Bollettino valanghe

Northwest Mountains

Crested Butte Avalanche Center
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Pericolo valanghe

Thu
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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In sintesi

Use caution as you gain elevation into more wind-affected terrain on the north side of the compass, where the snowpack is more variable and has more trigger points.  Old hard slabs remain a low likelihood, high-consequence threat.  Persistent slabs are most likely triggered where the snowpack is thinner, often near rocky areas, convexities, or in wind-affected terrain.  You can find a more trustworthy snowpack with generally good stability on the south side of the compass.  Stay alert for a few thin wind slabs that might have formed last night.    

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    2–2.5

    Old, hard slabs 2–3+ feet thick linger over buried weak layers but are becoming increasingly stubborn to trigger and less common. Without obvious warning signs, avalanches are most likely triggered where the slab is thinner and easier to impact the weak layer below. The most fragile layers are buried in the middle and lower portions of the snowpack on northerly aspects. Persistent slab avalanches are dangerous in size; they can propagate widely and be remotely triggered from adjacent low-angle terrain.  The most reliable way to manage this problem is to choose conservative slope angles or travel on southerly aspects, where the snowpack structure has become stronger and more spring-like. On northerly aspects, avoid steep terrain with a thin or variable snowpack, such as rocky areas, rollovers, or places where winds have created uneven slab thickness.

    In the Northwest Mountains, our most recent persistent slab activity occurred during the last week of February, including this pair of destructive slabs.  

    Credit: Irwin Guides

  • Wind Slabs

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

    Strong winds over the past 24 hours, generally out of the west to northwest, scraped what little soft snow was remaining off of northerly facing terrain and may have formed thin wind drifts below ridges or on crossloaded features at higher elevations.  New wind slabs are expected to be small and relatively rare, but they are likely stiff enough to knock you off your feet in consequential terrain.  Look for fresh deposits, indicated by smooth-looking, stiffer, thicker pockets of snow, and steer away from them in high-consequence terrain.  

    An example of a small, hard wind slab from early this season.  

Avalanche Discussion

Persistent slabs remain the most difficult problem to manage and can cause a dangerously large surprise. Statewide, these avalanches have been most active in the Vail/Summit County/Front Range, likely due to greater recent loading and a colder snowpack that avoided the late-February freezing rain event. Although we haven't observed persistent slab activity in our zones in the past week, we're only a stone's throw away from several destructive avalanches that ran above Conundrum Creek during last Friday's storm.   Closer to home, we continue to find the most concerning snowpack structures along the eastern side of the forecast area. Brush and Cement Creek areas, which typically had the weakest layers going into the February storms, now have thinner slabs that remain more susceptible to human triggers and thinner rain crusts in the upper snowpack. Stay vigilant: land mines exist across our entire forecast zone, and careful terrain selection remains essential.

Cooler temperatures should limit wet avalanche concerns today. Still, watch for isolated exceptions, especially if the upper snowpack becomes unsupportable and you begin punching through the surface.  After weeks of relatively tame winds, an extreme wind event blasted the mountains last night (yes, Mother Nature is angry this year). Cinnamon recorded its strongest winds since the late-February storms, averaging around 40 mph with gusts near 70.  In northerly alpine terrain, up to 10" of soft snow above the late February rain crust was available for transport. Although extreme wind events often cause more erosion than loading, thin, hard wind slabs may have formed in concave terrain features and just below ridgelines. These will be most sensitive where snow was cross-loaded onto weak surfaces on northerly aspects. Look for clues under your feet. Rippled sastrugi, shimmering rain crusts, or textured corn indicate you are not on a wind slab. Smooth, chalky, hard snow may signal that you've found one.  

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