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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, January 6th, 2018 - 7:00AM
Expires
Sun, January 7th, 2018 - 7:00AM
Forecaster
Heather Thamm
Avalanche risk The Bottom Line

The avalanche danger is  CONSIDERABLE  on all aspects and elevations above 2000′ where triggering a slab avalanches 1-3′ thick is likely today. At elevations above 3,000′ triggering a very large slab avalanche that breaks in weak snow near the ground is also possible.  Cautious route-finding and conservative decision-making are essential. Sticking to low consequence terrain less than 30 degrees is recommended.  

Below 2,000′ the danger is  MODERATE where crusts have formed, and an avalanche releasing from above is still possible. Below 1000′ the danger is LOW where very little snow exists.  

For a summary of avalanche conditions in Summit Lake click HERE.

Special Announcements

Turnagain Pass is now open to motorized use on the Western side of the Seward Highway. A Special Avalanche Bulletin has been issued due to elevated avalanche danger  combined with the anticipated crowds of snowmachine traffic in the Turnagain Pass. Human triggered avalanches are likely today on slopes steeper than 30 degrees in the alpine, above treeline and alder line.  

The Chugach National Forest wants to remind riders to avoid areas with thin snow cover like €œRookie Hill € and the Southern end of Turnagain Pass towards Bertha Creek. Johnson Pass remains closed due to lack of snow. You can keep tabs on the current riding status of other areas of the National Forest at bottom of this page.  

Sat, January 6th, 2018
Alpine
Above 2,500'
3 - Considerable
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. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential. 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
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.
Likelihood of Avalanches
Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.
Size of Avalanches
Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word Size (D scale) Simple Descriptor
Small 1 Unlikely to bury a person
Large 2 Can bury a person
Very Large 3 Can destroy a house
Historic 4 & 5 Can destroy part or all of a village
More info at Avalanche.org

Wind loading from yesterday and today is adding additional stress to an already tenuous snowpack. Several persistent weak layers are sitting under the New Year’s storm that ended on Wednesday and left 1-3 feet of snow across our region (Turnagain Pass and Girdwood.) This storm covered up a widespread layer of surface hoar that has been reactive in stability tests over the last few days. It is unknown how this layer will respond to a bigger trigger like a snowmachine. Only one steep slope has been tested by numerous skiers/boarders, Common Bowl on Tincan. No incidents were reported. Otherwise very limited information exists as to how well the snowpack is adjusting to it’s new load. We also don’t know how intact the surface hoar remains across any given slope. 

Be aware of newly forming wind slabs on leeward features. These slabs are like to be small and isolated, but could have high consequences should someone initiate a wind slab that steps down to a deeper layer of the snowpack. Wind loading is an additional reason we urge folks to use caution and chose low consequence terrain in the alpine, slopes less than 30 degrees in steepness.

Today keep in mind:

  1. Several tracks (snowmachine, ski, or snowboard) may be on a slope before someone triggers an avalanche
  2. Remote triggering from the top, side or bottom of a slope is possible
  3. The larger the slope the larger the avalanche
  4. Obvious signs like “whumpfing” and shooting cracks may or may not be present. 

 *In addition to the buried surface hoar problem, weak faceted snow sits near the ground in avalanche paths that released in early December. Places like the SW face of Sunburst and Seattle Creek Headwall are suspect of this structure. 

 

Moderate Easterly winds seen yesterday afternoon on Pyramid loading Westerly aspects into Seattle Creek side of mountain.

 

 

Propagation potential has been found in stability tests during and after the storm on a widespread layer of surface hoar.

 

Avalanche Problem 2
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.
Likelihood of Avalanches
Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.
Size of Avalanches
Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word Size (D scale) Simple Descriptor
Small 1 Unlikely to bury a person
Large 2 Can bury a person
Very Large 3 Can destroy a house
Historic 4 & 5 Can destroy part or all of a village
More info at Avalanche.org

In the Alpine, above 3,000’, a human triggered, large and dangerous deep slab avalanches is still possible. A hard slab, 3-5+ feet thick sits on weak sugary snow (basal facets) near the ground. This is a high consequence avalanche problem that is impossible to outsmart, and will take a long time to heal. A big trigger like a snowmachine or a slab avalanche in the upper layers of the snowpack may be enough force to initiate a deep slab avalanche. Likely trigger spots will be in thinner areas of the snowpack that are connected to large, loaded slopes. Cautious route-finding is essential. This includes thinking about the remote trigger potential from below.

Weather
Sat, January 6th, 2018

Yesterday skies were clear and Easterly ridge top winds increased in the afternoon to Moderate. Sunburst weather station averaged 14mph with a few gusts in the low 40s mph early this morning. An inch of snow was recorded overnight at Center Ridge Snotel in Turnagain Pass and temperatures have increased from the low 20F’s to 30’s F at 1000′ this morning.

Today 1-4 € of snow is possible (.2 € SWE) and Moderate Easterly winds are expected to decrease by late afternoon. Skies should remain overcast with limited visibility. Temperatures should remain in the low 30F’s at 1000′, and rain/snow could reach 600′.  

Snow flurries are possible tomorrow, but minimal accumulation is expected. Temperatures will be in the mid 20F’s and winds should be light and variable. Clearing skies and cooler temperatures are expected Monday into Tuesday.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 27   1   .1   44
Summit Lake (1400′) 13   0   0   16  
Alyeska Mid (1700′) 25   trace   .04   36  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23   ENE   14   41  
Seattle Ridge (2400′) 21   *n/a   *n/a     *n/a    
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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.
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