Bollettino valanghe

East Slopes Central

Northwest Avalanche Center
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Pericolo valanghe

Fri
Alta quota
4 · Forte
Limite del bosco
4 · Forte
Sotto il limite
3 · Marcato
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In sintesi

Very dangerous avalanche conditions will develop as a warm storm impacts the area. Generally avoid avalanche terrain on Thursday, especially in areas where persistent slab avalanches have been triggered over the last few days.   

Problemi valanghivi

  • Storm Slabs

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

    While there's a fair bit of uncertainty in the weather forecast tomorrow, the key ingredients for storm slabs are there - increasing snow and wind along with rising temperatures and snow levels.  While storm slabs may break in the new snow or on the old snow surface, what we're really concerned is about storm slabs stepping down to deeper weaker layers in the snowpack outlined below in the persistent slab problem.  Thursday is a good day to avoid avalanche terrain, especially near and above treeline where overlapping avalanche problems will create very dangerous avalanche conditions. 

    (12/20/2022)

    If you're seeing natural storm slabs like this below treeline, you already know conditions are touchy.

    Photo: Alexa Cathcart

  • Persistent Slabs

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

    Heading into this storm we had already had several consecutive days of people triggering persistent slab avalanches on weak layers buried 1-2' down in the snowpack.  These layers were most reactive in the near treeline band and especially in the Wenatchee mountains. Now we're going to up the ante and stress these buried weak layers with new dense snow and wind. 

    We know persistent slabs can break more widely than expected and around terrain features.  Thursday is not the day to mess with this avalanche problem (and likely not Friday either).   Let the storm do its thing and stick to simple lower angled slopes that are not connected to large avalanche paths. 

    (1/8/2023)

    Very wide shooting crack on Tronsen Head at Blewett Pass. This is typical of a persistent slab problem.

    Photo: PKS

  • Loose Wet

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

    As temperatures warm and snow changes to rain at lower elevations, human and natural wet loose avalanches will become more likely.  The travel advice is simple; if you see natural wet loose avalanches or find yourself sinking into wet surface snow more than a few inches, avoid steep slopes.  Small wet loose avalanches may gouge down to older weaker wet snow, becoming larger and more powerful.  Use extra care near terrain traps like cliffs, rocks, and other hazards that would amplify the consquences of a small avalanche. 

    As temperatures rise, you may see natural wet loose avalanche release on Thursday. 

    Photo: Public Observation

Avalanche Discussion

Over the last few days, the majority of persistent slab avalanches have occurred in the 5,000-6,000ft range and on N-E aspects. However, avalanches have also been triggered on WSW aspects, and instabilities found as low as 4,400ft.  Prior to this storm cycle, sSlabs could be anywhere from 1-2ft deep, or deeper in wind drifted terrain. 

The most recent known avalanche on the January 4th layer of buried surface hoar occurred on Tuesday in the Wenatchee Mountains on an NE aspect at 5500 ft.  This avalanche was small (D1), but slabs depths were approximately 1’ deep and clearly failed on a bed surface of feathery surface hoar.  Additionally, observers continue to find these weak layers scattered through the terrain and producing planar results and full propagation in snowpack tests. (Mission / Blewett Pass). 

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