Bollettino valanghe

East Slopes Central

Northwest Avalanche Center
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Pericolo valanghe

Tue
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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In sintesi

On north-facing terrain at higher elevations, you can trigger dangerous avalanches that may run at or near the ground. Before considering travel in steep terrain, dig down and identify any weak, sugary snow layers lurking below all the recent storm snow. If you find this weak snow or hear whumphing collapses in the snow beneath you, stick to low angle terrain.

Problemi valanghivi

  • Persistent Slabs

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

    At the base of the snowpack, an old layer of facets and surface hoar exists. Avalanches have been running near or down to the ground on these layers in and around Mission Ridge. Many other observations throughout the zone have noted audible collapsing in this weak-faceted snow. These types of avalanche problems are slow to heal. You could trigger avalanches 1.5-3 feet deep, and slides that run on rocky ground with stumps and obstacles to hit can further increase the consequences of being caught. It is possible to trigger these avalanches from low angle terrain below steep slopes or from adjacent slopes, so be sure to keep tuned into the surrounding terrain and identify steep slopes near or above you.While we don't expect natural activity on this layer, you can still easily trigger an avalanche on this layer by skiing, snowmobiling, or snowshoeing in steep terrain above 5,000ft, particularly on northern aspects. More recent observations indicate that conditions are potentially becoming less touchy, and reports of collapsing are decreasing. However, even if you don't get red flag signs of instability, it's important to dig down and identify these layers. If you find this snowpack set up, make conservative travel decisions.Be especially wary of ridgeline terrain if you observe actively blowing snow or find deep drifts of stiff wind blown snow. Active wind loading can quickly add stress to these deeper weak layers of snow. 

    Evidence is increasing that this weak layer may not be a problem on SE and SW aspects, but we need more information to boost confidence.

    (2/9/2021)

    Example of a persistent slab avalanche from 2021

    Photo: Matt Primomo

Avalanche Discussion

Evidence of various weak layers surrounding the 11/22 and 11/25 crusts continues to roll in as avalanche professionals and winter recreationists report finding this snowpack structure in many locations. Patrol at Mission Ridge reported touchy conditions on Saturday in terrain without previous avalanche control efforts. During control work, avalanches in the ski area ran on a thin crust from around 11/25, and many stepped down to or ran on the ground (or on basal facets from 11/22). On Thursday in Icicle Creek, there were multiple reports of whumphing tied to basal facets (11/22) in locations as low as 3,600ft on North facing slopes. Observers at Holden and Dirtyface also heard and felt whumphing on these deeply buried weak layers. All of this strongly suggests that this layer is not ready to be trusted, and could produce surprising and wide avalanches. 

It's still early season, and the snowpack is relatively shallow and unconsolidated. Tree wells are open, and plentiful buried snags, creeks, and rocks exist. If you are out and about in the mountains, let us know what you find by submitting an observation. 

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