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Issued
Tue, February 16th, 2021 - 7:00AM
Expires
Wed, February 17th, 2021 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

Today’s avalanche danger is MODERATE, and it is possible to trigger an avalanche on weak layers of snow buried 1-2’ deep. The most likely places to trigger an avalanche will be on steep slopes with stiff slabs near the surface. A dusting of light snow will make it more challenging to identify stiff slabs, so it will be important to be intentional with assessing the upper snowpack before committing to steep terrain.

Tue, February 16th, 2021
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
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. 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
  • 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

Today it is still possible to trigger an avalanche on weak layers of snow in the upper 2’ of the snowpack. These layers formed during dry spells in late January and early February, have recently produced avalanches (examples here and here) and continue to show poor results in stability tests. In some areas we are dealing with buried surface hoar, others are a combination of decomposing stellars and near-surface facets, and many places have both. These layers are particularly concerning where buried surface hoar sits on top of a rain crust at elevations up to 1200-2000′. The key to staying out of trouble today will be avoiding steep slopes where those weak layers are capped by stiff snow near the surface. A few inches of snow from last night will not increase the avalanche danger, but it may make it more difficult to identify slopes that were previously wind-loaded. You can still look for slabs as you travel, and it can be as easy as hopping off your machine and poking into the snow, or stepping off the skin track to probe around as you approach your objective.

Persistent weak layers will sometimes– not always– give clear warning signs prior to avalanching. If you notice shooting cracks or collapsing, these are sure signs that the snowpack is capable of avalanching and it is time to stick to low-angle terrain. These persistent weak layers become more difficult to anticipate in the absence of these warning signs. When we are dealing with uncertainty in the strength of the snowpack, and in the reactivity of persistent weak layers, it is important to increase our margin of safety by minimizing exposure to consequential terrain.

We are expecting to see more snow tonight and into tomorrow. Snow and winds are not expected to pick up until later tonight, but the active weather may increase avalanche danger. Pay attention to changing conditions later in the day today, keep your fingers crossed for snow, and stay tuned for more!

Sluffs: Be aware of dry loose avalanches on slopes that have been sheltered from the wind. While it is unlikely they would be big enough to bury a person, they can become dangerous if they carry you down steep slopes and into terrain traps.

Cornices: Large cornices have been peeling away from ridgelines, opening up large cracks. If you are traveling along ridges be sure to give them plenty of space, and minimize the amount of time you spend traveling below them.

Looking down at the natural avalanche on Eddies (first observed on 02.12). The avalanche failed on a layer of buried surface hoar and near-surface facets about 12″ deep, and quickly stepped down to a deeper layer of buried surface hoar. 02.15.2021

Two layers of buried surface hoar, plus a fresh layer on the surface which just got buried this morning. Eddies. 02.15.2021

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 have opened up throughout the area, and some are getting quite large. These avalanches are unpredictable and they are destructive since they involve the entire season’s snowpack. Avoid spending any time below glide cracks, as they can release unexpectedly. If you see any new glide activity, please let us know here.

Weather
Tue, February 16th, 2021

Yesterday: High temperatures reached the upper 20’s to low 30’s F under mostly cloudy skies. Light easterly winds were blowing 5-10 mph at ridgetops, with gusts of 10-15 mph. Snow began trickling in last night, bringing 1-2” low-density snow by this morning, with snow to sea level.

Today: Snowfall is expected to turn off during the day today, with light easterly winds blowing 5-10 mph at ridgetops. Temperatures will be in the low- to upper 20’s F under mostly cloudy skies.

Tomorrow: We are expecting more snow and increased winds tonight, with another 3-7” possible by tomorrow morning, and winds 15-25 mph out of the east at ridgetops. Overnight low temperatures will be in the upper teens to low 20’s F, and high temperatures tomorrow will be in the mid 20’s to low 30’s F.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 25 1 tr 116
Summit Lake (1400′) 21 1 0.1 43
Alyeska Mid (1700′) 25 3 0.15 111

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 20 ESE 4 10
Seattle Ridge (2400′) 20 E 7 14
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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.