Boletín de aludes

Northwest Mountains

Crested Butte Avalanche Center
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Peligro de aludes

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

Lo esencial

Westerly winds have averaged around 20mph and gusted into the 30's and 40's over the last 24 hours. More of the same can be expected today. Wind-loaded terrain will be the most dangerous areas, due to the combination of reactive wind slabs and the potential for avalanches to break into deeper persistent weak layers. In wind-sheltered locations, slab avalanches can release on persistent weak layers buried 2  to 3 feet deep on average. In those wind-sheltered areas, you can manage the avalanche problem through your terrain selection, or careful snowpack evaluations. 

Problemas de aludes

  • Wind Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Likely
    Tamaño
    1–2

    Westerly winds have formed reactive wind slabs on the lee sides of terrain features. These wind slabs could release in just the wind-loaded snow, or step down to deeper persistent weak layers. The potential for natural avalanche activity also exists on slopes that are being actively wind-loaded. That potential will increase this afternoon and into tonight once permeation rates pick back up. 

    An example of a large avalanche that failed on a wind-loaded terrain feature last week.

  • Persistent Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    1.5–2.5

    A new round of weak layers was buried on February 2nd. These weak layers consist of widespread surface hoar and facets on most east, north, and west-facing slopes, and along with some faceted crusts on higher elevation southerly aspects. Slabs 2 to 3 feet thick over these weak layers can be triggered by skiers and riders. This avalanche problem can be remotely triggered from flat terrain, or may propagate further than expected.  Shooting cracks or collapses are a clear sign of danger, but these obvious signs of instability will not always show themselves before an avalanche releases.  There is isolated potential to trigger older, larger slabs on weak layers deeper in the snowpack, most likely from shallow terrain features or step-down avalanches.  

     

    This skier-triggered persistent slab avalanche likely failed on a layer of buried surface hoar last weekend.

Avalanche Discussion

The most notable change in conditions has been the increased winds. Over the last 24 hours, 12,000ft winds were averaging around 20mph with gusts in the 30 to 50mph range. More of the same is expected for the next 24 hours. There is plenty of snow available for transport, forming fresh wind slabs and adding load to persistent weak layers. A layer of surface hoar was buried on February 2nd, and this weak layer has produced the majority of the avalanche activity over the last week. The most dangerous areas will be where this weak layer is intact, and being further loaded by the current winds. Those slopes would most commonly be found on easterly to northeasterly aspects around the near treeline elevation band. 

In wind-sheltered areas, the primary layer of concern is again the February 2nd interface consisting primarily of surface hoar and near surface facets. Since last weekend, those weak layers have had some time to adjust to the current weight they are holding, and have become less reactive. In the NW Mountains that interface is buried 2 to 3 feet deep on average, and hasn't been showing much for obvious signs of instability. However, stability tests are still showing propagating results and this avalanche problem is very real if you find the right trigger point. In the SE Mountains that interface is buried around 1 to 2 feet deep on average. Obvious signs of instability have still been common generally above 11,000ft where the previous winds had added some additional cohesion to the slabs above the weak layer. At lower elevations in the SE Mountains, cold temperatures combined with a clear sky earlier this week began faceting the thin slabs above the weak layer and made the persistent slab avalanche problem less common to encounter at lower elevations. 

With continued wind-loading, natural avalanche activity has become possible, especially once precipitation rates pick up later this afternoon and this evening. Keep this potential threat in mind if you are traveling in an avalanche track below a wind-loaded start zone. 

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