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Mon, February 8th, 2021 - 7:00AM
Tue, February 9th, 2021 - 7:00AM
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger remains MODERATE today above 1000’. Triggering a slab avalanche on a layer of weak snow, buried 1-2’ deep, is still possible in steep wind-loaded terrain. Assess the snowpack as you travel, identify areas of concern and evaluate terrain consequences. Give cornices a wide berth, limit time spent under glide cracks and watch your sluff.

The avalanche danger is LOW below 1000’.

Mon, February 8th, 2021
Above 2,500'
2 - Moderate
Avalanche risk
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. 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
  • Persistent Slabs
    Persistent Slabs
  • Certain
    Very Likely
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
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

While some enjoyed watching the Super Bowl yesterday others enjoyed skiing Super Bowl… and skiing and snowmachining many locations throughout the forecast area. There were no reports of human triggered avalanches but observers noted variable surface conditions from the wind event on Saturday.  Some terrain was completely spared, soft and fun, while other slopes had stiff breakable wind crust and hollow-sounding wind slabs. With another day of sunshine on tap, determining where the stiff slabs linger over weak snow will be the key to avoiding our current avalanche issue. If you follow the forecast consistently our messaging might be sounding like a bit of a broken record. This the nature of a persistent slab issue. Because there is weak snow (surface hoar and near surface facets) buried under the wind affected snow, the potential to trigger an avalanche won’t quickly heal up. Over time the cold clear weather can help slowly facet away the slab and ease tension. However, steep slopes with wind affected snow should be still be approached with caution today.  Slabs may be found near ridges, on wind-loaded slopes and in cross-loaded gullies. Watch for cracks shooting out from your snowmachine or skis, smooth rounded pillows of snow, and ‘punchy’ feeling stiff snow over weaker snow. Even shallow slabs can be quite dangerous in high consequence terrain and hard wind slabs may break above you as you travel out onto the slope. Look for signs of signs of wind effect and choose terrain carefully.

Loose Snow Avalanches (Sluffs): Continued cold and clear weather is keeping the surface snow dry and loose in areas that have been sheltered from the wind. Be aware of fast-moving and high-volume sluffs in steep terrain, especially if you are traveling near terrain traps like cliffs, rocks, trees, or gullies. Check out this observation from a week ago in the Library.

Cornices: When traveling along ridgelines be sure to give cornices plenty of space, and minimize the amount of time spent traveling below them. Cornices could fail under the weight of a person on skis or a snowmachine, and might trigger an avalanche if the slope below is wind-loaded. Also be aware that cracks have opened up along some ridgelines where the large cornices are peeling away.

Buried surface hoar and near surface facets, 2.7.21. Terrain that harbors a stiff slab of wind affected snow over this set-up is suspect today.

Shooting crack on Raggedtop. 2.6.21. Photo: Matti Silta

Wind affected snow along the Cornbiscuit ridgeline, 2.7.21.

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 continue to open up throughout the area. These cracks could release at any time and be very destructive. Avoid spending time below glide cracks. If you see any new glide activity, please let us know here.

Glide cracks in the Seattle Creek drainage, 2.7.21. Photo: Christina Twogood.

Mon, February 8th, 2021

Yesterday: Skies were clear with low stratus and valley fog along Turnagain Arm. There was an inversion with temperatures in the single digits to -10°F in valley bottoms and in the mid to high teens in the Alpine. Winds were west/northwesterly 5-10 mph with gusts into the teens. Overnight skies were clear with valley fog.  Temperatures were in the single digits above 0°F to a few degrees below 0°F in the valley bottoms and in the low teens in the Alpine. Winds were light and west/northwesterly.

Today: Skies will be mostly clear with continued patchy valley fog and low stratus which may clear out later today. Winds will be light and variable.  Temperatures will be in the single digits and teens. Overnight skies will be mostly clear with temperatures in the single digits above 0°F to a few degrees below 0°F. Winds remain light and variable.

Tomorrow: Skies will be sunny in the morning with increasing clouds and a chance of snow showers late in the day. Temperatures will be in the high teens to low 20°Fs. Winds will be light and easterly, increasing overnight.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 10 0 0 117
Summit Lake (1400′) 2 0 0 43
Alyeska Mid (1700′) 8 0 0 107

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 13 W 9 19
Seattle Ridge (2400′) 14 NW 2 9
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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.