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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Wed, January 31st, 2018 - 7:00AM
Expires
Thu, February 1st, 2018 - 7:00AM
Forecaster
Aleph Johnston-Bloom
The Bottom Line

The avalanche danger remains  MODERATE  above 1,000′ for triggering a slab avalanche 1-3′ thick.  Slopes with signs of wind loading will be the most suspect for triggering an avalanche. Additionally, watch for easily initiated  loose snow avalanches (sluffs) on steep slopes.  

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Wed, January 31st, 2018
Alpine
Above 2,500'
Moderate (2)
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
Moderate (2)
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
  • 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

There were a couple reports of people triggering small slabs on wind-loaded slopes yesterday, one skier and one snowmachiner. The 1-2′ of snow that fell at the end of last week loaded a layer of widespread surface hoar, that was buried on January 21st. There were a number of human triggered avalanches over the weekend but overall the storm snow has only been acting as a slab in areas that were affected by wind. This continues to be the case.  The slides yesterday occurred in the Johnson Pass area and on Eddies.  One group investigated and found that the weak layer in the slide they triggered was the January 21st buried surface hoar underneath wind-affected slab.  This is a good reminder that despite the cold and clear weather now slowly faceting away the slabs; there is still the possibility of finding and triggering lingering slabs in leeward terrain today.  Remember that the surface hoar is lurking underneath the recent snow and it’s important to assess areas affected by wind. Slabs can be deeper in loaded areas. Pay attention to slopes where the snow feels stiff, looks pillowed or sounds hollow and watch for shooting cracks. A small slab in the wrong terrain could have high consequences. 

Small slab in the Johnson Pass area, intentionally triggered in a ‘sled cut’ yesterday. 

It is important to keep this snowpack structure in mind today on slopes with wind-affected snow. Buried surface hoar could be underneath. 

These avalanches occurred on Eddies Saturday and were remote triggered from the ridge. They were noted again yesterday by a party in the area that had a ski cut produce a slab, 16″ deep and 30′ wide in similar terrain, that ran to the bench below. Photo: Joe Engel

 

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

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

As the cold weather continues to loosen and facet the surface snow, loose snow avalanches (sluffs) are becoming larger and faster by the day. Watch out for and manage your sluff in steep terrain features protected from wind.

Additional Concern
  • 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

Triggering a deep slab is becoming unlikely, but is still not out of the question above 3000′. In the high elevation snowpack there are a variety of weak layers in the mid pack and near the ground. Because of this poor structure, there is still a chance of triggering a deep slab if you find the wrong spot. The most likely trigger spots are in thin areas in the snow cover, often near rocks, or where the slope rolls over. 

 

Weather
Wed, January 31st, 2018

Yesterday was clear and sunny. Temperatures were in the single digits to mid teens. Winds were easterly and picked up a little in the afternoon gusting as high as 20 mph. Overnight the skies became partly cloudy.  

Today skies will be mostly to partly cloudy. Winds are forecast to be calm and temperatures will be in the mid to high teens. Skies will become clear again overnight.  

This weather pattern will persist into the weekend with more sunshine, cold temperatures and calm winds. From the NWS,  the “blocky”  pattern aloft maintains it`s hold over the regional weather  pattern.  There is still discussion of a pattern shift but a lot of uncertainty about timing and how much it will impact this region.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′)  14  0  0  65
Summit Lake (1400′)  4  0  0   18  
Alyeska Mid (1700′)  10  0  0   52  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 8   NE    8 20  
Seattle Ridge (2400′)  11  ESE  9 19  
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Updated Mon, November 30th, 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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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.