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Sat, November 27th, 2021 - 7:00AM
Sun, November 28th, 2021 - 7:00AM
John Sykes
Avalanche risk The Bottom Line

The avalanche danger remains CONSIDERABLE at higher elevations above 2500’, on all aspects. Human triggered slab avalanches 2-3’ thick are likely on slope angles above 35 degrees. Multiple layers of weak snow exist throughout the advisory area and have been buried by 20-34” of new snow this week. Avalanches can be triggered remotely from low angle terrain above, below, or to the sides of steeper slopes. Evaluating conditions carefully and cautious terrain selection is important for safe travel today. Several human triggered avalanches were reported yesterday, which the best indicator of unstable conditions.

The avalanche danger is MODERATE below 2500’, with similar snowpack conditions as higher elevations. Triggering a large slab avalanche is possible.

Special Announcements
  • The Turnagain Pass motorized area will open today Saturday, Nov 27th. Snowmachiners: We currently have limited information about the conditions in Seattle Creek Drainage. We do know there are widespread weak layers under 2-3 feet of snow. This is a time to have fun in the low angle terrain and be very suspect of steeper slopes as we gather more information.
Sat, November 27th, 2021
Above 2,500'
3 - Considerable
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. 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.
Recent Avalanches
  • Seattle Ridge – We received multiple reports of a possible remote triggered avalanche south of the Seattle Ridge uptrack. Currently we are uncertain whether reports are from one or two avalanches in the same area. One report stated that the avalanche released at 2500’ and was about 18” deep by 60’ wide and ran approximately 400’ (photos here, more details here).

Seattle Ridge slab avalanche 11.26.21. Photo from anonymous

  • Tincan Common – Skier triggered soft slab avalanche on skiers right hand side of common bowl on Tincan. This avalanche was about 24″ deep and was triggered from lower angle terrain above the crown.

Tincan Common slab avalanche 11.26.21. Photo from Matt Dietrick

  • Hope area – this is outside our forecast zone, but the conditions in this location appear to be similar to the Summit Lake area. The avalanche was triggered on a ridgeline with a firm wind slab over faceted snow or possibly buried surface hoar. Skier had previously descended near this same slope but triggered it while climbing back up. They were caught and carried about 400′ and able to arrest on the bed surface, but are ok.

Hope area avalanches, an old windslab over a persistent weak layer from 11.26.21. Photo from anonymous

  • Lipps Glide Avalanche – Observations of glide releases have slowed down in the past few days, but this new release was spotted yesterday on the south face of Lipps. We don’t know the exact release date but it was between 11.21.21 and 11.26.21.

Photo 11.26.21, exact release date unknown

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

Our primary concern today is persistent slab avalanches which could be triggered on a variety of weak layers (details here and here) that are becoming more active as the 20-34” of very light new snow from this week gradually settles into a more cohesive slab. These slabs can be 2-3′ deep and have the potential to propagate across an entire slope. Triggering an avalanche is more likely in areas where the wind has transported snow to create a firmer slab. The most recent pulse of snow came in on Wednesday (~10-12″) with no wind, which makes it very hard to see where the prior storm snow has been redistributed by wind. Assessing the slab character of each slope by travelling off trail and looking for red flags, such as collapsing (whumphing) and shooting cracks, is important to evaluate conditions today.

We recommend keeping a conservative mindset today because of the weak snowpack structure and a high level of uncertainty about where conditions are unstable. Persistent slabs can be tricky because it is not always the first person on the slope who triggers an avalanche. We have observed remote triggering in avalanches yesterday and expect similar conditions to remain throughout the weekend. If you want to keep it safe today, stay on terrain with less than 35 degree slope angle and pay attention to steeper slopes above you. It is important to travel one at a time in terrain above 30 degree slope angle and spot your partners to minimize the consequences if an avalanche is triggered.

Shooting cracks in new snow over faceted weak layer from 11.25.21 on Seattle Ridge

Snowpack summary from Sunburst on 11.26.21

Summit Lake Area – An avalanche reported in the Hope area yesterday seems to line up with conditions observed in Summit Lake recently. This avalanche was a hard wind slab over faceted snow that released along a ridgeline. Even though this is outside our forecast zone it is worth noting if you are planning to travel in Summit Lake today, where the new snow from this week only added up to 3-6” but old wind slabs could continue to be a problem.

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 avalanche activity has slowed down in the past week, but we observed a release on the south face of Lipps sometime in the past few days. This is a reminder the glides are still active in our area. It is key to minimize time underneath glide cracks as they can produce very large and unsurvivable avalanches. They can also be a hazard to fall into on skis or snowmachines and are harder to see in many locations because of the recent light storm snow.

Sat, November 27th, 2021

Yesterday: Temps in the single digits with winds less than 5 mph out of the west and gusts below 10 mph. Trace amounts of precipitation. Mostly cloudy with variable cloud layers throughout the day.

Today: Temps in the negative single digits with light northwest winds at 5-10 mph and gusts up to 15 mph. Partly to mostly cloudy throughout the day. Snow showers are possible with accumulation of less than an inch.

Tomorrow: Sunday looks like more cold temps and calm winds with scattered clouds. The next wave of precipitation should arrive during the day on Monday with a low pressure system moving into the area.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 6 N/A 0.2* 64
Summit Lake (1400′) 4 0 0 11*
Alyeska Mid (1700′) 6 N/A 0.02* N/A

* Precipitation sensors are struggling to accurately capture the snowfall totals from the past few days.

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 0 W 3 7
Seattle Ridge (2400′) 1 NNE 3 8
Recent Observations for Turnagain Pass
Date Region Location
11/16/23 Turnagain Observation: Tincan Common
11/14/23 Turnagain Observation: Sunburst/Magnum
11/14/23 Turnagain Observation: Seattle Ridge
11/13/23 Turnagain Avalanche: Tincan Trees
11/12/23 Turnagain Observation: Sunburst
11/12/23 Turnagain Avalanche: Goldpan – avalanche
11/11/23 Turnagain Observation: Tincan Common
11/11/23 Turnagain Observation: Taylor Pass – Sunburst
11/10/23 Turnagain Observation: Tincan
11/10/23 Turnagain Observation: Tincan Trees
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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.