Bollettino valanghe

West Slopes South

Northwest Avalanche Center
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Pericolo valanghe

Fri
Alta quota
4 · Forte
Limite del bosco
3 · Marcato
Sotto il limite
3 · Marcato
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Non aggiornato — questo bollettino è scaduto 3 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Blustery, windy, and snowy weather is scheduled to continue. It's another day when the avalanche danger should approach High in the southern half of the forecast zone. Avoid all avalanche terrain where you observe heavy snowfall, intense wind transport, widespread signs of instability, or natural avalanche activity.  

Problemi valanghivi

  • Wind Slabs

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

    An additional 6-12" of new snow is forecast to fall over the next 24 hours. This will add to the already impressive amount of cold, light snow on the ground, waiting to get strewn about by the wind. With a more westerly wind direction, there may be big differences in conditions between different parts of the zone. Some areas could see sustained strong winds drifting snow at even low elevations. Others may remain quite calm.

    Blowing and drifting snow should show exactly which steep slopes are loading up. If it stops blowing but keeps snowing, signs of wind transport could become obscured. Track differences in snow stiffness as you gain elevation and change aspects to help pin down where fresh wind slabs might be hiding. Also consider the fact that a smaller wind slab avalanche could trigger a larger one that fails in a deeper layer (see below).

     

    (2/24/2022) Evidence of wind transported snow can help anticipate possible wind slabs. Photo: Dallas Glass

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    2–3

    Persistent slab avalanche problems are tricky to pin down on the West Slope of the Cascades. When is there just a "layer" of persistent grains that causes a few avalanches and then goes away versus when is it truly a "problem?"

    At this point natural, human, and explosive-triggered avalanches - all suspected of failing at the crust-facet interface buried 12/30 - have been reported from around West South starting on Saturday, Jan 6th.  Other noteworthy observations have been wide propagation, and that generally light loads triggered large avalanches.  The nail in the coffin is when avalanche workers say things like "I have never seen this before" or "The new snow layers are stabilizing, but I just don't feel good about big slopes, especially unsupported ones."

    Weather conditions on Thursday will likely restrict travel in the kind of terrain where you could trigger one of these avalanches. On the other hand, locations that see the most new snow and strongest winds could see natural avalanche activity in big terrain overhead. Finally, calmer conditions heading into the weekend will open the door to the bigger, steeper, and unsupported terrain. Use caution if you are considering bigger slopes in the coming days. Get a handle on snowpack structure and recent avalanche activity before considering more complex terrain.  

    (1/10/2024)

    Crown of an explosive-triggered avalanche at Crystal. The weak layer was the crust-facet interface buried on 12/30.

    Photo: Crystal Mountain Ski Patrol

Avalanche Discussion

Avalanche size and consequence are bigger factors than likelihood when it comes to danger on Thursday. While the forecast snowfall amounts and wind speeds might not normally be enough to drive High danger, continual snowfall adding more and more load to established weak layers is something that can't be ignored.

Location is also a big factor in terms of the actual danger you encounter. If you are further north in the zone, you may find conditions barely meeting the criteria for Considerable. If farther south, say at St. Helens, you might find any backcountry travel a risky proposition. Wherever you end up in the mountains, start with a conservative plan in simple terrain. Get a handle on weather inputs, snowpack structure, and avalanche activity before considering anything more complex.

Regioni coperte