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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, December 24th, 2015 - 7:00AM
Expires
Fri, December 25th, 2015 - 7:00AM
Forecaster
Heather Thamm
Avalanche risk The Bottom Line

A MODERATE avalanche danger exists above treeline and in the upper elevations of Turnagain Pass where triggering a slab 2-4′ thick is possible in steep terrain. The Southern side of Turnagain Pass and periphery areas (Johnson Pass, Lynx Creek, Summit Lake) are particularly suspect due to a shallower snowpack. This is a low probability but a high consequence situation and requires evaluating the snow and terrain carefully. Likely trigger spots will be in shallow areas near rocks and on unsupported convex rollovers. Choose terrain wisely, do not put multiple snowmachines or skier/riders on or under steep slopes all at once, and watch for shooting cracks and recent avalanches.

*Should today’s winds exceed the forecasted 10-15mph be on the lookout for newly forming windslab and adjust to changing conditions.

Below treeline the danger is  LOW  where there has been less snow and less wind.    

Special Announcements

The Chugach National Forest has recently opened Johnson Pass trailheads (North and South) to motorized use.  Please see the “Riding Areas” table at the bottom of this page for current info on areas open to snowmachines.

Thu, December 24th, 2015
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
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 weak snow that formed in early December sitting over the Thanksgiving Rain Crust or old hard snow is now buried under 2-4′ of snow. This combination is our current cause for concern and heightened caution in the backcountry. Many large avalanches ran during the storm Saturday including a large D4 avalanche in Silvertip drainage

Yesterday several small skier triggered slab avalanches were reported near Pastoral above 3500’ on multiple aspects.  Two of these slabs may have been remotely triggered, a symptom of a persistent weak layer. No information is known about the weak layer/bed surface that caused these slides, but it is a good reminder that poor structure does exist within the top 2-4’ of the snowpack. This avalanche problem seems to be more reactive on the Southern side of Turnagain Pass and aligns with reports of snowmachine triggered slides in Johnson Pass and Lynx Creek two days ago.

Many people have been getting into steep terrain without issue, but these conditions warrant respect. Triggering a slab 2-4’ is possible in steep terrain and will be more likely in thinner areas near rocks and on unsupported convex features. Choose your terrain wisely and avoid slopes with high consequences. 

An avalanche that occured at the end of the Dec.19th storm had a long connected crown, just under 2 miles long on a ENE aspect of Twin Peaks ridge above Silvertip Creek. 

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

Multiple days of cold temperatures have been decomposing the top 8-10” of snow creating very weak surface snow. Yesterday a fast moving loose snow avalanche ‘sluff’ caused a skier to be knocked down and drug into an alder patch. Although it is unlikely that this avalanche problem will bury a person, it is capable of causing serious injury. The best way to manage a loose snow avalanche is by choosing terrain with low consequences and allowing the ‘sluff’ to pass by you while descending.

*Looking forward into the weekend a large storm is expected to arrive tomorrow morning and the current surface snow conditions (near surface facets and widespread surface hoar growth) will be a future layer of concern once burried.

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

Yesterday 15-30mph Westerly gusts were recorded at various ridgetop weather stations and several groups observed drifting snow along ridgetops. Although this wind appeared to be short lived, it did move some snow around in the upper elevations.  Be on the lookout for the following:

Cornices: Recent wind loading can make cornices extra tender. A cornice fall (like the photo below) could be a big enough force to awaken a persistent slab. Give these features plenty of space and be careful not to accidently drive your snowmachine out onto them or have your ski lunch break in the wrong spot. 

Windslabs: Should you see snow being transported by winds, this is a obvious sign that wind slabs are forming on leeward features. Pay attention for stiff, pillowed, sculpted snow and look for shooting cracks. 

A fresh looking slab was reported yesterday in Superbowl between Magnum and Cornbiscuit. The exact trigger and timing are unknown, but appear to have been triggered by a cornice. If anyone has information about this recent slab/cornice failure please submit an observation HERE

Weather
Thu, December 24th, 2015

Yesterday skies were clear and temperatures ranged from 5F to 15F. Westerly ridgetop winds averaged around 5-10mph and several gusts were recorded in 30’s mph on Seattle ridge. Overnight winds were light from the West and temps remained cold.

Today skies are expected to be clear and temps should stay in the teens F and start increases later in the day. Light Westerly winds will become Easterly and increase to moderate by the evening.

A Winter Storm Watch has been issued by the National Weather Service for Friday morning into Friday evening. Tomorrow heavy snow and moderate winds are expected on the Kenai Peninsula and could bring another 1-2′ of snow to the area. Temperatures are expected to warm up and the rain/snow line could reach as high as 1500′.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 13F   0   0   51  
Summit Lake (1400′) 7F   0 0   17  
Alyeska Mid (1700′) 13F   0   0   36  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 9F   WNW   5   23  
Seattle Ridge (2400′) 12F   WNW   8   35  
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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.