Bollettino valanghe

West Slopes North

Northwest Avalanche Center
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Pericolo valanghe

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

In sintesi

Although the avalanche danger rating eased, conditions still demand a very cautious mindset. The likelihood of triggering a deep persistent slab avalanche may have decreased, but the consequences remain high. Steer around steep open slopes at upper elevations, especially those with evidence of recent wind loading. 

Problemi valanghivi

  • Wind Slabs

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

    Variable winds may blow snow and build fresh slabs at upper elevations over the next 24 hours. Plenty of cold, low-density snow is already available for transport, and another 2-4in of new snow looks to accumulate by Saturday afternoon. Triggering an avalanche will be most likely on slopes 35 degrees and steeper at upper elevations with evidence of wind drifted snow. Avoid steep slopes below cornices and look and feel for sudden changes in snow surface texture and hardness. Watch for shooting cracks, collapses, and recent avalanches as well as signs of active loading like blowing snow. Fresh slabs Saturday may be softer and more subtle than your average firm and hollow-sounding wind drifts. In areas protected from the wind, small loose dry sluffs may also occur. Wind slab and loose dry avalanches could catch and carry you downslope and could even trigger a deeper weak layer in the snowpack, resulting in a very large avalanche. Seek out lower angled or less wind-affected terrain to reduce your risk.

    (2/1/2021) Example photo of shooting cracks on a small wind loaded test slope.

  • Deep Persistent Slabs

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

    It may still be possible to trigger a very large avalanche on a persistent weak layer buried 4-6 feet deep. The layer is slowly healing and becoming more difficult to trigger with time.  With such a deeply buried weak layer, it is unclear whether the weight of a person would cause an avalanche. However, large triggers like cornice fall or a snowmobile are more likely to trigger it at that depth. If an avalanche does occur, it will be very large and likely unsurvivable. 

    For this avalanche problem, there are no obvious signs of instability like you might find with other problems. You need to dig down to the mid-January crust and make note of the layer above it, observing the presence of persistent grains and how readily that layer fails. Broad open slopes and large features in areas steeper than 35 degrees are where you are most likely to find the weakness, so avoid those slopes unless you have investigated the strength of the weak layer. If you must get on steep slopes, your best option is to travel in areas where avalanches on this layer already occurred. 

    (2/11/2021) Natural slab avalanches that ran in early February along the east side of the 3 Fingers Mountain area in the West Central Zone.

Avalanche Discussion

It's been nine days since the last reported avalanche failed on the mid-January persistent weak layer in the West North Zone. Snowpack tests targeting this layer continue to show more favorable results, but a concerning snowpack structure and uncertainty both linger. Additional time is needed for this layer to heal. The current situation presents a low likelihood/ high consequence scenario and demands a very cautious mindset. Accurately assessing this layer with one snow pit or a few stability tests is nearly impossible and each observation must be placed within the context of a bigger picture of conditions. Build a wide margin for error and carefully consider the consequences of your terrain selection. Utilize good travel practices like riding one at a time and always keep eyes on your partners. In addition to a complex snowpack, cold temperatures this weekend will increase the consequences of any incident, avalanche-related or otherwise. 

Unfortunately, a snow biker triggered an avalanche on the deep persistent layer in the East Central zone on Monday and was buried and killed. For more details and information, see the Accident Reports page and the post-incident investigation video below. 

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.

Regioni coperte