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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, January 14th, 2019 - 7:00AM
Expires
Tue, January 15th, 2019 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is  CONSIDERABLE  on slopes above 1,000′.  Human triggered slab avalanches, 1-3′ thick, will be likely  and may be triggered remotely  in the Alpine and in areas exposed to wind in the Treeline elevation band.  Careful snowpack evaluation, cautious route finding and conservative decision-making are essential today.  In addition, glide cracks may release into avalanches; limit/avoid exposure under them.  

SUMMIT LAKE / JOHNSON PASS:  Keep in mind buried weak layers exist in the middle and base of the snowpack. More potential for triggering a large slab avalanche exists in this zone, especially in terrain that was recently wind-loaded.  

OVERHEAD HAZARDS: Heads up! With another day of above freezing temperatures near sea level, roof avalanches are a hazard in Girdwood and Portage.

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Mon, January 14th, 2019
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
Travel Advice Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern. Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches Natural and human-triggered avalanches unlikely. Natural avalanches unlikely; human-triggered avalanches possible. Natural avalanches possible; human-triggered avalanches likely. Natural avalanches likely; human-triggered avalanches very likely. Natural and human-triggered avalanches certain.
Avalanche Size and Distribution Small avalanches in isolated areas or extreme terrain. Small avalanches in specific areas; or large avalanches in isolated areas. Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas. Large avalanches in many areas; or very large avalanches in specific areas. Very large avalanches in many areas.
Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    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

Human triggered (skier or snowmachiner) wind slabs are likely today on wind-loaded slopes steeper than 30 degrees. Be on the lookout for recent avalanches and shooting cracks and listen for hollow sounds and whumpfs, these are all Red Flags that an avalanche will be easy to trigger. Ridgetop winds have been blowing strong from the east (20-30mph with gusts up to 60 mph) for a day and half. This combined with old soft snow available for transport and the new snow that fell over the weekend is a perfect recipe for wind slabs. Observers yesterday reported shooting cracks and touchy “connected” wind slabs. There is very weak snow (“sugary” facets and surface hoar that formed during our cold/clear period) under the wind-affected snow. Due to this set-up remote triggering is a possibility. Choose terrain carefully. Slabs are likely to be in the foot thick range in general, but could reach as thick as 2-3′ along the tops of ridgelines and could range from soft to stiff. Upper elevations in Girdwood Valley received more snow than Turnagain Pass, expect slabs to be thicker in this zone. 

Storm Slabs: Warming temperatures caused an upside-down storm, which may have created shallow storm slabs in areas protected from the wind. Again, watch for cracking and carefully evaluate the storm snow today.

Cornices: Wind and new snow combined with the rapid rise in temperature may have made cornices sensitive to triggering. Cornice falls are possible today and may trigger slabs under them. 

***Note the Sunburst winds on January 12th and 13th.

Weak snow (surface hoar and near surface facets) is now buried under new and/or wind affected snow, photo from 1-12-19 at the start of the storm. 

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    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

South of Turnagain – Johnson Pass/Summit Lake zone: A poor snowpack structure exists in these areas and strong winds over the past two days have loaded leeward slopes.  In addition to the recently buried weak surface snow the Christmas buried surface hoar has been found; as well as concerning facet/crust combinations in the bottom of the snowpack. Avalanches may initiate near the ground and be quite dangerous. If you’re headed this way, evaluate terrain exposure and the snowpack as you travel. Be on the lookout for signs of instability. 

Additional Concern
  • Glide Avalanches
    Glide Avalanches
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
More info at Avalanche.org

Glide cracks are likely to become hard to identify with snow/wind filling them in. Known areas with cracks are the southerly facing slopes on Eddies, Tincan, Sunburst, Magnum, Cornbiscuit, Lipps, Johnson Pass, Girdwood Valley and a few on the easterly slopes on Seattle Ridge. Not to mention the Lynx creek glide extravaganza. Avoiding/limiting time under these features is prudent as they can release into an avalanche at any time and are completely unpredictable. The weather pattern change of the past couple days may or may not cause an increase in glide activity. 

Old glide release in Lynx Creek with additional cracks below. Much of the terrain in Lynx is threatened by glide cracks, 1-12-19. Photo: Wendy Wagner

Weather
Mon, January 14th, 2019

Yesterday: Skies were overcast to obscured with rain/snow showers on and off throughout the day. Rain/snow line rose to around 1500′ late in the day. Winds stayed strong 20-30 mph gusting as high as 60 mph. Temperatures were in the high 30Fs/low 40Fs at sea level and mid 20Fs at upper elevations. Overnight skies remained cloudy and winds eased off a bit around midnight. Temperatures remained in the high 30Fs to mid 20Fs depending on elevation.  

Today: Mostly cloudy with a chance of rain/snow showers. Rain/snow line is forecast to stay around 1500′. Temperatures will be in the high 30Fs/low 40Fs at sea level and mid to high 20Fs at 3000′. Winds will be easterly 10-20 mph with gusts into the 40s. Skies will clear overnight and winds will become calm.  There is a little cooling as skies clear but temperatures rise again as clouds increase Tuesday.

Tomorrow: Partly sunny skies and calm winds in the morning becoming cloudy in the afternoon as the next storm system arrives. Stay tuned for storm details as the models are not in good agreement on this on yet! The pattern looks to remain active through the week.  

*Seattle Ridge weather station was heavily rimed and the anemometer (wind sensor) was destroyed. We are currently working to replace it.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 34    1  0.1 54  
Summit Lake (1400′)  32      1   0.1    22    
Alyeska Mid (1700′)  33       1     0.3   42  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 25    NE  21 60  
Seattle Ridge (2400′)

 30    

 *N/A   *N/A   *N/A  
Observations
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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.
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