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Wed, December 1st, 2021 - 7:00AM
Thu, December 2nd, 2021 - 7:00AM
John Sykes
Avalanche risk The Bottom Line

The avalanche danger is MODERATE at all elevations. Weak layers are distributed throughout the advisory area and are creating the potential for large human triggered avalanches 2-3′ deep that can release more widely than expected. Human triggered avalanches are possible and evaluating the potential for avalanche release is difficult due to the deeply buried weak layers. Selecting smaller terrain features and lower slope angles is recommended to minimize exposure to this avalanche hazard. In areas above 2500′ that saw strong winds yesterday morning it could still be possible to trigger an avalanche in wind loaded terrain.

Special Announcements
  • The Glacier and Seward Ranger Districts have opened all motorized zones to motorized use. NOTE: There is limited parking for Placer Valley due to road construction on the Portage curve. Please see this map for details.
  • Snowmachiners: Although there is little snowpack information, signs are pointing to potentially dangerous avalanche conditions in many of the areas opening to snowmachines. This includes Placer and Skookum Valleys, Johnson Pass, Lynx Creek, Lost Lake, and others. Keeping off of the steep slopes and maximizing the powder in the mellow terrain will be a good way to have a safe fun opener wherever you head!
Wed, December 1st, 2021
Above 2,500'
2 - Moderate
Avalanche risk
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. 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.
Recent Avalanches

Girdwood area – Alyeska snow safety triggered a 2′ deep and 500′ wide avalanche at low elevation (~1600′) during avalanche mitigation operations. The avalanche was triggered by the fourth person on the slope and released down to the facet and Halloween crust interface that we have observed throughout the advisory area.

Avalanche on facets over Halloween crust with wide propagation between chair 4 and chair 6 at Alyeska. Avalanche was triggered during control work. Photo: Alyeska Ski Patrol 11.30.21

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

Weak faceted snow buried 2-3′ deep is our main avalanche concern. This type of avalanche can often lie dormant for days to weeks before getting the right combination of snow load and trigger to release. The recent avalanche at Alyeska yesterday is a reminder that persistent slabs can release on lower angle terrain than expected, propagate more widely than expected, and be stubborn to trigger, with the fourth skier on the slope having triggered the avalanche. Taking a conservative approach to selecting terrain (smaller terrain features, slope angles below 35 degrees), travelling one at a time in avalanche terrain, and spotting your partners is essential to manage this avalanche hazard. If you are travelling to one of the newly opened motorized areas (Placer, Skookum, Lynx, Johnson Pass, Lost Lake, ect) keep in mind that we have very limited information and recommend a conservative approach while we can start to visit more areas and get more public observations.

So far the new snow loads from the past 48 hours and during the week of Thanksgiving have not been enough to produce significant natural avalanches on these persistent weak layers. Instead they have incrementally loaded these weak layers to the point where the size of the avalanche that could be triggered has increased. Based on our exploration of the snowpack on Tincan yesterday, it seems like the weak layers are more prevalent below 3000′ where the Halloween melt freeze layer is thicker and a thicker layer of weak faceted snow remains from the cold spell in mid-November. Most of the avalanches reported in the advisory area in the past week have been at or below 3000′ (Seattle ridge, Girdwood, Tincan, Alyeska yesterday).

Digging a snowpit can be a good way to check on persistent weak layers in the area you are travelling. Keep in mind that snowpits only give you information about a single point and can vary greatly across a slope, so choosing a representative location is key. Stability tests can struggle to initiate a failure in deeply buried weak layers and can provide confusing results. The bigger value is just to understand the snowpack structure in your area and determine whether persistent weak layers are present and how sugary and soft they are.

Wind Slabs: In higher elevation areas, above 2500′, that saw the brunt of the winds from early yesterday morning it is still possible to trigger a wind slab. Look for firm and hollow feeling snow and signs of active wind transport. These are especially likely along ridgelines or in passes at higher elevations.

Snowpack structure near treeline on Tincan at 2300′ on 11.30.21. This pit has a well defined slab and weak layer combination, with very sugary weak snow (fist hardness) in the weak layers.

Snowpack structure from 3000′ on Tincan on 11.30.21. This area had more wind effect in the upper snowpack and a thinner layer of faceted snow that had smaller grain size and were stronger than what we saw at 2300′.

Recent wind scouring with rocks exposed on ridge features along north face of Sunburst from strong winds in early morning hours of 11.30.21

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 activity has continued to decline and many of the lingering glide cracks are slowly getting filled in by the recent storms. They can still be a hazard to fall into and have to potential to spontaneously release. Minimize time underneath open glide cracks to avoid this unlikely but catastrophic avalanche hazard.

Wed, December 1st, 2021

Yesterday: Cloud cover was in and out yesterday, with scattered snowfall but little accumulation. Temperatures were in the teens to mid-twenties. Winds were strong in the early morning hours, with gusts close to 40 mph on Sunburst, but calmed down to 5-10 mph with gusts to 15 mph for most of the day.

Today: The tail end of the storm could drop an additional 1-4″ of snow today with mostly cloudy conditions. Winds will range from 5-15 mph with gusts into the 20’s starting in the east in the morning and then switching to the northwest in the afternoon/evening. Temperatures will be in the teens and drop into the single digits this evening.

Tomorrow: Precipitation will taper off and temperatures will drop back into the positive and negative single digits. As the current low pressure moves out of the area we could see some increased winds, but the trend will be toward cold and calm again while we wait for the next storm system.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 25 0* 0* 68-70*
Summit Lake (1400′) 20 1 0.1 12
Alyeska Mid (1700′) 24 ~6″* 0.4 N/A

* Most snow depth and precipitation sensors area still giving errors in Center Ridge and Alyeska

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 18 ENE 6 27
Seattle Ridge (2400′) 21 E 2 7
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Date Region Location
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05/07/24 Turnagain Observation: Turnagain Pass Wet Slabs
04/29/24 Turnagain Avalanche: Turnagain aerial obs
04/27/24 Turnagain Observation: Johnson Pass
04/23/24 Turnagain Observation: Turnagain Sunny Side
04/21/24 Turnagain Observation: Bertha Creek
04/20/24 Turnagain Avalanche: Spokane Creek
04/16/24 Turnagain Observation: Cornbiscuit
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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.