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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, December 21st, 2016 - 7:00AM
Expires
Thu, December 22nd, 2016 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

Today the avalanche danger is  MODERATE  in the Alpine where trigging a slab up to 2′ thick is still possible on steep wind-loaded terrain features. Evaluate snow and terrain carefully and manage your terrain choices with safe travel protocols. At Treeline and below the avalanche danger is LOW.  Remember to avoid terrain traps where an avalanche of any size could have high consequences.

If you are headed to Summit Lake check out the Saturday Summary and recent Summit observations.  

Special Announcements

***Carter Lake and Snug Harbor areas are now open to motorized use as of yesterday, Tuesday Dec. 20th. Please respect other closures across the Forest. The Forest Service is monitoring conditions daily and will open more areas just as soon as there is enough snow to prevent resource damage to underlying vegetation. Thanks for your patience!  

OUTSIDE AREAS:  Recent snowfall has created dangerous avalanche conditions around Southcentral and Interior, Alaska. Keep this in mind if you are headed to Snug Harbor and Carter today and please send in an  observation  if you see any obvious signs of instability. Check out these avalanche observations from  Hatcher Pass  and  Petersville  region  where recent avalanche activity was observed over the weekend.

Wed, December 21st, 2016
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
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. 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

The touchy snowpack was that was observed over the weekend has calmed down as the new snow has settled and the snowpack is showing signs that it is adjusting. However, it is still very important to remember that the 12”-14” of new snow that accumulated incrementally fell onto suspect weak layers. A widespread layer of weak faceted snow remains our primary layer of concern and was the culprit in many skier triggered avalanches over the weekend. Monday, an observation from Sunburst showed propagation potential on a layer of buried surface hoar (buried Dec.15th) which is sitting on a wind hardened bed surface along some ridgelines. Slopes that were wind loaded due to the storm snow arriving with moderate to strong Easterly ridgetop winds have the most potential to be hazardous as the slabs are deeper and stiffer.  Watch out for areas with hard over soft snow, wind pillows or drifts and be on the look out for obvious signs like cracking and whumpfing sounds. Steep unsupported slopes and areas with stiffer connected slab over the facets or buried surface hoar should be avoided. Triggering a dangerous slab avalanche is still possible with our current snowpack set-up.  

Snow pit from Monday on Sunburst showing the December 15th layer of buried surface hoar. Photo: Heather Thamm

Avalanche Problem 2
  • Dry Loose
    Dry Loose
Dry Loose
Dry Loose avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.
More info at Avalanche.org

“Sluff” may also be fast moving on steep terrain features where the snow is loose and unconsolidated.  Be aware of terrain features that could have high consequences if knocked off your feet.

Additional Concern
  • Cornice
    Cornice
Cornice
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
More info at Avalanche.org

Expect cornices to be sensitive and easy to break off. They also could trigger a slab avalanche below. If you choose to walk a ridgeline today, give these a wide berth and be aware of people below you.

 

 

 

A small piece of cornice fall pulled out the surface snow below yesterday in Zero Bowl. 

 

 

Weather
Wed, December 21st, 2016

Yesterday was partly cloudy with afternoon sunshine. Temperatures were in the high teens and low 20Fs and winds were calm. Overnight temperatures and winds were similar.

Today and tonight will be mostly cloudy with a chance of snow. Winds will be light and northerly and temperatures will be in the 20Fs during the day dropping into the low teens/single digits overnight.  

There is cooling and clearing trend for the remainder of the week with a chance for snow returning for the Holiday weekend.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 22    0  0  28
Summit Lake (1400′) 10  0  0 9  
Alyeska Mid (1700′)  24  0  0  17

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  16 ENE   7   15  
Seattle Ridge (2400′)  21  SE  3 6  
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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.