Bollettino valanghe

Northwest Mountains

Crested Butte Avalanche Center
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Pericolo valanghe

Sun
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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In sintesi

Keep an eye out for small slabs on previously wind-loaded terrain features, such as near ridgelines and cross-loaded slopes lower in the terrain. These slabs will include old hard slabs that formed early in the week and soft slabs that may have formed yesterday, with what little snow was available for transport. Observers yesterday were able to find examples of these avalanche problems in both forecast areas (1&2). The softest snow surface, and probably the best skiing and riding, can be found in wind-sheltered locations with no slab avalanche concerns. 

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1

    Any new slab formation will be a stretch with what little snow was available for transport when yesterday's small storm moved through. The most suspect areas will be where a small slab formed on the bed surface of an old avalanche. The old snow is often exceptionally weak in those areas, and a small avalanche could entrain additional volume.  The photo to the right is the perfect example of this type of location. 

    An example of a small wind slab that released below Schuylkill Ridge earlier this week. 

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    1–1.5

    During the first half of the week, a northwest wind event combined with an average of 4" to 12" of new snow and created slabs on a few wind-affected terrain features. These slabs have mostly been found in concave terrain features such as cross-loaded gullies or rollovers in the terrain. These slabs could also be found below the lee sides of some ridgelines. More commonly, the snow was blown away and eroded during this wind event, leaving these lingering slabs less common across the terrain and more isolated in their big-picture distribution. 

    In very isolated areas, an avalanche could release several feet deep on weak layers near the ground. These areas would have needed to retain the full-season snowpack without that snowpack having been removed by the widespread avalanche cycle that occurred in late November. 

     

     

    You can see the outline of a slab that tried to displace but didn't move far downhill. This is a cross-loaded terrain feature at 11,500ft. Note the wind-eroded snow by the skier and smoother wind-loaded snow in front of the skier. 

Avalanche Discussion

The avalanche danger has been riding the line between LOW and MODERATE this week. This is because the old snowpack is very weak due to its shallow depth and the prolonged drought following Thanksgiving. When we have such a weak foundation, even small weather changes can temporarily make new avalanche problems more reactive. So, to start the weekend, we'll keep an eye out for any new slab formation over these old weak layers. 

Snow accumulation in the Gunnison River Basin has basically flatlined since November 28th. Click here to see the graph. This extended drought has greatly reduced concerns for avalanche activity involving the Thanksgiving Slab or Gizzard Blizzard. We have now transitioned into a period where each small storm we receive creates a new round of wind-related slabs over the old, weak snow surface. The NW wind event that started of the week, did more wind erosion than slab formation. The slabs that are left over from this event are covered in the current persistent slab avalanche problem. These slabs formed over small near-surface facets and, in some cases, on large depth hoars where that slab developed on an old bed surface from the avalanche activity back in November.  An example of some of these slabs can be seen in this observation on Thursday. 

Looking ahead, the next storm on Sunday is looking like a dud. So unless there is some notable new wind slab formation that is observed from this last storm, the avalanche danger will again be on a decreasing trend. 

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