Boletín de aludes

Northwest Mountains

Crested Butte Avalanche Center
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Peligro de aludes

Tue
Alta montaña
3 · Notable
Límite del bosque
3 · Notable
Bajo el límite
3 · Notable
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Desactualizado: este boletín caducó 6 years ago. Estás viendo condiciones pasadas, no el boletín vigente.

Lo esencial

The Northwest Mountains saw a cycle of large persistent slab avalanches over the weekend.     It will be easy to trigger similar avalanches today that break on buried weak layers several feet thick.  Cautious route-finding is essential: you can remotely trigger avalanches from below steep slopes and we may not be out of the woods yet for natural activity.  You can safely enjoy the powder on slopes less than 30 degrees that aren't connected to steeper terrain above you.  

Problemas de aludes

  • Loose Wet

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    1

    Dozens of wet loose avalanches ran yesterday once the snow surface became moist on southerly aspects.  A number of these triggered large persistent slab avalanches.  If surface crusts thaw into wet snow this afternoon, be alert for continued sluffing.  Rollerballs, pinwheels, and natural sluffing from rock outcrops are all warning signs of impending wet loose avalanches.  Move to shadier aspects to avoid this problem.   

  • Persistent Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Likely
    Tamaño
    2–3

    Dozens of large and dangerous persistent slabs have run in the past two days.   Slabs are breaking several feet on facets and faceted crusts that formed during our January drought.   This large avalanche triggered exemplifies how persistent slabs can be remotely triggered from below a steep slope and propagate widely across the terrain.  Avalanche activity this weekend was most widespread on the east quadrant of the compass at all elevations.  Slides are breaking larger on higher elevation, leeward aspects.  Do not expect to see obvious signs of instability before triggering an avalanche: we had numerous reports of "no signs of cracking or collapsing" yesterday while witnessing ongoing natural avalanches.   Give yourself a wide margin for error when assessing and managing persistent slabs.  

    1/31: Natural persistent slabs that were triggered on Sunday by solar warming

  • Loose Dry

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

    On shaded aspects protected from the wind, you can easily trigger shallow sluffs in powdery snow on steep terrain.   These problems are small and generally harmless by themselves unless they push you into a terrain trap.  However, numerous persistent slabs were triggered by natural sluffs yesterday.    Consider the potential for a small sluff - natural or human triggered - to step down and trigger a larger avalanche.    

    1/31: Natural sluffing triggered by solar warming on Cascade Peak.

Avalanche Discussion

NW Mountains Discussion

We've seen a doozy of persistent slab avalanche activity in the Northwest Mountains this weekend. See the gallery below or our observations page for photos.  The first wave of avalanches ran on Saturday from exceptional storm totals exceeding 20" in the Ruby Range, paired with bouts of gusty westerly winds.  Yesterday's activity was caused by a rapidly consolidating snowpack under strong solar and warming temperatures.  Ongoing natural loose snow avalanches triggered a number of these avalanches.   This video captures a sequence of persistent slab releases on Mt. Bellview throughout the day yesterday. Wild!  Even with our strong warnings and a special avalanche bulletin yesterday, we were still surprised at how much natural persistent slab activity released during the day.  We even saw natural avalanches running on east aspects well after the slopes had moved into the shade.  With the initial heat shock now past us, we are expecting a downturn in natural activity today but I'm not sure we're out of the woods yet for naturals.  We'll see warm temperatures again today, and high clouds moving in by midday. Clouds may put a damper on heating, or they may amplify heating with the greenhouse effect.  Either way, buried weak layers remain sensitive to human triggers, so choose your terrain carefully.  The focus of avalanche activity yesterday was on east, southeast, and a few south-facing slopes.    The more southerly aspects will offer better stability this morning while surface crusts remain frozen, but if the snow surfaces thaw later today, consider them on the scary list as well.  Where is the safest riding today?  You guessed it, lower slope angles and small, rolling terrain features where slides will be smaller if they break.  Remember that you can remotely trigger an avalanche that is connected to steeper terrain above.  

SE Mountains Discussion

We haven't observed any natural persistent slabs in the Southeast Mountains (yet) from this event.  I expect a few ran on Saturday from windloading.  The Southeast mountains missed out on Saturday's bounty of snowfall (they picked up 1" to 3" from this storm), so persistent slab problems remain localized to wind drifted terrain, mostly near and above treeline.  The distribution rose reflects the slopes where you are most likely to find well-developed slabs over weak layers in the middle and bottom of the snowpack.  The likelihood of triggering a persistent slab is lower, and the expected size is smaller, thus the danger is lower in the Southeast Mountains.  If you're riding in the alpine and find a slope with deep and attractive snow coverage, it is probably suspect.  Below treeline, reactive persistent slab structures are rare - slabs haven't developed in most of the terrain.  Below treeline exceptions are most likely to be found where winds have drifted snow or where there are thin crusts below this week's snow to help communicate failures across slopes.  

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