Bollettino valanghe

Northwest Mountains

Crested Butte Avalanche Center
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Pericolo valanghe

Sun
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
3 · Marcato
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In sintesi

An AVALANCHE WATCH is in effect for the Northwest Mountains.  Avalanche conditions are becoming increasingly dangerous and more complicated.  Up to 2 feet of new and drifting snow is forming easily triggered storm slabs and overloading weak layers buried deeper in the snowpack.  Large natural and triggered avalanches are likely today, especially at mid to upper elevations facing north or east.  Bring a conservative mindset to the backcountry today.   The danger will rise to HIGH tonight.  

Problemi valanghivi

  • Persistent Slabs

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

    Each of our past two storms has caused a clear pattern of persistent slab avalanches on the north half of the compass near and above treeline.  At mid and upper elevations, accumulating and drifting snow will increase the threat of large natural and triggered slabs breaking on old weak layers.  The problem is less developed below treeline but could switch on under our current loading.  Slabs are typically 2 to 4 feet thick and have the potential to break wider than you might expect, or can be triggered from flat slopes below steep terrain.   Shooting cracks or collapses are a clear sign of danger.  Choose conservative terrain with slope angles less than 30 degrees where you find a cohesive slab over weak faceted snow.  

    One of many large persistent slabs that failed naturally during last weekend's storm.  Similar, albeit larger, activity is expected this weekend.

  • Storm Slabs

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

    Storm totals are approaching two feet in the Northwest Mountains.  The new snow is accumulating on a mix of crusts, near-surface facets, and low-density snow that fell at the start of the storm.  Observers yesterday noted storm slabs starting to form (Example A, Example B); the problem will become thicker and more widespread today, especially in leeward terrain.   Monitor the size and bonding of storm slabs on small, steep test slopes.  On the north half of the compass, storm slabs are likely to entrain old, weak snow at lower elevations or step down to trigger larger persistent slab avalanches at higher elevations.   That's a dangerous setup that is easily avoided on slope angles less than about 30 degrees.  

    An example of a small test slope with an unstable storm slab, from the archives.

Avalanche Discussion

The avalanche danger is rising more dramatically in the Northwest Mountains today, where storm totals are approaching two feet in the Slate and Kebler areas.   Heavy snowfall and increasing winds tonight and tomorrow are expected to spur a large and very dangerous natural avalanche cycle in the deeper zone.  The Southeast Mountains are lagging behind on storm totals (~5 to 9" so far) so avalanche problems aren't as large or widespread today.  The danger will increase significantly in our shallower zone tonight and tomorrow as well, but doesn't appear to cross the threshold into High danger (Level 4).  

Our buried weak layers have proven to be surprising and quite active so far this season, so give yourself a wide margin for error during this major loading period.   Last weekend, a persistent slab avalanche in Marble broke widely and engulfed four skiers, burying three of them.  Thankfully everyone survived (read the full accident report here).  Avalanche conditions are even more complicated this weekend, and they are becoming significantly more dangerous as this storm evolves.  You have multiple problems to manage now, and instabilities span around the compass. And like last weekend, slopes with the deepest coverage for early season turns are also the most problematic for avalanches.  Fear not, you can still enjoy the powder! Choose smaller terrain features and lower slope angles free of overhead hazards.  Avalanche hazards are smaller and more manageable in wind-sheltered terrain in the Southeast Mountains, though coverage is still quite thin for riding there.    

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