Bollettino valanghe

Northwest Mountains

Crested Butte Avalanche Center
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Pericolo valanghe

Sun
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Limite del bosco
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Sotto il limite
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In sintesi

Heavy snowfall on multiple weak layers in the snowpack is creating dangerous avalanche conditions.   It will be easy to trigger avalanches in the new snow, and some slopes will fail naturally.   Less obvious and less common, there is potential for larger avalanches to break on older weak layers several feet thick.   Instabilities will be smaller and easier to manage on wind-sheltered southerly aspects or smaller terrain features without terrain traps.  The safest riding is on slope angles less than 30 degrees.  

Problemi valanghivi

  • Storm Slabs

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

    About a foot of new snow will form easily triggered and naturally failing soft slabs on some steep terrain features.  Fresh slabs will be most sensitive on wind loaded slopes at higher elevations or on the north half of the compass where they are forming over fragile weak layers.  Carefully evaluate slab size and terrain consequences before riding on slopes steeper than about 30 degrees. Cracking or collapsing in the new snow is a clear sign of unstable snow.  Storm slabs could be remotely triggered because they are forming on surface hoar.   Instabilities in the upper snowpack have the potential to step down and trigger larger and more dangerous persistent slabs where the problems overlap.  

    The new snow is accumulating on weak layers of surface hoar (shown here) and near surface facets on west, north, and east facing slopes, and crusts on southerly slopes.  

  • Persistent Slabs

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

    Today's new snow will make old layers deeper in the snowpack more susceptible to human and natural triggers than earlier this week.   Persistent slabs up to several feet thick have the potential to fail widely or be remotely triggered from near or below steep slopes.  These dense old slabs are unlikely to give warning signs before releasing.   The problem is less concerning on southerly aspects where the recent warmup created thick surface crusts (see video).   The easiest and most reliable way to manage this problem is to ride on slopes consistently less than 30 degrees in steepness.  You are most likely to trigger a persistent slab where the snowpack is shallow or variable in depth, or if you trigger an avalanche in the new snow that gains enough mass to step down.  

    An example of a large persistent slab that ran earlier this week in the Elk Mountains.  The current loading pattern will make avalanches like these more likely to fail.  

Avalanche Discussion

The storm is currently underproducing and looks to fall short of our thresholds for High danger, so the Avalanche Watch was canceled this morning.   As of 5 a.m., storm totals range from 4" to 8" of dense snow, which could double in the Northwest Mountains today.  Today's most obvious and common issue will be fresh storm slabs or wind slabs breaking on surface hoar, near-surface facets, and crusts.  This video describes the storm interface.  You can manage this issue by monitoring storm totals and using small test slopes to gather feedback on how the storm snow is behaving.  We're expecting slabs to be most sensitive in wind-drifted terrain or where you find more than a foot of new snow over surface hoar.  Shooting cracks are a telltale sign that the snowpack is unstable.  

The trickier issue is our persistent slab problem, which won't give easy-to-recognize or reliable feedback today.  Our travel advice is similar to the last week: shallow slopes and slopes with a variable snowpack are most concerning.  However, with new snow adding stress, those old layers will be a little angrier today than they have been all week. Furthermore, there's potential for smaller avalanches in the fresh snow to step down.  We saw that occur on a few slopes earlier this week with wet loose avalanches.   Our problematic drought layers have been productive with every storm and windloading event this winter.  Conservative decision-making and careful snowpack evaluation are essential today.  

Events: Our next Fireside Chat is this Sunday, February 4 at 6 p.m. at High Alpine Brewing Company in Gunnison. A local snowmobiler will talk about his experience with an avalanche burial and rescue while snowmobiling last year.

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