Bulletin d’avalanche

Stevens Pass

Feb 10 – 11, 2019Northwest Avalanche Center
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Danger d’avalanche

Tue
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
3 · Marqué
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Points saillants

Dangerous avalanche conditions are to be expected. This storm pattern has been unusual, and will continue to produce low density snow along with storm slabs for Monday. We are tracking a commonly found persistent weak layer. This layer may produce deeper and more destructive avalanches and they may be triggered from a distance. Seek out slopes less than 33 degrees if you find slab structure over facets. 

Problèmes avalancheux

  • Persistent Slabs

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

    Heads up, persistent slabs are back in the picture for Stevens Pass. A layer of facets rest on top of a crust beneath the weekends storm snow. It may only be a matter of time before this layer begins to fail in a widespread fashion. Will tomorrow be the day? It's hard to say with any certainty but we are certainly not getting any further away from it. If you hear collapsing or see deep shooting cracks, watch out. Use caution on and beneath any steep slopes where you find strong over weak. Slabs may be triggered from a distance if they rest over these facets. In the few places out of wind effected locations, the low density of the snow may not have enough cohesion to act as a slab, yet. Previously formed slabs up to 2 feet deep can be expected, underneath whatever new snow falls tonight. This new snow will mask the old, hollow slabs. The snow may feel incredible underfoot, and it may feel stable, but don't get lulled to complacency just because we haven't seen much activity on this layer yet. If you trigger one of these slabs they will be surprising and easily could be big enough to kill a person.

  • Storm Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1

    The new snow could pile up very quickly with these temperatures. All we need to form storm slabs is for the new snow to be ever so slightly different than the previous snow surface. This may or may not happen, but chances are it will be. Look for shooting cracks, recent avalanches on small cutbanks, and use hand shears to check how the new snow is bonding to the underlying surface. Is there strong over weak or are you seeing shooting cracks? Then there is slab structure. 

Discussion

Snow and Avalanche Discussion:

Prior to the storm cycle that started on February 8, the very cold temperatures turned a layer of low density snow into facets. These facets rest over a mostly supportable crust, and are fairly widespread above 5,000ft. Observers have reported multiple collapses on a wide range of aspects in recently wind loaded areas the past two days. As the fluffy snow becomes more cohesive above this layer and the storm snow continues to pile up, the size of the potential avalanches are increasing, and once the slab begins to communicate a fracture across the terrain, watch out. This is a fairly unusual situation for the Cascades, and one to be taken very seriously as the cold temperatures and snowfall continues into the week. Tell your friends to be safe out there, and hold them to it. This forecast may be updated at 0630 on Monday based on overnight storm totals. 

Facets are angular snow grains that bond poorly to other grains around it. Once formed, buried, and preserved, they tend to persist in the snowpack. They are often the culprit of deep and destructive avalanches that result in accidents. 

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