Boletín de aludes

Lutak

Haines Avalanche Center
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Peligro de aludes

Alta montaña
2 · Moderado
Límite del bosque
2 · Moderado
Bajo el límite
2 · Moderado
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Desactualizado: este boletín caducó 3 years ago. Estás viendo condiciones pasadas, no el boletín vigente.

Lo esencial

The avalanche danger is MODERATE on all aspects and elevations.  Slopes greater than 30 degrees could be difficut to trigger but have unexpectedly wide propagation with high consequences. Use visual clues to look for loading and avoid terrain such as starting zones, run outs, and terrain traps to increase your safety margins.

 

Problemas de aludes

  • Persistent Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Unlikely
    Tamaño
    1–3

    You are mostly likely to trigger a slide where there has been previous wind loading and a dense slab formed. Last week's north-northwest winds buried a combination of weak surface snow, or near-surface facets and surface hoar that formed Jan 6-7th. Cold temperatures made things worse by driving the weakening of snow grains, known as faceting.  Faceting has accelerated around persistent weak layers AND deeper into the pack snow around melt-freeze layers. 

    Persistent problems are tricky and may not provide obvious red flags- and when they fail, they produce destructive avalanches with wide propagation. With fresh snow luring us in, they are not a problem to try and outsmart. Slopes greater than 30 degrees have the potential to propagate an avalanche from a thin area of snow to a deeper one that fails on buried weak layers.  Talk to your partners about what terrain you should avoid and where you are most likely to initiate a slide from. Travel one at a time to limit exposure to slopes on the way up and down.  Snow that is getting denser and more consolidated is a clue that you are venturing out on a slab, also shooting cracks and collapsing/whumphing are indicators too.

    Avoid hazards such as terrain traps like rocks, cliffs, trees, gullies and road cuts that could increase the consequences of being caught in a slide, or where snow could pile up quickly. With multiple weak interfaces,  the possibility of an avalanche stepping down from one layer into another is a reason to use wider safety margins at this time.

    This surface hoar grew during the calm, clear and cold weatherJan 6-7th. It was buried Jan 8th by 1-3" of snow. Any slab over this and other layers near the surface could produce wider than expected propagation.

Avalanche Discussion

The intensity of faceting last week produced a highly unstable sliding surface in the upper snowpack depending on the next loading event (wind, snowfall, warming, rain, etc.).

Freezing cold temperatures are accelerated the weakening of snow grains throughout the snowpack: at the surface, below wind slabs, and above and below melt-freeze crusts. We should treat these layers as guilty until proven innocent and continue to monitor, track, and assess stability on a slope-by-slope basis. Melt-freeze crusts are a dangerous weakness in a snowpack and have played a role in multiple avalanche accidents, even within our forecast zones.

What we know about the EOY crust is that above freezing temperatures at all elevations on December 30th caused the formation of a thick ( 1" +/-), widespread melt-freeze crust down 20-50cm. The strength of this layer and it's thick and thin variability is dependent on duration and intensity of the warm and inverted EOY event. Very weak faceted grains have been observed around the EOY crust, and the cold is making them worse. 

Other layers to note are from earlier this season include a few melt-freeze layers that are present throughout the snowpack formed during other warming, or rain on-snow events. Recent observations have confirmed a noteworthy December 24th Christmas Eve crust located down 60-90cm from when freezing temperatures reached ~3,000ft.

The wide propagation and dangerous nature of persistent slabs are best mitigated with safe travel techniques and terrain management- only exposing one person to a slope at a time, avoid grouping up in run-out zones and re-consider islands of safety to truly limit your group's exposure. The people you go out with, the way you communicate, and the way you practice risk reduction as a team throughout the day is equally important as the conditions.

Help us gather information about weather, snowpack, and avalanche activity by reading and submitting field observations. Useful observations can be anonymous and include: 

  • Natural or human-triggered avalanche activity
  • Signs of instability: Cracking, collapsing, or whumphing
  • Red Flag Weather: Precipitation, wind loading, warming temperatures and rain-on-snow
  • Surface hoar growth, snow depth, persistent weak layers, etc
  • Snow stability tests, or pit result and location, elevation, aspect, etc 

Were you caught, carried, injured or buried in a slide? We are all human and can benefit from lessons learned, please report incidents to Haines Avalanche Center staff confidentially.

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