Bollettino valanghe

Northwest Mountains

Crested Butte Avalanche Center
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Pericolo valanghe

Thu
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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Non aggiornato — questo bollettino è scaduto 6 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Gusty winds yesterday and last night added stress to a weak layer that has produced daily avalanche activity since Saturday's storm, including this near miss on Monday.   Natural avalanche activity is ending as winds die down, but steep, windloaded terrain is dangerous for human triggers today.   Slabs are smaller and less reactive in windsheltered terrain.  

Problemi valanghivi

  • Persistent Slabs

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

    There are two flavors of persistent slabs that you can trigger today.  The problems are most dangerous on windloaded slopes where slabs are thicker. The youngest generation of persistent slabs is more widespread, on steep slopes where recent and windblown snow formed soft slabs over weak facets and faceted crusts in the upper snowpack.   The older generation involves stubborn hard slabs that could fail on weak layers near the ground - a more dangerous but more isolated problem.  Persistent slabs can be remotely triggered from below a steep slope or propagate widely across the terrain.  Shooting cracks and collapses are clear warning signs of this problem; however, persistent slabs are notorious for giving unreliable feedback.  For example, there were numerous tracks on the slope before a skier was nearly caught in the large avalanche on Schuylkill Ridge.  Give yourself a wide margin for error when assessing and managing persistent slabs.  

  • Wind Slabs

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

    Yesterday, forecasters were able to trigger thin wind slabs as winds started to increase in the afternoon.  With moderate to strong winds continuing overnight, expect wind slabs to be thicker and larger today.  With good visibility today, they should be fairly easy to identify.  Look for and avoid smooth, rounded drifts below ridgelines and rollovers or in crossloaded gulleys.  The snow will feel thicker and denser where it is windloaded.  You can avoid the problem by seeking out softer, wind protected snow.  

    1/27: A shallow wind slab triggered on a wind drifted slope below treeline. Expect slabs like this to be thicker at higher elevations.

Avalanche Discussion

The big news is increased winds.  West to northwest winds picked up yesterday afternoon, blowing 20-25 mph with gusts in the 30's and 40's at alpine stations.  Last night's winds were loading snow onto our most problematic aspects: northeast, east, and southeast.  These aspects have been the most active for slabs breaking on both the 1/19 weak layer in the upper snowpack this week, as well as the 12/10 weak layer near the ground all winter.  With upwards of two feet of low-density snow since Saturday, a missing ingredient for slab formation in some cases has been the wind.  This video demonstrates why windloaded slopes are the most dangerous.  The windloading event has now ended, but slabs will be most sensitive to human triggers where the combination of recent and wind drifted snow overlap with a buried layer of facets or faceted crusts.  Not all slopes have those ingredients, so with careful snowpack assessments, you can hunt out steeper terrain.  Wind sheltered, below-treeline slopes have less slab development, especially near and east of town in the Southeast Mountains.  In the Northwest Mountains, persistent slabs extend into below treeline terrain where storm totals are deepest.   South and southwest aspects held stronger, thicker crusts prior to our recent storm, but you'll need to manage for fresh windslab problems on those aspects.  Where the recent snow accumulated on hard, thick windboard, it has also been less reactive than slopes where wind crusts are thin or non-existent.  This video highlights what the problematic 1/19 weak layers looked like before our recent storm.   

With poor visibility and continuous snowfall for almost a week, forecaster and public travel have largely been confined to below treeline.  Our observations and documented avalanche activity from the alpine have been very limited since Saturday, and thus our forecast confidence is low.  If you are traveling to higher elevations with today's clearing skies, bring a conservative mindset.  Remember that we picked up 20% of our season's snowpack this week, culminating with a windloading event onto two unnerving weak layers in our snowpack.  Avalanches that fail in the alpine will be larger, wider propagating, and more dangerous than avalanche activity that we have been documenting below treeline this week.   

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