Bollettino valanghe

Northwest Mountains

Crested Butte Avalanche Center
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Pericolo valanghe

Thu
Alta quota
4 · Forte
Limite del bosco
4 · Forte
Sotto il limite
4 · Forte
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In sintesi

An extreme wind event has combined with accumulating snow to create very-dangerous avalanche conditions. Natural avalanche activity is expected to already be occurring in wind-loaded terrain and may further extend to wind-sheltered terrain. Some avalanches could step down to old weak layers and become very large in size. Travel in avalanche terrain is not recommended. Periods of blizzard conditions and limited visibility will further make it difficult to recognize potential overhead hazards if you are traveling in low-angled terrain. 

Problemi valanghivi

  • Storm Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Very Likely
    Dimensione
    1.5–2.5

    The strong to extreme wind event is expected to drive a natural avalanche cycle in wind-loaded terrain. In wind-sheltered areas, natural avalanches will be possible, while human-triggered avalanches will become likely. Those slabs may break around 12" deep within the recent storm snow, or 15" to 30" deep at the old snow surface. The old snow surface (3/20 interface) consists of near-surface facets and a collapsible crust. Once those weak layers become reactive, these slab avalanches may propagate widely through the terrain and be remotely triggered. Low elevation due south-facing slopes having thicker crusts and they will be less likely to collapse. A triggered avalanche will have lots of snow available for entertainment and quickly grow in size as it runs through the terrain. This isn't an avalanche problem that you can outsmart with a specific snowpack test and a high skill level of terrain management will be required. 

    A large avalanche that ran into lower-angled terrain earlier this winter. 

  • Persistent Slabs

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

    There are several faceted crusts in the upper snowpack that have produced dangerous avalanche activity in the last week, with slabs breaking several feet thick or more.  Most persistent slabs have failed on east and southeast aspects, though we suspect similar crust-related issues span to south and west aspects.   This problem has been particularly challenging because it has not provided feedback underfoot (like collapses) and stability tests are not giving reliable results.   You can reduce your risk by traveling to aspects where the problem hasn't been as active.  Choose simpler terrain when in doubt.  

    A small natural or human-triggered avalanche in the recent storm snow could step down to these deeper weak layers and produce an unsurvivable avalanche. 

     

Avalanche Discussion

This week we have seen 1 to 2 feet of snow accumulate across the forecast area, with more on the way today. There is a significant amount of snow available for transport, and today's extreme wind event will be redistributing that snow onto the lee sides of terrain features. Strong wind gusts are expected to penetrate down to valley elevations as well and further lead to wind-loading in unusual locations. It's an easy call on the expectation that natural avalanches will occur in wind-loaded terrain today. There is lower confidence in just how widespread the natural avalanche cycle will be in wind shelter locations at below treeline elevations. However, the likelihood of human-triggered avalanches in those locations will still create dangerous avalanche conditions. 

It was a rough weekend for our backcountry community. Persistent slab issues have caught numerous groups by surprise this week in the Elk Mountains, and unfortunately, two of the accidents resulted in fatal outcomes. One accident was near Marble and the other near Aspen Highlands.  In the deeper zone, the issues appear to be tied to a near-crust faceted layer at the base of our two recent storms (March 10th interface), about 2 to 3 feet deep on average.  Our forecast team checked out the impressive crown on Mt. Baldy,  which was notably thicker due to extensive wind drifting.  This issue has been particularly challenging to forecast for because the layer was not obvious on the snow surface prior to its burial, so we don't have a clear map of its distribution.  Furthermore, it has provided very limited feedback in the way of collapses or unstable pit results.  Our best tool right now for plotting the problem is observing avalanche activity patterns, which have been most common on east-to-south aspects. In our shallower SE Forecast area, we are painting a broader distribution because we are also concerned about issues on northerly terrain where the snowpack is shallow and/or variable in depth.  Sunday's accident in Maroon Bowl is a good example of that. With the significant loading currently occurring the potential has increased that avalanches will step down into these deeper weak layers. 

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