Bollettino valanghe

West Slopes Central

Northwest Avalanche Center
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Pericolo valanghe

Thu
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 4 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Slab avalanches in wind drifted snow are the primary concern and are more likely in the afternoon when winds are stronger. Wet avalanches on sun-exposed slopes are possible in the morning before the sun gives way to cloud cover. Pay attention to changing conditions and travel in simpler terrain to reduce uncertainty.

Problemi valanghivi

  • Wind Slabs

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

    Weather stations across West Central recorded 6-12” of new snow at treeline. Increasing winds will redistribute abundant new snow and build wind slabs on leeward slopes. Alpine terrain will have more wind slab development in the afternoon but expect to see some at mid-elevations as well. In addition to new wind slabs, you can trigger day-old slabs, some of which formed over buried surface hoar.  One remotely triggered avalanche was reported on Tuesday from northerly, near treeline terrain. Though intact surface hoar layers are likely spotty in distribution, they are present and could be the weak layer in slab avalanches in the coming days. Remember that avalanches that fail in surface hoar layers can release above you, when you are in the middle of the slope or surprise you when you least expect it.

    Blowing snow will be the best indicator of active wind loading in the surrounding terrain.  Also note the pattern of wind scoured versus loaded slopes to help identify where wind slabs are building. Track the change in snow depth as you move in and out of wind affected terrain and look for stiff slabs overlying weaker snow below. Hand pits and test slopes can help you gauge the bond of shallow wind slabs, but assessing a deeper layer of surface hoar often requires snowpack tests. When in doubt just navigate around steep features and slopes where you suspect wind loading or that simply have deep new snow.

    Example of wind transport in alpine terrain.

    Photo: Jason Speer

Avalanche Discussion

Wednesday should start clear and sunny with increasing clouds mid-morning. Though freezing levels will start low, temperatures will rise through the day and the April sun doesn’t need much time to affect the snow surface. Wet loose avalanches are possible if new snow warms, becomes wet, and weakens on sun-exposed slopes or at low elevations. Minimize travel on and below slopes where you observe wet loose avalanches or other signs of instability like pinwheels or rollerballs. If skies quickly cloud over in the morning then wet loose avalanches are less likely.

Observers in West North and Snoqualmie and Stevens Passes reported small natural and human triggered slab avalanches in new snow layers. While areas to the north received less sun and saw much less wet loose activity, those avalanches were more widely observed in areas south and east. Of greater concern is a small but deeper slab (~2’) that released in a layer of preserved surface hoar in the Baker Backcountry in West North. Though the avalanche occurred on a small terrain feature, it was remotely triggered from below. The lack of large natural avalanches during the storm shows buried surface hoar is probably not widespread - but the jury is still out without more observations. If you were to find buried surface hoar it would be on northerly, mid and upper elevation slopes where it managed to survive sun and warmer temperatures. In a snowpit you would find surface hoar below the most recent snow but above a melt-freeze crust from early April. Clouds limited direct sun in some areas but others saw significant solar radiation. Warm temperatures affected the snow on all aspects to at least 4500'. With clear skies and cooling overnight expect lower elevation and southerly slopes to sprout a crust Wednesday morning.

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