Boletín de aludes

Northwest Mountains

Crested Butte Avalanche Center
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Peligro de aludes

Fri
Alta montaña
2 · Moderado
Límite del bosque
2 · Moderado
Bajo el límite
2 · Moderado
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Desactualizado: este boletín caducó 2 years ago. Estás viendo condiciones pasadas, no el boletín vigente.

Lo esencial

Wind speeds are forecasted to increase out of the east today, and begin forming wind slabs in unusual or atypical locations. Keep an eye out for shallow instabilities in the recently accumulated snow, especially on those drifted features at higher elevations. There are lingering concerns for triggered avalanches releasing deeper in the snowpack in areas where the total snowpack height is below average and variable in depth, like this avalanche on Mt Baldy.

Problemas de aludes

  • Wind Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    1–1.5

    Last Tuesday, westerly winds were strong enough to produce wind slab avalanche activity on lee terrain features. We have since seen the wind direction vary from the southwest, on around to the west, and eventually north last night. Winds are now forecasted to increase today out of the east. This changing wind direction has formed wind slabs around much of the compass. Low-density snow that accumulated earlier this week may make small wind slabs surprisingly reactive in some locations today. Look for cracking underfoot or blowing snow to identify signs of instability. Wind slabs will be most common below corniced ridgelines or in cross-loaded terrain featuers. Those fresh drifts will be the most dangerous above terrain traps like cliffs, trees, or gullies. 

    A couple of small wind slabs that ran naturally on Gothic Mtn last Tuesday. 3/12.   

  • Persistent Slabs

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

    In the Northwest Mountains, high-consequence slabs breaking on older weak layers have become isolated and difficult to trigger. Persistent slabs are most likely to be triggered from a shallow, previously wind-eroded slope where the total snowpack height is below average or shallow, and from random cornice falls. Persistent slab avalanches can be remotely triggered from low-angle terrain and break deeper and wider than you might expect.  The most problematic structures are on shallower slopes on the north half of the compass and a few high, southerly aspects.  Mid to low-elevation, southerly aspects have the most trustworthy snowpack.  Slope angles consistently less than 30 degrees are a reliable way to avoid the problem when in doubt.  

    The crown of a very large persistent slab remotely triggered by snowmobiles on March 8.  This slope is west facing and eroded by seasonal winds, making for a shallower and more problematic snowpack than other slopes. 

Avalanche Discussion

A layer of very low-density snow accumulated earlier this week. Below 11,000ft on southerly facing slopes, this snow has been cooked away. While on some northerly facing slopes, it was preserved under the recent new snow. We don't currently expect to have enough new snow to create much of a widespread storm slab avalanche problem, but we may see some soft slab avalanche activity become surprisingly reactive if this layer of snow is preserved under fresh slabs. This will most likely occur where the wind has added some additional wind-loading. Yesterday an observer reported fast-running sluffs in steep terrain, and we can expect more of the same today, in more wind-sheltered areas. 

Snowfall over the past few days is gradually adding small amounts of stress to our persistent slab problem.  The pattern of triggered and natural persistent slab avalanches over the past 2 weeks clearly points to areas with shallower snowpacks being the most problematic.  Most of the avalanche activity has been in the shallower part of our forecast area (the Southeast Mountains), and the few persistent slabs in the deeper zone occurred on windward slopes or on slopes that have repeatedly avalanched this season keeping the snow depth unusually shallow.  It's an appropriate time to step out if you are choosing slopes with consistently deep snowpacks (>200cm), or choosing southerly aspects with strong crusts near the snow surface.  This video of a recent persistent slab on Gothic shows how shallow spots and variable snow depths are more common in wind-affected, alpine terrain.   

CBAC Event:  Don't forget the 50th Al Johnson Race is this Sunday, March 17th.  Registration and information can be found at cbavalanchecenter.org.  All proceeds to benefit the CBAC.

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