Bollettino valanghe

Northwest Mountains

Crested Butte Avalanche Center
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Pericolo valanghe

Sat
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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In sintesi

More snow and strengthening winds are creating dangerous conditions at higher elevations. This video summarizes the situation.  Natural and triggered slab avalanches breaking up to several feet thick have increased in frequency over the past few days, especially on wind-loaded slopes.  You can find safer riding at lower elevations protected from the wind and free of overhead hazards.  

Problemi valanghivi

  • Persistent Slabs

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

    Snow and wind-loading in the past week have formed a new generation of slabs up to several feet thick over small-grained facets in the upper snowpack.  There have been numerous natural and triggered slabs on this layer in the past few days.  Slabs are larger and most sensitive near and above treeline on windloaded slopes, especially on colder aspects.  Give wind loaded slopes a cautious buffer by choosing lower slope angles or smaller terrain features, while staying alert for isolated instabilities on wind-sheltered terrain.  

    On unusually shallow slopes, most commonly in the Southeast Mountains, there is still a small potential to trigger a destructive avalanche that breaks on deeper weak layers. The most concerning structures are on northerly aspects near and above treeline where snowdepths are less than 170 cm.  

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–2

    Up to a foot of new snow and moderate to strong alpine winds will form sensitive drifts today on leeward terrain features at higher elevations.  Wind slabs are forming below corniced ridgelines and in crossloaded gullies.  Blowing snow and cracking in the snow are signs of danger.  Wind slabs could be dangerous by themselves in bigger or high consequence terrain features, or they could step down and trigger a larger avalanche several feet deep.  Monitor for developing storm slab issues in wind protected terrain in areas that see more than about 10" of new snow.  

    A recent wind slab avalanche on Augusta. 

     

Avalanche Discussion

Over the past 36 hours, increased winds brought more slopes to their tipping point, evidenced by a handful of large naturals that likely ran late Wednesday or early Thursday: Whetstone, Gothic, Richmond, Peeler, and Ruby.  Some patterns are emerging with avalanche activity ramping up over the past few days. Much of the recent slab avalanche activity has failed on our March dry-spell layer: facets or faceted crusts that exist on northerly aspects and potentially wrap around to some east or southeast-facing slopes at high elevations.  Larger storm totals in the Northwest Mountains have caused the issue to be more common in the deeper zone, while the Southeast Mountains are just starting to get in on the action with increased wind transport thickening slabs in the alpine.  There has been significantly more activity near and above treeline, due to some combination of higher storm totals, more wind drifting, and more suitable weak layer formation/preservation conditions.  We were describing the problem as a storm slab issue earlier this week, but we are moving it into the persistent slab bin today as slabs get thicker and denser, with more potential for remote triggering or wide propagation.  Today's continued snowfall and wind transport will likely cause more natural activity and keep conditions sensitive to human triggers on slopes with the right combination of slab and weak layer.  There have been several close calls around Colorado the last few days, including a full burial near Ashcroft on Wednesday.   Keep the recent avalanche activity and close calls at the forefront of your decision-making this weekend.  The snowpack has been quieter below treeline and there are more opportunities to ride safely in steeper terrain (see this forecaster observation yesterday).  At those lower elevations, continue to assess the snowpack, terrain consequences, and overhead hazards.  

Secondary concerns today include fresh wind slabs breaking in today's new and windblown snow with up to a foot of snow.  In areas that see favored snowfall, we may see thin storm slabs forming in wind-protected terrain.  In shallower parts of our forecast areas, there is still potential for avalanches to break on deeper weak layers. Locally, the last avalanche that broke deeply was in Terminator Chutes on March 16th. More recent examples come from the Aspen side, such as this close call near Highlands Ridge last weekend.   The most concerning structures are for this low likelihood issue are near and above treeline on northerly aspects where the total snowpack height between 80 and 170cm. 

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