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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, December 18th, 2021 - 7:00AM
Expires
Sun, December 19th, 2021 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger remains MODERATE above 1000′. It is possible to trigger a very large avalanche on a layer of weak sugary snow buried 3-5′ deep. These avalanches can also be triggered remotely (from above, below, or to the sides of steeper terrain). Conservative terrain selection and awareness of steeper slopes above you is the best way to manage this avalanche hazard.

The avalanche danger is LOW below 1000′. Be aware of steeper slopes above you because avalanches releasing at higher elevations could run to valley bottoms. There is thin snow cover at lower elevations and the main hazards are shallowly buried vegetation causing difficult travel and partially frozen creeks and wetlands.

Sat, December 18th, 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. 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
  • Lynx Creek – Looks like a new glide avalanche released on the lower portion of Lynx creek sometime in the past 48 hours. It is hard to believe and hard to see because of the cloud sandwich in this photo, but we received multiple reports from reliable sources.

Photo taken from Cornbiscuit on 12.17.21

Avalanche Problem 1
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.

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 will bring another day of calm weather, with cloud cover being the limiting factor but the visibility should improve throughout the day. It has been over a week since our last significant snowfall and other than some wind loading events the forecast area has been largely quiet since then in terms of avalanche activity. While the faceted weak layers buried 3-5′ deep have been gaining some strength during that time it typically takes weeks or longer for this type of weak layer to stop being possible to trigger. The lack of recent avalanche activity could start to lure us into a false sense of security about the stability of our faceted weak layer, but it is important to remember that deep persistent slabs are defined by their lack of feedback before producing large and destructive avalanches.

Here are some of the common characteristics of a deep persistent slabs:

  • Difficult to trigger – stubborn
  • Capable of producing very large avalanches that could propagate across terrain features
  • No signs of instability before a slope releases
  • Can be triggered after many people have been on a slope
  • Can be triggered remotely (from the side, top or below)
  • The slab itself is more than 3′ deep and is composed of hard snow

These characteristics are what make deep persistent slabs so hard to evaluate and manage. The potential to trigger one of these avalanches has probably decreased over the last week but we are still seeing poor structure in our snowpack and signs of propagation in stability tests (videos from Magnum and Cornbiscuit). Triggering a deep slab is easier in areas with a thin snowpack, which is the case in the southern portion of the forecast area and even more so as you get closer to Summit Lake. Most of the avalanche activity we have seen on this layer is in the 1,500 to 3,000′ elevation band.

To minimize your exposure to this avalanche problem we recommend steering clear of large slopes with slope angles greater than 30° and being aware of steeper slopes above you. If you decide to step out into avalanche terrain it is wise to follow safe travel protocols, like having only one person at a time in avalanche terrain and always spotting your partners.

Our basic snowpack structure between 1500 – 3000′. Photo from Cornbiscuit on 12.15.21

Weather
Sat, December 18th, 2021

Yesterday: Temperatures were downright comfortable yesterday staying in the teens to low twenties throughout the day. Winds were light and came from variable directions. No new snow. Cloud cover was in and out, obscuring the ridgelines and occasionally dropping down to the level of the pass.

Today: It looks like it will be cloudy to start but cloud cover should diminish throughout the day. Temperatures are forecast to remain in the teens to low twenties. Winds should be out of the west from 5-15 mph with gusts into the 20s. We could see a trace of snow.

Tomorrow: Sunday and Monday look largely similar to today. Winds should be light out of the west and temperatures will remain in the teens to twenties. Visibility should be the best on Sunday. No new snow in the forecast until Monday night or Tuesday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 21 0 0 70
Summit Lake (1400′) 14 0 0 22
Alyeska Mid (1700′) 19 0* 0.02 43

* No change in snow depth recorded but a small amount of precipitation

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 16 W until 1600, then E 4 15
Seattle Ridge (2400′) 17 W until 1500, then E 5 17
Observations
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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.