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Sat, December 20th, 2014 - 7:00AM
Sun, December 21st, 2014 - 7:00AM
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger remains  CONSIDERABLE at elevations above 2,500′ on all aspects. At these above treeline elevations, human triggered slab avalanches 2-4′ in depth continue to be likely on slopes over 35 degrees. At the treeline elevations there is a MODERATE danger where it is possible to trigger a slab avalanche in the 2,000′ – 2,500′ zone. Below 2,000′ triggering an avalanche will be unlikely.

If skies clear enough for travel above treeline today, we recommend to keep close tabs on your powder fever. We are approaching a period of lower probability of triggering an avalanche but continued high consequences. Choosing terrain wisely will be essential. This means avoiding slopes over 35 degrees, steep rollovers and thin snow cover areas below or adjacent to steeper slopes.

Sat, December 20th, 2014
Above 2,500'
3 - Considerable
Avalanche risk
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
0 - No Rating
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. Dangerous avalanche conditions. 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

It was a relatively quiet day in the backcountry yesterday; one day after an eventful Thursday where two large human triggered avalanches occurred with one resulting in a full burial – details on those HERE and HERE. One reason for the quiet day was possibly the low visibility hampering travel too far from treeline and the other possibly, the one we hope, is that people are sticking to lower angle slopes due to the poor snowpack stability. 

Today marks the fourth day since a 3 day storm brought 24-40+” to Turnagain Pass and surrounding zones. Buried under the storm snow is a widespread and intact layer of buried surface hoar that is responsible for the recent avalanche activity. We have been tracking this buried persistent weak layer and it is showing signs of becoming stronger (hence, harder to trigger an avalanche). HOWEVER, we have only been able to access very few areas. These are the same areas folks have been getting into – namely low angle slopes on the Tincan and Sunburst ridges. In fact, our staff deemed the risk too high to investigate the Sunburst avalanche yesterday becuase of the required exposure to steep slope angles. This leaves a lot of unknowns for when skies clear and travel is possible to further reaches of the forecast zone. All signs point to conservative decision making. What we do know is that the buried surface hoar exists to the ridgetops and is covered with a slab that ranges widely: from 4-5′ in the Girdwood Valley to 2-4′ in the Turnagain Pass zone and 1-2′ further south in the Summit Lake area.

For today, continued patience is recommended. Enjoying quality snow conditions can be had by sticking to low angle slopes, avoiding steep rollovers and staying away from the steeper terrain. Click this hyperlink for a definition of a “Persistent Slab” and the Travel Advice associated with it.


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

The winds kicked up into the 15-20mph range from the North and East yesterday. With plenty of loose snow available for transport, soft wind slabs and drifts formed on leeward aspects. This concern is trumped by the primary concern above, yet is worth noting for two reasons. One, it adds additional load to the slab resting on the buried surface hoar. And two, wind slabs may be found below 2,500′ where triggering a larger slab is less likely.

Sat, December 20th, 2014

Yesterday brought intermittent snow showers and partly cloudy skies. Around 1″ snow accumulated with light to moderate winds from the East (details in the charts below). Temperatures were mild, yet cold enough for snow to ~500.  

For today, we should see partly sunny skies and intermittent cloud cover as a series of low pressure systems spin to our South. Since we are above the action, ridgetop winds are expected to be light from the East and temperatures mild (mid 30’s F at 1,000′ and the low 20’s to 30F on the ridgetops).

The exciting news is for Sunday and into Monday where a good chance for snow region-wide is developing! Stay tuned, but as of now the models are pointing toward a low pressure system over the Kenai with just enough associated dynamics to push moist air over the Kenai and Chugach mountains and into Anchorage and other areas of mainland Southcentral. Temperatures will be cold enough for snow to sea level.

For the early part of this week, a break between storms is likely for Tuesday into Wednesday with another large low pressure moving in for Christmas Day.  

PRECIPITATION 24-hour data (6am – 6am)

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 32   1   0.1   28  
Summit Lake (1400′) 30   0   0   4  
Alyeska Mid (1700′) 31   0   0   18  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 25    E 15   39  
Seattle Ridge (2400′) 26    N/A  N/A N/A  
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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.