Avalanche forecast

Northwest Mountains

Crested Butte Avalanche Center
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Avalanche danger

Thu
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
3 · Considerable
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Highlights

Numerous large natural and skier-triggered avalanches ran yesterday and the snowpack is screaming "Danger!" with rumbling collapses and shooting cracks.  Choose conservative routes that avoid traveling on or below steep slopes that have old, weak snow at the bottom of the snowpack.  The danger will rise to High again tonight as another pulse of snow and increasing winds look to spur another natural cycle.   

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1.5–2.5

    Over a foot of recent snow has made persistent slabs very sensitive to human triggers, and some could fail naturally.  There's a widespread weak layer of facets and faceted crusts buried by 2 to 4 foot slabs on west to north to southeast aspects. The slabs are largest in leeward terrain at higher elevations.  The problem is less common, but still possible, on windward terrain or south facing slopes that hold old snow layers near the ground.  Choose routes that keep you off and out from under steep terrain that holds old snow below our recent storms.  Collapses and shooting cracks are clear signs of danger warning you to stay on slope angles that are consistently less than 30 degrees.  

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    Up to 18" snow fell in the past 24 hours, with moderate winds during the storm yesterday.  You are most likely to trigger a storm slab avalanche at higher elevations where winds have stiffened the recent snow. At lower elevations, loose dry avalanches are more common.  Relatively shallow releases in the storm snow threaten to step down and trigger a larger persistent slab avalanche where the two problems overlap.  

Avalanche Discussion

Yesterday's storm delivered upwards of 15" of low-density snow with efficient wind drifting speeds in the alpine.  Another 3" or so fell overnight under light winds, and we should see a couple more inches during daylight hours today.  With a lull in snowfall and wind today, natural activity should take a significant downturn compared to yesterday.  However, persistent slabs remain very sensitive to human triggers.   Persistent slabs have been producing rumbling collapses and shooting cracks everywhere in our forecast area all week.  Even though the deepest coverage is on those slopes that held old snow, make a plan to go elsewhere or choose low-angle objectives.  Slab avalanches are now breaking 2 to 4 feet thick, and are stiff enough to connect across terrain features.  For example, yesterday I triggered a collapse on Schuylkill Ridge that propagated into an adjacent bowl and triggered avalanches in both bowls.  (See video).   

The snowpack isn't nearly as dangerous on slopes that were bare dirt prior to our recent storms.  These two observations from the Northwest Mountains and Southeast Mountains document the distribution of our problematic weak layer that formed during November's dry spell.  In terrain that's lacking the troublesome old snow layer, you'll need to manage for shallow storm instabilities that developed during yesterday's heavy squalls and gusty winds.  

 The final pulse from this prolonged storm system pushes through tonight with increasing wind speeds.  If snowfall and wind forecasts verify, we're looking at yet another round of significant natural avalanche activity that will continue into tomorrow's daylight hours.   With the added loading tonight, expect avalanche size to increase to very large (D3) on some slopes in the Northwest Mountains.  A few slopes may be there already.   Today's Avalanche Warning addresses the sensitive conditions to human triggers today and the increasing threat of natural avalanches tonight.

 

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