Bollettino valanghe

East Slopes Central

Northwest Avalanche Center
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Pericolo valanghe

Sun
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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Non aggiornato — questo bollettino è scaduto 6 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Expect dangerous conditions at upper elevations with actively forming wind slabs over slick surfaces. Stay off of slopes steeper than 35 degrees if you find more than 8" of new snow and signs of instability such as shooting cracks or whumphing collapses. Wind slabs will be thickest in upper elevation terrain and in areas open to wind exposure. Very large persistent slab avalanches are still possible on these same high elevation slopes.

Problemi valanghivi

  • Wind Slabs

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

    Some 5-10" of new snow is expected with the storm into Saturday afternoon. Strong winds with cool temps will create reactive windslabs over the most recent (12/21) melt freeze crust. This new snow is stacking up over a variety of weak surfaces including the slick crust, surface hoar, near surface facets and wind broken particles. Freshly formed wind slabs could be surprisingly reactive, and may only need a persons weight to trigger one. Look for blowing snow, and instability such as shooting cracks. Use test slopes to check how well the new snow is bonding. Avoid steep open slopes if you find signs of instability or fresh avalanches. Slabs will be deeper and widest in higher elevation, wind exposed terrain, and near ridgelines. You can avoid dangerous slopes by seeking out sheltered slopes and terrain less than 35 degrees in angle.  

    (12/25/2020) Blowing snow can help show you which aspects may harbor wind slabs.

  • Persistent Slabs

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

    Persistent slab avalanches could be very large and could bury or kill you. These dangerous and surprising avalanches can catch even the most experienced backcountry travelers. The added load of new snow and wind drifting may make triggering these deeper slides possible. With uncertainty about how easily you can trigger these avalanches, it's important to give them some time to heal. The best way to stay safe is to avoid where these avalanches exist. Keep your distance from slopes 35 degrees and steeper that are open enough and tall enough to produce large avalanches. Watch for obvious signs of instability like recent avalanches, whumphing collapses, or cracks shooting through the snow.

    Persistent slabs are more likely closer to the Cascade crest, in the western and northern portions of the zone. If you'll be traveling there, check out the neighboring Steven's Pass and East North zones. The most dangerous slopes will be at the highest elevations, where snow stayed mostly dry and where southwest to west wind drifted snow over the past few days. You can find a thin weak layer of small facets (and in places surface hoar) above a crust buried about 1.5-3 feet below the surface. Further east, and in the Wenatchee mountains, there's less snow over this layer. 

    (12/23/2020) A crown of an avalanche that ran between Dec 19-20th on a NE aspect near 7,000ft in the Alpine Lakes Wilderness.

Avalanche Discussion

We've issued a Special Avalanche Bulletin for the neighboring East North zone due to lingering persistent slab instabilities. Expect light to moderate snow with strong southwest winds on Saturday. Underneath the new snow, the recent crust could still make for some slick and challenging travel.  More snow will fall near the crest in areas such as the Salmon la Sac and Icicle drainages. There is uncertainty about how the new snow will bond to existing surfaces, but chances are it won't bond well initially. 

Observers have confirmed that a widespread cycle of large to very large natural avalanches occurred last weekend. These avalanches likely involved a crust and weak layer buried in early December. An observer reported a very large slab avalanche (at least D2.5) in Big Slide creek on an ESE aspect at about 7,300ft, near the Icicle drainage. This avalanche likely occurred Dec 19-20th. While observations have been limited, we've received good confirmation from neighboring zones to the north and west. In adjacent zones, a prolonged cycle of widespread avalanches occurred from the 18-21st. There is a lot of variability in snowpack depth just east of the crest where you may find 6ft of snow at 6000ft, but east of Highway 97 in the Wenatchee mountains you may only find a few feet at the same elevation.  

During the ongoing Covid-19 pandemic, continue to follow the latest guidance from local, state, and federal health officials. Avoid unnecessary risks that could impact first responders and stress medical resources. Wear a mask when appropriate, practice social distancing, and consider recreating closer to home.

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