Lawinenlagebericht

East Slopes Central

Northwest Avalanche Center
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Lawinengefahr

Wed
Hochgebirge
3 · Erheblich
Waldgrenze
3 · Erheblich
Unterhalb
2 · Mässig
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Kernpunkte

There is plenty of low-density snow to blow around, and strong westerly winds will build deep slabs below ridgelines. This additional wind-driven snow may overload buried weak snow near the base of the snowpack, and you can easily trigger large avalanches that run at our near the ground. Use your probe or pole to feel for stiff snow over soft snow and heed warning signs such as shooting cracks or collapsing. With this complicated and weak snowpack, avoid steep ridgeline terrain on Tuesday, especially northern aspects above 5000ft. 

Lawinenprobleme

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Likely
    Größe
    1–2

    Strong winds will transport snow to the lee sides of ridges and may overload an old layer of facets and surface hoar. 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. During the last significant storm, avalanches ran near or down to the ground on these layers at Mission Ridge Ski Area. Observers throughout the zone continue to report audible collapsing in this weak-faceted snow. Most of our information is from lower elevations. However, we expect a similar structure at higher elevations. With additional wind-driven snow loading, triggering an avalanche at upper elevations may be even easier. Northern aspects are suspected to be the most likely place to trigger these avalanches, but we need more information before ruling out SW and SE aspects. These types of avalanche problems are slow to heal.Look and listen for red flag signs of instability, such as recent avalanches, cracking, or collapsing. However, even if you don't get these loud warning signs, it's important to dig down and identify any weak, sugary layers in the snowpack. In areas with this weak snowpack, make conservative terrain choices and stick to lower angle slopes sheltered from the wind.

    (2/9/2021)

    Example of a persistent slab avalanche from 2021

    Photo: Matt Primomo

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Likely
    Größe
    1–2

    It is going to be a blustery day on Tuesday. Strong ridgeline winds are in the forecast, with wind speeds between 20-30mph. Areas in the Wenatchee Mountains near Mission Ridge could see stronger gusts into the 50mph range. Even though it has been a few days since the last significant precipitation, the cold temperatures have maintained plenty of loose, dry snow that the wind can blow around. With weak snow buried in the snowpack, rapid wind loading could quickly increase avalanche danger on Tuesday. There is potential for even a small wind slab to step down to deeper weak layers, creating a large and dangerous avalanche. Consider sticking to wind sheltered terrain and avoid steep slopes near ridges. Look for signs of blowing and drifting snow and use small hand pits to identify stronger layers of wind-driven snow over soft powder snow.

    (12/3/2022) 

    Drifting snow that quickly fills in tracks, like this photo, is a sign of rapid wind loading. 

    Photo: Joe Dellaporta

Avalanche Discussion

We are still gaining information on how widespread and reactive these various weak layers surrounding the 11/22 and 11/25 crusts are throughout the zone. Evidence continues to roll in as avalanche professionals and winter recreationists report finding this snowpack structure in many locations. But observations from elevations above 5000ft are lacking. As a result, we are still determining how easily you can trigger an avalanche on this layer. Therefore, conservative terrain choices are warranted until we gain more information. If you are out and about in the mountains, you can help us by submitting an observation. 

 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. As recently as Sunday, an observer in the Stuart range also reported a weak snowpack structure and collapsing. These sugary, faceted snow layers take a long time to gain strength, and they are not ready to be trusted. We have yet to put this layer to the test with another big storm, but this wind event may give us additional information on how sensitive this layer is to triggering. 

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. 

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