Bulletin d’avalanche

Blues

Wallowa Avalanche Center
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Danger d’avalanche

Fri
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
1 · Faible
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Périmé — ce bulletin a expiré 2 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

February's 1-1.5' of snow sits on a foundation of weak faceted old snow grains, creating a dangerous recipe for a persistent slab avalanche.  This exists on all aspects above 4000' where human triggered avalanches are Possible on slopes greater than 30 degrees.  Evaluate snow and terrain carefully, identify features of concern, especially terrain traps.  

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Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–2

    Upon finding facets and poor structure on multiple E-S-W aspects, we've expanded the distribution of this problem to southerlies.  Below 4000', there is not enough of a slab overlying the persistent weak layer (PWL) to warrant listing.  Continued observations will help us refine the location of this problem, especially on north aspects.  Persistent slabs are notoriously tricky to manage, and often offer little in the way of signs of instability before it's too late.  

    The Persistent Slab problem will pose the highest complexity and consequence in the Northern areas of the Blue Mountains which have the greatest snow totals in February.  

Avalanche Discussion

A persistent slab needs two ingredients: a weak or soft layer below (the facets we've been finding and discussing for weeks) and a stronger or harder layer above (the new snow this week).   As the name suggests, the persisting layer will stick around and be slow to strengthen.  The slab above it gained a major load on Tuesday night, tipping the scales and triggering natural avalanches on some slopes.  Although we've had minimal precipitation since then, an inch here and there adds up, as does wind amplifying the slab's load.  Slopes that didn't avalanche already are closer to the breaking point.  

  • On shady and wind-sheltered northerly aspects, February's snow will not bond well to the 10-12" of weak, sugary facets and/or surface hoar. 
  • On solar aspects, a buried melt-freeze crust may provide a slick bed surface and certainly harbors weak sugary snow below it.  We have had one confirmation from the S Wallowas of a persistent slab triggering below the 1/31 melt-freeze crust.  

What to do:   A persistent slab avalanche needs two more ingredients: a slope greater than 30 degrees to initiate, and a natural or human trigger.  With uncertainty about which slopes have the right combination of slab ingredients to be unstable enough, you can hedge your bets by choosing low-angle terrain or, at the least, inconsequential pieces of avalanche terrain. 

As always, be on the lookout for the freebie signs of unstable snow: recent natural avalanches, shooting cracks, and whumphing of the snowpack.   With a persistent slab problem, the absence of these clues does not mean that a slope is safe to ride, and careful evaluation should be used moving forward.   If you're not in avalanche terrain, you can't get caught in an avalanche.  

Observations from the Blues on  Feb 4 and Feb 2 did not include avalanches, but they found poor structure and propagating results (including grains spitting out of a pit wall, see video).   

Add value to your avalanche forecast.  Help us reduce uncertainty.  Please submit an observation if you get out in the backcountry.  

Régions couvertes