Bollettino valanghe

Northwest Mountains

Crested Butte Avalanche Center
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Pericolo valanghe

Fri
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
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In sintesi

The snowpack in the NW Mountains is generally deep, and buried weak layers have become notably more difficult to affect with the weight of a human. The most likely area to trigger a large to very large persistent slab avalanche will be from an area where the snowpack thins to a below-average depth. Those trigger points could be near rock outcrops or in wind-effected areas with variable snowpack depths as a couple of examples. Normal caution is expected for managing any new snow avalanche problems like the potential for small wind slabs in the alpine. 

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    2–3

    In the Northwest Mountains, there has been surprisingly little avalanche activity observed in the last week. Buried weak layers appear to have held up well to last weekend's loading event. Since early February the weak layer that produced the most avalanches was a layer of buried surface hoar and small facets, several feet deep on the north half of the compass. There is also potential for avalanches to break on faceted crust layers on some higher, southerly aspects, or depth hoar layers near the ground.  Persistent slab avalanches can still be triggered after multiple people have traveled across a slope, and will be more likely to be triggered from a trigger point where the snowpack thins to a below-average depth. Avalanches breaking into old weak layers are unlikely on low elevation, southerly aspects.  Slope angles consistently less than 30 degrees away from runout zones are a reliable way to avoid the problem.  

    A slab avalanche in Redwell Basin on the Shield.  Visibility has been somewhat limited the past few days, but this avalanche represents the only evidence of recent natural activity in the Northwest Mountains.

    Photo: Eric Murrow

Avalanche Discussion

The total snowpack depth is playing a key role in areas where the persistent slab avalanche problem is more or less reactive. In the last week, the bullseye for avalanche activity has been in the SE Mountain Forecast Area. During this period there have been 11 persistent slab avalanches documented in the SE Mountains and 1 persistent slab avalanche documented in the NW Mountains. Of course, we can't see every mountain and record every avalanche, so those numbers are never going to be perfect. Most notably we haven't had the best views along the spine of the Ruby Range. However, we have had enough views of both forecast areas to get a general sense of recent avalanche activity. Where the snowpack is deep, weak layers are both harder to affect with the weight of a person, and those buried weak layers continue to slowly gain strength. Where the snowpack is shallower, those weak layers haven't gained much strength and their shallower depth is easier to effect. It's difficult to put a number to what could be considered generally deep or generally shallow, as there are many variables. However, for some guidance, a deep snowpack where the persistent slab avalanche problem is less reactive and more difficult to trigger will have a total snowpack height of over 200cm. A shallow snowpack and somewhere that the persistent slab avalanche problem is more reactive will have a total snowpack height somewhere between 100 and 180cm.

CBAC forecasters were on the hunt for a reactive snowpack yesterday to verify avalanche conditions. They remotely triggered multiple avalanches just traveling near avalanche terrain. That report is located here. 

An avalanche accident occurred just outside of the CBAC forecast area on Wednesday, March 6th. A snowmobiler was caught, buried, and seriously injured in an avalanche in the Elk Mountains northeast of Lambertson Peak. His partners were able to extricate him and performed CPR. Crested Butte Mountain Rescue and Western Mountain Rescue Team evacuated him to a waiting ambulance, and he was transferred to the hospital. We wish the snowmobiler a full recovery, and our thoughts are with him and his friends and family. CAIC forecasters will visit the accident site on Thursday. More information will be provided in the coming days.

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