Avalanche forecast

Blues

Wallowa Avalanche Center
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Avalanche danger

Sat
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
1 · Low
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Out of date — this forecast expired 2 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

February's 1-1.5' of snow sits on a foundation of weak old snow grains, creating a dangerous recipe for a persistent slab avalanche.  This exists on all aspects above 4000'.   While natural avalanches are Unlikely, human triggered avalanches are Possible on slopes greater than 30 degrees.  Evaluate snow and terrain carefully, identifying features of concern, especially trauma boosters like trees or burial intensifiers like gullies.  

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Avalanche problems

  • Persistent Slabs

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

    Upon finding facets and poor structure on multiple E-S-W aspects Tuesday, 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.  

    The thickness of the overlying slab varies from 1-1.5' (or more with wind loading). The persistent weak layer below can be summarized:

    • On shady and wind-sheltered northerly aspects, February's snow has not bonded 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 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.  

Avalanche Discussion

A persistent slab needs two ingredients: a weak or soft layer of persistent grains (the facets we've been finding for weeks) and a stronger or harder layer above (the new snow this month).   As the name suggests, the persisting layer will stick around and be slow to strengthen.  Inch by inch, the slab above it continues to grow, destabilising it.  

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.   

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).   

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