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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Tue, February 13th, 2018 - 7:00AM
Expires
Wed, February 14th, 2018 - 7:00AM
Forecaster
Aleph Johnston-Bloom
The Bottom Line

There is a  CONSIDERABLE  avalanche danger above 1000′ due to recent snowfall and strong winds overnight. Human triggered avalanches remain likely. Fresh wind slabs up to 2 feet thick, should be expected on slopes with recent wind loading. Additionally, weak layers deeper in the snowpack may be triggered, creating a much larger avalanche.

Below 1,000′ the danger is MODERATE.

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Tue, February 13th, 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

Yesterday saw continued snowfall adding a couple more inches of new snow. Rain/snow line fluctuated from 200′-500′.  Overnight winds shifted to the north and gusted into the 60s on Seattle Ridge. The storm total of 6-12″ of snow fell on weak surface snow and/or added load to slopes harboring buried weak layers. We have been talking about the buried January 21st surface hoar needing more of a slab to be reactive and produce large avalanches. The storm snow combined with wind has likely created that slab. We do not yet have enough information about how the snowpack will behave after this loading event. Small avalanches at the old snow/new snow interface could ‘step down’ and release a much larger and unmanageable slide. Don’t let the sunshine blur your judgment. Today is a day to be extra cautious and evaluate the snowpack carefully. Look for recent avalanches, shooting cracks and listen for ‘whumpfs’. 

Crown profile from Twin Peaks shows the January 21st buried surface hoar layer that may now be reactive with added load. 

This avalanche in the North Cornbiscuit Chutes occurred Sunday on recently buried surface hoar and the cracking in the bed surface is believed to be on the January 21st buried surface hoar layer. This illustrates the potential for failure on more than one weak layer.  Photo: Mike Records

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

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

Moderate to strong winds coupled with 6-12″ of new snow have likely formed wind slabs 1-2′ thick. Because these slabs are likely sitting on weak old snow, they are expected to be quite sensitive and easy to trigger. The winds shifted from south to north last night so multiple aspects may be loaded. It will be important to pay attention to where the snow feels stiff, looks pillowed or sounds hollow and watch for shooting cracks. 

Cornices: Cornices are unpredictable and can break further back along a ridge than expected. Give these features plenty of space.

Loose snow sluffs: Sluffs on steep slopes are likely with the recent new snow. Warm temperatures and sun hitting steep southerly slopes may also trigger roller balls that may progress to natural loose snow avalanches as they entrain surface snow. 

 

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

The recent snowfall is a relatively small load on top of our generally weak snowpack structure in the Alpine (above 3,000′). However, even a small load combined with strong winds and warming temperatures could help tip the balance. Someone might be able to trigger a large deep slab that breaks in the bottom half of the snowpack. It’s good to remember that multiple layers of old buried surface hoar, facets and crusts exist deep in the pack and near the ground. Small incremental loading can sometimes be just enough to ‘wake up’ dormant deep layers. The overall poor structure is worth keeping in mind, as outliers can happen like last week’s Twin Peaks slide.

 

Weather
Tue, February 13th, 2018

Yesterday skies were obscured and light snow/rain fell throughout the day depending on elevation. Temperatures were in the 30Fs at sea level and 20s at upper elevations. Winds started out southerly and shifted to the north last night. Winds overnight were NW 20-30 mph gusting into the 60s.  

Today will be mostly clear and sunny with some valley fog. Temperatures will range from the 30Fs to the 20Fs as you go up in elevation. Winds will remain elevated from the north this morning with gusting into the 40s but should calm down this afternoon. Temperatures cool into the teens overnight.

Tomorrow is forecast to be partly cloudy with a slight chance of snow and then clearing again on Thursday. There is another chance for snow over the weekend. Stay tuned!  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′)  33 2   0.1   67  
Summit Lake (1400′)  28 2   0.1   26  
Alyeska Mid (1700′)  30  2.8 0.19   59  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 20   NE-NW    12 37
Seattle Ridge (2400′) 25   SE-NNW    22 66  
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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.