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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Tue, December 27th, 2016 - 7:00AM
Expires
Wed, December 28th, 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 on slopes steeper than 35 degrees. An avalanche this size could be triggered remotely on adjacent slopes and has the potential to step down into a deeper layer of the snowpack. Avoid being on or under steep slopes and pay attention for other groups traveling near you. Cautious route-finding and conservative decision making are essential today.  

A MODERATE avalanche danger exists below 1000′ where an avalanche from above in steep or channeled terrain could run. This will be extra important in areas like Portage Valley.

Check out the Summit Lake Summary  HERE.  

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Tue, December 27th, 2016
Alpine
Above 2,500'
Considerable (3)
Avalanche risk
Treeline
1,000'-2,500'
Considerable (3)
Avalanche risk
Below Treeline
Below 1,000'
Moderate (2)
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
Moderate (2)
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

A three-day storm that ended yesterday morning dumped 2-3 feet of new snow on top of a very weak snowpack throughout Turnagain Pass. Numerous human triggered avalanches (both skier and snowmachine triggered) were observed yesterday directly following this rapid loading event. This also coincided with the first day of snowmachining at Turngain Pass. Most of these avalanches were small and proportionate to the smaller terrain folks were riding/skiing in. One exception was a larger avalanche on the NW shoulder of Sunburst that was likely triggered remotely from the skin track above. 

AVOID BEING ON OR UNDER STEEP SLOPES! As the snowpack settles and the slab becomes more cohesive and supportable, there is increasing potential for more connected avalanches today. Weak faceted snow and buried surface hoar are widespread under the new snow and triggering a slab 2-3’ deep is plenty large enough to bury a person or multiple people. Areas like Repeat Offender are suspect and with busy holiday crowds anticipated, it will be difficult to practice safe travel habits like only exposing one person at a time.  We recommend giving the snowpack more time to heal-up before venturing near steep terrain.

Today it will be extra important to keep our natural human tendencies in check and avoid the lure of avalanche terrain. 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.

Do not ignore obvious signs like recent avalanches, ’whumpfing” sounds or shooting cracks. Remember that MOST AVALANCHE FATALITIES HAPPEN WHEN THE DANGER RATING IS CONSIDERABLE.

An avalanche that occured yesterday on the NW shoulder of Sunburst at about 2500′. This occured sometime after 11am and is suspected to be remotely triggered from the skin track that was put in on the ridge above. 

One of several 2′ slabs triggered yesterday in the Tincan trees below 2300′. Photo compliments of Tully LaBelle-Hamer

 

Yesterday touchy avalanche conditions were obsevered in the mellow terrain below Seattle Ridge.  Photo by Nick Olzenak

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 and three separate layers of buried surface hoar. There is much uncertainty about the upper elevations where the total snowpack is closer to 6’ thick. It will be in this elevation band where trigging a deeper slab, 3’ or thicker will have very high consequences. 

Three very distinct layers of buried surface hoar can be seen in this photo taken yesterday by Jesse Haffener and Sam Galoob. This observation was made yesterday in the Tincan trees, where the snowpack is more uniform. In the alpine where snow depths vary these layers have been reactive in many test pits over the last few weeks. 

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.

Weather
Tue, December 27th, 2016

Yesterday morning a storm ended leaving 30″ of snow at Turnagain Pass over a three day period (2.8″ of snow water equivelent.) After 6am yesterday only brief snow flurries were observed, but visibility cleared and winds were light from the East. Temperatures remained around 32F at sea level and high 20F’s in the upper elevations.  

Overnight temperatures dropped below freezing in the lower elvations. Winds were calm and only a trace of new precipitation was recorded.  

Today expect temperature to remain the 20F’s with light Easterly ridgetop winds. Visibliity may be obscured at times with the possiblity of scattered snow showers. Not much accumulation is expected.  

*** Sunburst Wx station is temporarily down and will be fixed upon safe access for a repair. Seattle Ridge anenometer is likely covered in rime and currently not working.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 28   1   .1   47  
Summit Lake (1400′) 28   0    0 11  
Alyeska Mid (1700′)  30 1   .05   29  

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′) 25F   ***n/a   n/a   n/a  
Observations
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Riding Areas
Updated Mon, December 02nd, 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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