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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Fri, January 5th, 2018 - 7:00AM
Expires
Sat, January 6th, 2018 - 7:00AM
Forecaster
Heather Thamm
The Bottom Line

The avalanche danger is  CONSIDERABLE  on all aspects and elevations above 2,000′ where  human triggered slab avalanches 1-3′ thick are likely today.   It may be possible to remote trigger an avalanche from above, below or adjacent to a steep slope. 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 due to a complicated snowpack.  

Below 2,000′ the danger is  MODERATE  where an avalanche from above may send debris.

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Fri, January 5th, 2018
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
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    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

Several weak layers are sitting under new snow from the New Years storm. Slab depths range from 15” – 30” with deeper amounts in the alpine, on the Northern end of Turnagain Pass, and in Girdwood. This poor set-up plus good visibility is the perfect recipe for human triggered avalanches today. Please understand there are several different dragons lurking in the snowpack right now. 1) a widespread layer of buried surface hoar that is showing signs of being reactive. 2.) weak facets near the ground in slide paths that released during the early December storm cycle, for example the SW face of Sunburst  3.) weak snow (basal facets) near the ground in the upper elevation (above 3,000’)

Rain fell during the end of the New Year’s storm in the lower and mid elevation zones, which has now formed crusts in the top foot of the snow pack up to 2000’. Trigging an avalanche is more likely in upper portion of the Treeline zone where the snow quality quickly improves. Keep in mind today: 

  1. Human triggered avalanches 1-3’ thick are likely above 2,000′
  2. Several tracks may be on a slope before someone finds a trigger point and the whole slope release
  3. Remote triggering from the top, side or bottom of a slope is possible
  4. Avalanche size is expected to be large enough to bury and kill a person 
  5. The larger the slope the larger the avalanche

Additionaly, obvious signs of instability like “whumpfing” and shooting cracks may or may not be present. If winds increase above the forecasted 20mph, and you observe any blowing snow, this could form new wind slabs and add stress to the persistent slab problem.  

 

Two different people traveling near our pit location at 2750′ experienced “whumpfing” due the buried surface hoar collapsing under the new snow. 


Avalanche Problem 2
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    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’, weak sugary snow (basal facets) sits near the ground. The storms over the past few days have added additional load to slopes that already have a hard slab, 3-5+ feet thick. In the upper elevations, a human triggered, large and dangerous deep slab avalanches is still possible. This is a high consequence avalanche problem that is impossible to outsmart and can take a long time to heal. Keep this in mind if clear skies allow for travel into the Alpine. It is really important to remember that triggering an avalanche in the upper layers of the snowpack may then initiate a deep slab avalanche. Cautious route-finding is essential. This includes thinking about the remote trigger potential from below. 

Weather
Fri, January 5th, 2018

Yesterday skies were mostly cloudy in Turnagain Pass with clear skies in Girdwood. Temperatures along ridgetops were in the low 20’s (F) and upper teens yesterday and overnight. Temps at sea level were in the high 20F’s overnight. Winds were light and variable and no precipitation was recorded.  

Clear skies are expected today along with temperatures in the 20F’s at ridgtops and low 30F’s at sea level. Periods of valley fog are possible. Easterly ridge top winds could range from 5-20mph and no precipitation is expected.

Snow showers are possible this weekend, but not a lot of accumulation is expected. Temperatures could range from the low 20F’s at ridge tops to mid 30F’s at sea level. Easterly winds may bump up to moderate at times, 10-25mph on Saturday.

*Seattle Ridge anemometer is rimed.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 23   0   0   43  
Summit Lake (1400′) 13   0   0   16  
Alyeska Mid (1700′) 23    0 0   37  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 17   Variable   4   13  
Seattle Ridge (2400′) 19   *n/a   *n/a     *n/a    
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Riding Areas
Updated Fri, May 01st, 2020

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
Placer River
Closed
Skookum Drainage
Closed
Turnagain Pass
Closed
Closed as of May 1. Thanks for a fun, safe season!
Twentymile
Closed
Seward District
Carter Lake
Closed
Lost Lake Trail
Closed
Primrose Trail
Closed
Resurrection Pass Trail
Closed
Snug Harbor
Closed
South Fork Snow River Corridor
Closed
Summit Lake
Closed

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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.