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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Thu, January 11th, 2018 - 7:00AM
Expires
Fri, January 12th, 2018 - 7:00AM
Forecaster
Aleph Johnston-Bloom
The Bottom Line

The avalanche danger is  MODERATE  today due to new snow falling on weak surface snow and increasing winds.  Watch for changing conditions as the storm moves.  In the Alpine triggering a slab avalanche, breaking 1-3′ deep on a weak layer of snow remains possible. Additionally, triggering a larger slab breaking near the ground remains possible at elevations above 3,000′.    

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Thu, January 11th, 2018
Alpine
Above 2,500'
Moderate (2)
Avalanche risk
Treeline
1,000'-2,500'
Moderate (2)
Avalanche risk
Below Treeline
Below 1,000'
Moderate (2)
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
Moderate (2)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Avalanche Problem 1
  • Storm Slabs
    Storm Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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

Today is a transition day. We have had mostly cold, clear weather this week. Snow is forecasted to start falling this morning and winds are increasing, gusting into the 30s. The new snow will be falling on very weak surface snow. There is widespread surface hoar, with near-surface facets below.  In the terrain around  2000′ and below, this weak snow is more developed and is sitting on the New Year’s melt-freeze crust. This has the potential to act as a bed surface. As the weather comes in today it will be important to pay attention to changing conditions. Slabs may form quickly, especially in leeward terrain. Quick hand pits or riding small terrain features can help check whether or not the new snow is sticking to the old snow surfaces. In steep terrain the weak surface snow can also act as a few inches of fast moving loose surface snow “sluff” that could catch you by surprise if you’re not expecting it. 

Surface hoar in the sunshine on Tuesday at 1700′ on Sunburst, photo: Ray Koleser.

Avalanche Problem 2
  • 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

As we anticipate the weak snow on the surface getting buried today don’t forget the old buried weak layers.  The chances of triggering a persistent slab avalanche have been decreasing. However, it is not something that should totally be ruled out yet. We know that the snow from the New Year’s storm snow is sitting on either buried surface hoar, buried near-surface facets or in thin snowpack areas, facets near the ground. This means there is poor snowpack structure and a persistent slab set-up above 2500′. Snowpack tests over the past few days have shown variable results but point to the continued potential for triggering an avalanche.

For those riders and skiers headed out today:

  • Triggering a slab is most likely in the upper elevations above 2,500′, where NO crusts exist in the top foot of the snowpack.
  • Several tracks may be on the slope before a slab releases and no signs saying ‘the slope is unstable’ are likely to be present.
  • Larger slopes, 35 degrees and steeper are more suspect and those with rocky features. 
  • Practice safe travel habits: expose only one person at a time, have an escape route planned, watch your buddies closely and view slopes as avalanche paths. If the snow does slide, where will it go? Avoid terrain traps. 

Watch Wendy’s Extended Column Test from yesterday, January 10th, on Seattle Ridge. It took some harder hits but still propagated (failed across the weak layer). This layer could still produce an avalanche.

Wendy’s pit yesterday was near her pit location on Sunday. She had very similar results. The slab yesterday was a little deeper. 

 

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

A poor snowpack structure still exists at high elevations above 3,000’, human triggered large and dangerous, deep slab persistent avalanches are still possible. Weak sugary snow (basal facets) near the ground is creating a low probability/high consequence avalanche problem. Likely trigger spots will be in thinner areas of the snowpack that are connected to large, loaded slopes. Signs of instability will not likely be present and there may be tracks on the slope. The possibility of a deep slab avalanche should still be part of your decision-making before committing to big terrain in the Alpine. 

 

Weather
Thu, January 11th, 2018

Yesterday was a mixture of obscured to overcast to broken skies. Winds were in easterly 10-20 mph, gusting to 30. There were scattered, very light snow showers in the morning. Temperatures were in the teens to low 20s. Overnight temperatures rose as warm air moved into the area.  

Today will be mostly cloudy with snow, 4-9″ forecasted. Winds will be easterly 20-30 mph gusting into the 40s. Temperatures will be in the high 20s. Snow will continue overnight into Friday.

Friday afternoon the temperatures are forecasted to rise and Saturday looks like a shift to liquid precipitation at lower elevations. The pattern of the warm southerly flow combined with a series of low pressure systems could persist into next week.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 26    0 0 42  
Summit Lake (1400′) 15    0 0   14
Alyeska Mid (1700′)  22  0 0   34  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  13  ENE 11   31  
Seattle Ridge (2400′)  17  SE 16 31  
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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.