Bollettino valanghe

West Slopes South

Northwest Avalanche Center
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Pericolo valanghe

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

A complicated snowpack will need time to adjust to its recent load, so stick to conservative terrain on Sunday. Avoid slopes greater than 35 degrees if you see cracking in the snow or suspect wind-loading. Weak layers in the snowpack are now buried deeply and have the potential to create very large and destructive avalanches.

Problemi valanghivi

  • Storm Slabs

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

    Storm Slabs were sensitive to human triggers on Saturday, with many recreationalists and professionals getting surprised by how easily the storm snow was failing. Slabs were failing easily at all elevation bands and should continue to do so on Sunday. The winds have been blowing from the east and have played a part in creating thicker and possibly more reactive slabs at all elevations, loading some unusual features. These slabs may break in surprising ways, propagate far distances, or even be triggered remotely. Use hand pits, uptrack tests, and test slopes as you travel to determine the reactivity of the new snow. Feel for subtle differences in the snow to help you determine where wind loaded pockets may be buried by freshly fallen snow. Convex rollovers, steep open slopes, and rocky terrain near gullies and cliffs will be the most likely locations for you to trigger an avalanche. If you notice the snow surface cracking while you travel or blocks of snow failing easily in your tests, then avoid slopes over 35 degrees.

    (2/13/2021) This slab failed due to a Dry Loose avalanche which a skier triggered further up the slope. The slide happened on Saturday (2/13) near White Pass.

  • Deep Persistent Slabs

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

    This storm has now buried our persistent weak layers deep enough in the snowpack across the zone that you will not be able to see signs of danger on this layer as you travel. You could travel all day or even several days in a row without seeing any activity on these layers, giving you a false sense of security. We believe that the most likely places for these slides to run are large open terrain features and start zones over 30 degrees, with a higher likelihood as you ascend in elevation. Don’t get lulled into these areas thinking that this problem has gone away. The weak snowpack structure is still in place, but we have some uncertainty around what it would take to trigger it at this point. We do know that any avalanche occurring on these buried weak layers would be very large and destructive.

    (2/6/2021) This slab was intentionally triggered by a small cornice drop on Saturday (2/6) in the Crystal backcountry. It propagated widely and is believed to have run on the Mid-January crust/facet combo. [E aspect, 6300'. Photo: Jeremy Allyn]

Avalanche Discussion

Do not ignore the large snow totals and relatively cold weather we are experiencing in the West South. The cold weather will not help the snowpack to heal as quickly as you may be used to, and persistent weak layers heal even more slowly than standard storm instabilities. Reports indicate that the new snow was not well bonded to the old snow surface on Saturday. Crystal Pro Patrol reported slabs up to 2 feet deep were easily triggered during control work, while White Pass Pro Patrol reported touchy slabs at all elevation bands. A subtle layer of weaker snow buried on 2/11 may be to blame for avalanches being so sensitive in both areas. The wind certainly was also a major player in creating slabs, possibly on aspects or elevations you may not be used to. Recent avalanches are a great red-flag indicator that the snowpack is unstable, and we saw a lot of them on Saturday.

While the new snow instabilities and recent avalanches discussed above are enough on their own to drive Considerable danger (which is the level at which most avalanche accidents occur), persistent weak layers lurking deeper in the snowpack should have you dialing back your terrain choices from what you would normally pursue at Considerable danger. These layers have been the cause of a week’s worth of near-misses and avalanche involvements, including a fatality earlier this week. It’s difficult to predict if or when this new snow will be enough to tip the scales and create very large avalanches on these layers again. Don’t leave it to chance, remain aware of the high consequence slides that are possible on those deeper layers, and dial back your terrain choices. 

With so much recent snow, even a simple fall or getting knocked off your feet near small terrain traps can be consequential. Learn more about snow immersion hazard here.

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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