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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Wed, December 28th, 2016 - 7:00AM
Expires
Thu, December 29th, 2016 - 7:00AM
Forecaster
Heather Thamm
The Bottom Line

The avalanche danger is CONSIDERABLE in the alpine and treeline elevations where triggering an avalanche 2-3′ thick is likely to have high consequences. An avalanche this size also has the potential to step down into a deeper layer of the snowpack in the upper elevations. Avoid being on or under steep slopes and pay attention for other groups traveling near you. Be aware of any changing conditions like increasing winds, which will make triggering this avalanche problem more likely today.  

 A LOW avalanche danger exists below 1000′ where the snowpack has a firm crust and triggering an avalanche is unlikely.  

 Check out the Summit Lake Summary  HERE.  

 

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Wed, December 28th, 2016
Alpine
Above 2,500'
Considerable (3)
Avalanche risk
Treeline
1,000'-2,500'
Considerable (3)
Avalanche risk
Below Treeline
Below 1,000'
Low (1)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
Considerable (3)
Treeline
1,000'-2,500'
Avalanche risk
Considerable (3)
Below Treeline
Below 1,000'
Avalanche risk
Low (1)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
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

Great work everyone! No human triggered avalanches were observed yesterday. In general folks were keeping slope choices mellow on both sides of the road. The Seattle Ridge up-track was fully established and once riders ascended the ridge most people were sticking to low angle terrain. Big props to all those using radios for communication to minimize exposure both on the up-track and when stuck. This is a great tool for maintaining good communication with your partners and practicing safe travel habits in avalanche terrain. 

With that said we will encourage more patience and avoidance of large steep slopes. Yesterday observers were getting consistent test results on a widespread layer of buried surface hoar 2.5’ deep in a variety of locations and elevations in Turnagain Pass. Also noticible yesterday was how much the slab has settled and gained strength. This means it will be harder to trigger the weak layer, (especially for a skier or snowboarder) but this also means there is more potential for an avalanche to release once well out onto a slope. The most likely trigger spots will be in thinner areas of the snowpack near ridges and in rocky areas. A snowmachine is also more likely to trigger this weak layer due to its sheer weight and ability to penetrate deeper into the snowpack.  This particular avalanche problem, a Persistent Slab, can take awhile to heal. A slab 2-3’ is likely to be large and connected and bigger terrain will have much higher consequences. 

Just because someone has ridden/skied a steep slope does not deem it ‘safe.’  It is not out of the question for the 2nd, 3rd or 5th person to find just the right ‘trigger spot’ or even trigger a larger avalanche in a deeper layer layer of the snowpack. More on this in Secondary Concerns.

In addition, should winds pick up, it could increase the likelihood for triggering a persistent slab. Practice safe travel habits and be on the look out for blowing snow, shooting cracks or ‘wumpfing’ – these are obvious signs the snowpack is unstable.

 

 

Buried surface hoar is easy to indentify in pits right now. Here is what it looks like on the bottom of the slab. Photo by Mik Dalpes

 

A view of the Seattle Ridge up-track as seen from Tincan yesterday. Avoid skiing/riding on the upper part of Repeat Offender above the up-track to help minimize exposure to anyone below.

 

 

 

 

Avalanche Problem 2
  • Deep Persistent Slabs
    Deep Persistent Slabs
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.
More info at Avalanche.org

Our current snowpack structure is complex. Multiple weak layers exist within the entire snowpack including weak faceted snow near the ground and three separate layers of buried surface hoar. This avalanche problem may be lurking in the upper elevations above 2500’, where the total snowpack is closer to 6’. The likelihood of triggering this type of avalanche is low, but the consequence could be destructive. The places where triggering a deeper layer will be in shallow areas of the snowpack near rocks or very scoured areas. This is one more reason to keep your slope angles in check and avoid being on or under large slopes. Click HERE for an excellent write up from CAIC center published yesterday that describes more about the risks of Deep Slab avalanches.

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. Be extra cautious near ridgelines today where cornices can be challenging to navigate.  If you suspect a corniced ridge give these a wide berth and be aware of people below you.

Cornice triggered slab avalanche in Warm-up bowl that occured mid to late storm last weekend. Photo by Wendy Wagner

Weather
Wed, December 28th, 2016

Yesterday visibility was in/out for portions of the day with light snow flurries. Less than an inch of snow was recorded. Temperatures dropped into the low to mid 20F’s and winds were light from North.  

Today expect this pattern to continue along with light snow showers. Not much accumulation is expected. Temperatures should dip into the teens F. Ridgetop winds are expected to be light from the Northwest, but may increase this evening, 10-25mph from the Northwest.  

A high pressure system appears to be moving into our region by the end of the week bringing clear skies and cold temperatures.

*** Sunburst Wx station is temporarily down and will be fixed upon safe access for a repair. Rime was cleaned off of the  Seattle Ridge anenometer  yesterday afternoon, and wind data from 1pm to 11pm was available.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 22    trace .1    46
Summit Lake (1400′) 23   0   0   11  
Alyeska Mid (1700′) 24   trace   .06   27  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) ***n/a   ***n/a     ***n/a     ***n/a    
Seattle Ridge (2400′) 19   ***NW   ***2   ***8  
Observations
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Riding Areas
Updated Wed, December 11th, 2019

Status of riding areas across the Chugach NF is managed by the Glacier and Seward Ranger Districts, not avalanche center staff. Riding area information is posted as a public service to our users and updated based on snow depth and snow density to prevent resource damage at trailhead locations. Riding area questions contact: mailroom_r10_chugach@fs.fed.us

Area Status Weather & Riding Conditions
Glacier District
Johnson Pass
Closed
Closed.
Placer River
Closed
Closed.
Skookum Drainage
Closed
Closed.
Turnagain Pass
Closed
Closed.
Twentymile
Closed
Closed.
Seward District
Carter Lake
Closed
Closed.
Lost Lake Trail
Closed
Closed.
Primrose Trail
Closed
Closed.
Resurrection Pass Trail
Closed
Closed. Will be open for the 2019/20 season pending adequate snow cover.
Snug Harbor
Closed
Closed.
South Fork Snow River Corridor
Closed
Closed.
Summit Lake
Closed
Closed.

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