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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, February 4th, 2021 - 7:00AM
Expires
Fri, February 5th, 2021 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger remains MODERATE today above 1000’. Human triggered slab avalanches are still possible in steep terrain that has seen prior wind effect due to a layer of weak snow, buried 1-2’ deep. Evaluate the snowpack and terrain carefully. Give cornices a wide berth, limit time spent under glide cracks and watch your sluff.

The avalanche danger is LOW below 1000’.

SUMMIT LAKE: This area was more impacted by the NW wind event on Sunday and Monday with natural avalanches observed. There are multiple buried weak layers in the snowpack.  Extra caution is advised. Choose terrain carefully.

LOST LAKE/SNUG/SEWARD: With recent snow, strong winds and buried weak snow, large avalanches may be triggered by a person on skis or snowmachine in these areas as well.

Special Announcements
  • We are very sad to report that three hikers lost their lives in an avalanche on Bear Mountain near Chugiak on Tuesday.  We extend our deepest condolences to the families, friends, and everyone affected by this accident. https://www.adn.com/…/three-hikers-killed-in-avalanche…/
  • Heading to Hatcher Pass? Be sure to check the Thursday Conditions Summary at hpavalanche.org.
Thu, February 4th, 2021
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. 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
    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

With clearing skies and light winds, travel into the upper elevation terrain will be appealing today. While there is plenty of soft snow to enjoy it will important to pay attention to and look for past wind-loading.  This might take a bit of investigation due to the few inches of snow that fell Tuesday night without wind. The goal is to avoid steep slopes with the prior loading. There is a weak layer of snow (surface hoar and near surface facets) that formed early last week that was subsequently buried by the snow that fell at the end of the week. After the snow there was wind from both the east and the west which formed wind slabs over the weak layer on different aspects. Over the weekend there were human triggered wind slab avalanches and a couple of natural avalanches in Summit Lake on Monday. As time has passed since the wind-loading and because of the type of weak layer, we are now dealing with a persistent slab issue. The most suspect areas are loaded slopes at higher elevations.  Keep in mind that the amount of snow that fell was also really variable across the forecast area. As you choose where is go today it will be important to determine whether the wind formed a slab of stiffer snow above the weak layer or not and how deep the slab is. As you travel is the snow loose and sugary or stiff and supportable? Do you sink into soft snow on your skis or snowmachine or not?  Be on the lookout for signs of instability like cracking, whumpfing. Also know that signs of instability may not be present even though avalanche danger lingers when you are dealing with a persistent weak layer in the snowpack.  Slab or no slab? That is the question to answer as you travel today!

Loose snow avalanches (sluffs): Sluffs are likely to be high volume and fast running. They could entrain the few inches of new snow as well as the faceted snow that sits below it. These could have serious consequences if they carry you into terrain traps like cliffs, trees, or rocks.

Cornices: Large cornices are peeling away from ridgelines and cracks are opening. Be sure to give them plenty of space along the ridge and minimize the amount of time you spend below them.

Wind moving snow off the ridge, and down the slope, Glacier Gulch, 2.1.21

Extended column test showing propagation on a layer of buried near surface facets in Summit Lake, 2.2.21.

 

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 exist across the forecast area. Remember it is important to limit time spent underneath them. Glide avalanches are totally unpredictable, not triggered by people and are the entire snowpack sliding at the ground. This type of avalanche could be large and unsurvivable if you happened to be in wrong place when one releases. If you see recent glide activity please let us know.

Weather
Thu, February 4th, 2021

Yesterday: Skies were mostly cloudy in the morning becoming partly cloudy in the afternoon with areas valley fog. Winds were light and westerly and temperatures were in the teens to mid 20°Fs. Overnight skies were partly cloudy, winds were calm and temperatures were in the single digits to mid teens.

Today: Skies will be partly cloudy becoming mostly sunny in the afternoon with some continued valley fog. Winds will be light and northwesterly and temperatures will be in the teens to mid 20°Fs. Overnight skies will be partly cloudy with calm winds and temperatures in the single digits to low teens.

Tomorrow: Skies will be mostly cloudy with a chance of snow showers in the afternoon and evening.  Temperatures will be in the 20°Fs and winds will be light and easterly.  Looking ahead to the weekend the clouds will move out on Saturday with some sunshine in the forecast for afternoon and on Sunday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 14 0 0 121
Summit Lake (1400′) 12 0 0 45
Alyeska Mid (1700′) 13 0 0 107

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 13 SW 4 20
Seattle Ridge (2400′) 16 VAR 2 8
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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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