Bulletin d’avalanche

Stevens Pass

Feb 12 – 13, 2019Northwest Avalanche Center
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Danger d’avalanche

Wed
Alpin
4 · Fort
Limite forestière
4 · Fort
Sous la limite
4 · Fort
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Points saillants

Heavy snowfall and winds will create very dangerous avalanche conditions for Tuesday. An avalanche cycle is expected tonight into tomorrow. This unusually cold and snowy storm pattern continues to stress a deeper weak layer, even at low elevations. Travel in avalanche terrain is not recommended, as avalanches may run large from upper elevations. Avalanches may be triggered from below.

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    2

    Persistent slabs are getting deeper and nearing a tipping point at Stevens Pass. A layer of facets rest on top of a crust beneath the weekends storm snow and can now be found from 2 to 3 feet deep. With the heavy storm and snowfall into Tuesday, we believe this layer may begin to fail in a widespread fashion. Will this storm be the one? Signs appear to be hinting in that direction. If you go into the mountains you may hear huge whumphs, and or see deep shooting cracks. Watch out, because slabs may be triggered from a distance. 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.

    This new snow will mask the old, hollow slabs that rest above this layer. If you trigger one of these slabs they may be big enough to destroy a large semi-truck.

  • Storm Slabs

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

    The new snow will be piling up very quickly with these temperatures, and the winds will easily drift it into slabs. Storm slabs are thickening already this evening and will continue to get deeper and more reactive into the night. Shooting cracks, recent avalanches, and heavy snowfall rates are all red flag signs for avalanches. Avoid entering or going below slopes steeper than 30 degrees. Make sure you don't travel in potential avalanche runout zones, as slides could be triggered from a distance and may travel very far with large powder clouds. 

    Loose dry avalanches may be a concern at lower elevations. These may be large by Tuesday. Watch for fan shaped avalanches, and avoid hanging out in places where snow normally sheds off steep slopes from above. 

Discussion

Snow and Avalanche Discussion:

Morning Update: Avalanche Warning. HIGH Danger at all elevations

Observers continue to report multiple collapses on a wide range of aspects, and even a remotely triggered slide at Snoqualmie Pass. The collapses have been on a wide range of aspects, but appear to be most prevalent where the snow had previously been drifted onto. A skier was able to trigger a small slab that ran on the February 8th layer of facets over a crust near Lanham Lakes. These signs are bullseye pieces of information. Ding, ding, ding...

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.  As the fluffy snow continues to pile up, it is becoming more and more cohesive. 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 picks up Monday night into Tuesday. Tell your friends to be safe out there, and hold them to it. Honestly, tomorrow is not a good day to venture out of bounds and into avalanche terrain.

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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