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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sat, January 20th, 2018 - 7:00AM
Expires
Sun, January 21st, 2018 - 7:00AM
Forecaster
Heather Thamm
The Bottom Line

A MODERATE  avalanche danger exists above 2,500′ where triggering a large Deep Slab avalanche (3-8+ feet) remains possible. This is a scary avalanche problem where multiple people can ski/ride/snowmanchine on a slope before an unlucky person finds a thin spot and the whole slope releases.   Steer clear of cornices by giving them extra space, and don’t be surprised by fast moving loose surface snow.  Evaluate snow and terrain carefully.

Below 2500′, the avalanche danger is LOW where triggering a slab avalanche is unlikely due to a stout crust that has formed. LOW danger does not mean no danger, a loose snow avalanche could knock you off your feet. Be aware of runout zones in the event that someone triggers a very large avalanche in the alpine that could run into the mid elevation zone.  

For information about Summit Lake avalanche conditions click  HERE.

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Sat, January 20th, 2018
Alpine
Above 2,500'
Moderate (2)
Avalanche risk
Treeline
1,000'-2,500'
Low (1)
Avalanche risk
Below Treeline
Below 1,000'
Low (1)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
Moderate (2)
Treeline
1,000'-2,500'
Avalanche risk
Low (1)
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
  • 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

A high consequence, stubborn to trigger avalanche problem lurks above 3000’, where it will be easy to assume a false sense of stable conditions. A stout crust has stabilized the snowpack in the mid and lower elevations, and this crust disappears under the new snow somewhere around 3000’. In the Alpine several weak layers (buried surface hoar and facets) remain preserved deep within the snowpack, under 3-8+ feet of snow. Basically there is an invisible gray line between Treeline and the Alpine where the snowpack goes from stable to a Deep Persistent Slab problem without notice.

Several tracks may be on a slope before someone finds a trigger spot, a thinner area of the snowpack, that takes the whole slope. Because this problem is impossible to assess without x-ray vision, an element of luck will be involved if you ride/ski in big terrain without incident. You can manage this problem with your elevation by sticking to terrain below 3000’ or by choosing low-consequence terrain in the alpine. Don’t forget the bigger your objective, the bigger the size of a potential avalanche. 

Keep in mind:

  • Obvious signs of instability like collapsing “whumpfing” may not be present until it’s too late
  • Be aware of people above and below you, an avalanche from above could run much further than expected
  • Remote triggering is possible from a thinner area of the snowpack
  • Snow pits and stability tests may not be representative of the slope you are evaluating

 

A natural avalanche on Sunburst released mid-storm, likely Tuesday (1/16), and serves as a good reminder that slopes that have already avalanched, during one of the many storms this season, should not be considered safe. This recent avalanche was 3/4 of mile wide, ran 2000’, and filled up the entire drainage of Taylor Creek.

 

 

The 1st pit was dug just above the crown on Sunburst at 3400′ in a thinner area of the snowpack. The 2nd pit was a few hundred feet away in the bed surface. This is a good example of the poor structure that is buried 3-8+ deep in many places. 

 

 

Avalanche Problem 2
  • Cornice
    Cornice
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
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.

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

Cornices have been growing over the last week and could be easy to trigger. Give these features a lot of space and remember they can break further back from a ridge than expected.  A cornice fall also has the potential to trigger a very large avalanche on the slope below.

Additional Concern
  • Dry Loose
    Dry Loose
Dry Loose
Dry Loose avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.
More info at Avalanche.org

Fast moving surface snow “sluff” was observed yesterday and it could knock a skier or rider down if you’re not expecting it. You can manage this problem by slowing down and letting the snow move past you. This of course will be more problematic in steep high consiquence terrain where falling would be undesirable and is a secondary risk to the bigger problem mentioned above. 

Weather
Sat, January 20th, 2018

Yesterday skies were partly cloudy and temperatures were between 15-25F. Winds were light and variable and no precipitation was recorded.  

 Today looks similar, partly cloudy temperatures ranging from mid to low teens at ridge tops to mid 20F’s near sea level. Ridge top winds are expected to remain light and variable.  

High pressure over main land Alaska is causing cold clear, and rather benign weather in Southcentral Alaska. A similar trend is expected this week and temperatures  may dip into the single digits at times.  

**Seattle Ridge weather station stopped recording wind data on 1/17/18 due to rime covering the anemometer.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 20   0   0   55  
Summit Lake (1400′) 18   0   0   15  
Alyeska Mid (1700′) 23   0   0   43  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 18   variable   3   12  
Seattle Ridge (2400′) 20   **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

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Closed as of May 1. Thanks for a fun, safe season!
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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.