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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, January 13th, 2016 - 7:00AM
Expires
Thu, January 14th, 2016 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is  MODERATE  today at and above Treeline where cornices are looming large over wind-loaded terrain.  In addition, triggering a wind slab 1-3′ in depth is possible on steep  (greater than 35 degrees), leeward, upper elevation slopes.

At Treeline, the terrain is littered with glide cracks right now, some of which have avalanched to the ground over the last several days, these pose an unpredictable and potentially very hazardous threat.

Below Treeline the danger is  LOW  where triggering an avalanche is unlikely.

In Summit Lake,  the snowpack is shallower and harbors more weak layers. Click  HERE  to read Saturday’s Summit Lake Summary.  

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Wed, January 13th, 2016
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
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. Dangerous avalanche conditions. 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
  • 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

Today at Treeline (the 1,000’ – 2,500’ elevation band) on all aspects, pay attention to and avoid glide cracks. These can lead to glide avalanches that are very unpredictable.  Weather and other triggers such as humans or even explosives that we associate with other dangerous avalanche problems don’t seem to effect glides in the same way.  There is no discernable pattern to predict a failure as they tend to fail naturally and on their own schedule. Warm temperatures can trigger them and so can cooling temperatures. Cracks can form and release in seconds or days later or sometimes a glide crack won’t release at all, and benignly just fill back in with snow.  

Glide cracks are best to be given a wide berth.  Limit your exposure time spent underneath and if skiing or riding in terrain with glide cracks, try and map them out before your travels so as not to end up directly on top of or inside one.  Remember, when these do fail, they tend to be destructive, failing to the ground bringing the entirety of the snowpack with them. 

Glide crack on Petes North. Photo: Sean Fallon

Avalanche Problem 2
  • Cornice
    Cornice
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.
More info at Avalanche.org

Cornices have grown considerably during the past few weeks. Not only do they have the potential to fall naturally, but also to be triggered by the weight of a person or snowmachine. They can also trigger an avalanche on the slope below when they fall. Travel under or on them should be avoided. Remember they have the tendency to break farther back from the ridge than expected. 

Image from the National Avalanche Center

 

Additional Concern
  • Wind Slabs
    Wind Slabs
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.
More info at Avalanche.org

We saw substantial winds over the weekend (30 – 70 mph from the east) that actively stripped windward slopes and built stiff wind slabs in the alpine.  These may be in the 1-3’ range today and are most likely to be triggered in steep, leeward terrain. Wind loaded slopes are visibly fat right now, be on the look out for shooting cracks, hollow feeling or sounding snow and stiff snow over softer snow. Remember a small pocket of wind slab in can be very hazardous in high consequence terrain. 

Weather
Wed, January 13th, 2016

Yesterday was mostly overcast with a few breaks in the clouds. There were light snow showers, highs were in the mid-20Fs to mid 30Fs. Winds were light and variable.

Today will be mostly cloudy with snow showers throughout the day.  New snow accumulation of around 1-3 inches possible. Temperatures will be in the low 30Fs to mid-20Fs.  Winds will be light and shift from east to north. Snow showers continue overnight with a drying trend into tomorrow.

Thursday is forecasted to be mostly sunny and slightly cooler with light northerly winds as we see a break in the active weather pattern that has been affecting the region.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 29    1 .1    86
Summit Lake (1400′)  30  0  0  25
Alyeska Mid (1700′)  31  1  .06  62

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 25   NE*   10*   41*  
Seattle Ridge (2400′)  26 n/a   n/a    n/a

*Sunburst anemometer stopped recording at 2 pm yesterday. Wind data is from 6am-2pm.

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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.