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Mon, December 17th, 2012 - 7:00AM
Tue, December 18th, 2012 - 7:00AM
Graham Predeger
Avalanche risk The Bottom Line

The avalanche danger today remains MODERATE with human triggered avalanches possible above treeline.   Our snowpack is still comprised of weak faceted snow overlain by a stiff wind slab at the higher elevations.   These weak facets are still showing reactive to human travel as recently as yesterday on the southwest face of Sunburst.   Below treeline the danger is LOW due to the lack of a cohesive slab.

Mon, December 17th, 2012
Above 2,500'
2 - Moderate
Avalanche risk
1 - Low
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
Travel Advice Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern. Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches Natural and human-triggered avalanches unlikely. Natural avalanches unlikely; human-triggered avalanches possible. Natural avalanches possible; human-triggered avalanches likely. Natural avalanches likely; human-triggered avalanches very likely. Natural and human-triggered avalanches certain.
Avalanche Size and Distribution Small avalanches in isolated areas or extreme terrain. Small avalanches in specific areas; or large avalanches in isolated areas. Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas. Large avalanches in many areas; or very large avalanches in specific areas. Very large avalanches in many areas.
Avalanche Problem 1
  • Persistent Slabs
    Persistent Slabs
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.
More info at Avalanche.org

The persistent weak layers that make up the lower half of our snowpack are becoming more stubborn to trigger but the very nature of these means they can persist long after any significant wind or precipitation.  All it takes is the right trigger in the wrong spot to unleash a destructive slide.  Keep this in mind as you travel through the mountains today and for the rest of the season, as this weak layer is widespread and likely to be with us for a while.  Below treeline from the ground up, our snowpack is one persistent weak layer (buried surface hoar and facets of varying sizes).  This does not pose a concern until we see significant wind or precipitation at these lower elevations to form an overlying slab.

Avalanche Problem 2
  • Wind Slabs
    Wind Slabs
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
More info at Avalanche.org

This past weekend our snowpack has proven reactive to a skiers weight with several skier triggered wind slabs both on Tin Can and the SW aspect of Sunburst.  These were all triggered between 2700-3000’ in wind-loaded terrain on west and southwest aspects.  I don’t expect this slab to build much in the coming days given there is very little snow available for transport, but a skier or snowmachiner may still be able to trigger small avalanches in specific areas or large avalanches in isolated areas of big terrain.  Continue to travel through the mountains with heightened concern today and do not disregard the red flags the mountains give us.  Recent avalanches and loud whumphing are two you may expect to see today. 

Mon, December 17th, 2012

Temperatures have begun to rise from yesterdays sub-zero lows but it’ll feel anything but warm in the backcountry today as you can expect a brisk north wind in the 17-35mph range for much of the day.   Tomorrow looks to be our only chance to pick up a few flakes as winds become SE and a weak low pressure moves over Kodiak, but don’t expect much as the track appears to be well south of us.   Looking out toward the weekend, models suggest a continuation of cold and dry conditions over southcentral Alaska… Don’t shoot the messenger!

This is a general backcountry avalanche advisory issued for Turnagain Arm with Turnagain Pass as the core advisory area (this advisory does not apply to highways, railroads, or operating ski areas).  

Wendy will issue the next advisory Tuesday morning, December 18th.

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This is a general backcountry avalanche advisory issued for Turnagain Arm with Turnagain Pass as the core advisory area. This advisory does not apply to highways, railroads or operating ski areas.