.  Start zones with rocks or vegetation can often help pump meltwater into the snowpack as well.  Mushy surfaces and collapsing are clear signs of unstable Wet Slab conditions.

    A natural Wet Slab avalanche from Monday 2/28.  Meltwater from warm temperatures and solar radiation infiltrated the facets from February and caused the failure of this dangerous slab.

Avalanche Discussion

Tricky avalanche conditions persist around Crested Butte.  Let's begin with the dry snow avalanche problem.  The February facet layer remains a concern for human-triggered avalanches.  Slopes facing the north half of the compass at near and below treeline elevations, that do not have evidence of natural avalanche activity, are highly suspect for a triggered avalanche.  An observation from yesterday pointed to some collapses feeling like "small earthquakes".  Signs of instability like this reinforce the need to take a conservative approach to steep terrain that did not already avalanche.  The tremendous number of slopes that avalanched naturally last week has reduced the number of places where the slab and weak layer exist which is the reason for the likelihood at POSSIBLE.  Above treeline the weak layer was not as weak or widely distributed, but uncertainty exists about how sensitive the problem is to human triggers.  Your 'silver bullet' bullet, as always, is to choose lower slope angles and manage overhead hazards during times of uncertainty.  The snowpack continues to adjust to last week's storm and the likelihood of triggered avalanches slooooowly decreases, but I personally remain wary of shady avalanche terrain without signs of recent natural avalanches.

On the south half of the compass, meltwater from very warm temperatures is pumping into the snowpack.  Yesterday on Mount Emmons, numerous large collapses and long-running cracks were experienced by midday from meltwater flooding weak layers in the snowpack.  You can check out this observation here.  The natural avalanche in Red Lady Bowl from Monday was confirmed as a dangerous and hard to predict, Wet Slab.  The signs of instability for Wet Slabs often follow Wet Loose avalanches.  Rollerballs, pinwheels, and mushy surface conditions indicate free water flowing into the snowpack.  Additionally, Wet Slabs are often preceded by collapsing on nearby slopes.  Heed these warning signs.  The good news is the weather is forecasted to become cooler and 'winter-like' sometime on Friday; this means this period of concern about Wet Slabs should pass in the next few days.

We are in a two-part transitional period with a slow reduction in the likelihood of triggering a dry slab avalanche on northerly-facing slopes and meltwater entering the snowpack on the south half of the compass.  Even though the danger rating is MODERATE, I would describe it as the upper end of MODERATE for the reasons and descriptions above.  A skeptical mindset will serve you well if you are considering steep avalanche terrain today.

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