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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Thu, December 29th, 2016 - 7:00AM
Expires
Fri, December 30th, 2016 - 7:00AM
Forecaster
Aleph Johnston-Bloom
The Bottom Line

The avalanche danger is CONSIDERABLE in the Alpine and human triggered wind slabs are likely on leeward slopes. It will be also possible to trigger a slab 2-3′ thick on older buried weak layers.  

At Treeline in the mid-elevation band (1000′-2500′) the avalanche danger is MODERATE. It is possible to trigger a 2-3′ slab on older buried weak layers and it may slide to the ground.

Overall the snowpack is getting less likely to trigger but the consequences remain high if an avalanche is initiated.

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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Thu, December 29th, 2016
Alpine
Above 2,500'
Considerable (3)
Avalanche risk
Treeline
1,000'-2,500'
Moderate (2)
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
Moderate (2)
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
  • 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

Yesterday a westerly flow moved over area in the early afternoon and winds increased into the 20s gusting into the 40s. This continued through the evening. The way this wind direction shows up in terrain can cause unusual loading patterns. The winds blew from the South on the Sunburst/Tincan side of the road loading N slopes. On Seattle Ridge the winds were from the W/NW and loaded more easterly slopes. These patterns were varied around Girdwood, Portage and Summit Lake as well. There was a few inches of light fluffy new snow that was easily moved as well as soft settling snow from the Xmas storm. Expect fresh wind slab in leeward areas. Look for cracking and pillowed snow. These may be quite tender and reactive. The winds also had the potential to stiffen the slab sitting over older weak layers, add load to the snowpack and make it possible to trigger a deeper avalanche.  

 Wind on Sunburst yesterday.

 

 

 

Avalanche Problem 2
  • 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

Many people have been out enjoying the recent snow with no incident and for the most part avoiding steep slopes. There have been no reported human triggered avalanches since Monday and the natural avalanche cycle associated with the storm also ended that day. As visibility has improved is easy to see the remnants of this cycle. Many areas have had large slides that we suspect ran on the December 24th surface hoar, the December 15th layer of surface hoar or buried facets deeper in the pack. In addition there was a number of mid-elevation band slides that also ran on buried weak layers, some even running on facets just above the ground. The slides at all elevations are mostly 2-4′ thick. There are areas adjacent to those that slid that may still have the potential to go today. It is important to remember thinner areas are often the place that old weak layers are initiated. The snowpack is still complex and it may be harder to initiate but the slides could still be large and deadly. Observers yesterday still reported large whumpfs and shooting cracks. These are red flags and a good indication to stay away from avalanche terrain if encountered. As you choose terrain to ride or ski today think about how large and connected it might be. It may not be the 1st snowmachine rider or skier onto the slope that triggers it. Triggering a deep persistent slab is possible and winds from yesterday may have added stress/load.

Slide on North side of Magnum believed to have run Monday December 26th. Photo: Andy Moderow

The December 24th layer of buried surface hoar visible in the snowpack on Sunburst and still reactive in instability tests.

 

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. Due to the unsusal wind loading yesterday look for new cornices or unsual loading patterns. 

 

 

Sunburst Cornice 12.28.16

Weather
Thu, December 29th, 2016

Yesterday was mostly cloudy with a few light snowflakes falling. Winds were WNW and increased in the afternoon gusting into the 40s.Temperatures were in the low 20Fs and teens. Temperatures dropped to single digits overnight.

Today will mostly cloudy with the snow starting in the late afternoon. Temperatures will rise again into the 20s. Overnight the temperatures at lower elevations may rise into the low 30s and a rain/snow mix is expected with the storm system as it moves into the area. 2-6″ of snow is possible overnight. Winds will be light and Southerly.

Tomorrow snow showers will continue and temperatures will drop back into the low 20s. Winds will be variable as the system passes through. The pattern continues to be unsettled into next week as a series of fronts move off of the Pacific.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 19   0    0  39
Summit Lake (1400′)  15  0  0 10  
Alyeska Mid (1700′)  20  0  0  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′)  14 NW    22 42  
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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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