Bollettino valanghe

Southeast Mountains

Crested Butte Avalanche Center
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Pericolo valanghe

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

The avalanche danger will slowly rise through the day and is expected to reach Considerable tonight.   Skiers and riders have been finding the safest riding and best powder where the snowpack is deep and protected from the sun and wind on northerly aspects: watch for newly developing storm slabs in that type of terrain by later today.   Several large natural and triggered persistent slab avalanches have occurred on sunnier terrain with buried crusts (east, south, and westerly facing)  and on northerly aspects with thin and variable snow depths.  It was a tragic weekend in the Elk Mountains, with two fatal accidents since Friday.  Bring an acute sense of distrust to the backcountry today; this isn't our typical "spring snowpack."  

Problemi valanghivi

  • Storm Slabs

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

    Today's snowfall will start to develop thin storm slabs that will be sensitive to ski cuts.  Slabs will start to form later today at higher elevations where winds are stiffening and thickening the new snow.  Monitor new storm totals and anticipate the new snow to bond poorly to current snow surfaces.  Today's storm slabs could cause trouble if they drag you into terrain traps such as gullies or trees.  Use small steep test slopes to assess how the new snow is behaving.  

    Example of the type of storm slabs we are expecting to develop by later today. 

  • Persistent Slabs

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

    There are several faceted weak layers in the snowpack that remain a concern in some terrain. The most concerning slopes include those that held an unusually shallow and weak snowpack prior to our March storms (like Round Mountain), where snow depths are currently less than about 5 feet (150cm) deep, or in wind-effected terrain with variable snow depths (like Teo or Carbonate Hill).  There are also recently buried faceted crusts in the upper snowpack on easterly and westerly-facing slopes that are just developing an issue (like Whetstone). Carefully assess the snowpack structure and give yourself a margin for error in handling this problem. There is a high degree of variability across the region and on different aspects.  Choose simpler terrain when in doubt.  

     

     

     

    This very large persistent slab was triggered by a snowmobiler on 3/18.  

Avalanche Discussion

We are in the early stages of a major loading event and the avalanche danger will start rising through the day.  The danger is expected to rise to Considerable overnight; the Northwest Mountains might get there before sunset if we get the high end of snow and wind forecasts.  The avalanche danger looks to peak on Wednesday during the culmination of heavy snowfall and intense wind loading.  New snow instabilities will start developing today.  They're most likely to form as slabs at higher elevations where snowfall accumulations are deepest and winds will be stiffening the new snow.   On northerly aspects, sluffs or slabs in the new snow will entrain more snow because the snow surface is currently loose and cohesionless.  Our current snow surface is weak and faceted, so instabilities should develop fairly quickly as soon as the new snow becomes thick enough to form a slab; somewhere in the 6" to 10" range.  

It's been 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 yesterday,  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.  With significant loading over the next few days, we are in a stepping-back mindset, with more uncertainty on where and when this problem will act up again.  In our shallower zone, 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.  Yesterday's accident in Maroon Bowl is a good example of that.  

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