The avalanche danger remains MODERATE at all elevations. With higher wind speeds today and light snowfall forecast to start this afternoon, triggering a 1′ deep avalanche in wind loaded areas is possible. The larger concern is a deeper layer of weak sugary snow buried 3-5′ which could create very large avalanches that are possible to be triggered by a smaller wind slab or by a person. These deeper avalanches could be triggered remotely – from the bottom, sides, or above a steeper slope. Conservative terrain selection is the best way to avoid exposing yourself to the potential of triggering a large avalanche.
Thu, December 16th, 2021
Alpine
Above 2,500'
2 - Moderate
Treeline
1,000'-2,500'
2 - Moderate
Below Treeline
Below 1,000'
2 - Moderate
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
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.
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
Wind Slabs
Certain
Very Likely
Likely
Possible
Unlikely
Likelihood
Historic (D4-5)
Very Large (D3)
Large (D2)
Small (D1)
Size
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
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.
Winds picked up into the teens overnight with gusts into the twenties out of the east as a weak storm system moved into the area. The combination of increased wind speeds, 1-4″ of new snowfall forecast this afternoon, and existing light snow on the surface in many areas will make wind slabs up to a foot deep possible to trigger today. On the lower slopes of Cornbiscuit yesterday we came across some wind slabs from the last wind event over the weekend that were still somewhat reactive, with about 6″ of firm wind slab on top of soft snow. With the potential for additional wind loading and new snow these could become easier to trigger today. Look for active wind transport, firm over soft snow, and shooting cracks to identify areas with reactive wind slabs. A small wind slab has the potential to step down into a deeper layer of buried facets and create a very large avalanche today (see problem 2 for more details).
Portage and Placer valleys are likely to receive more new snowfall today, with up to 4-10″ possible, which could result in bigger wind slabs forming in exposed areas.
Wind affected snow along ridgeline of Sunburst from 12.12.21
Avalanche Problem 2
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.
The deep persistent slab is still foremost in our minds when making decisions in the backcountry, and will remain there for the foreseeable future. Patience is critical for managing this type of avalanche problem because the consequences of triggering one of these is so high. This type of avalanche problem is known for providing little or no feedback, in terms of ongoing avalanche activity or stability test results, but has the potential to create very large and destructive avalanches. In Girdwood and Turnagain Pass the most common deep persistent weak layer is weak sugary facets over a melt freeze crust which are buried 3-5′ deep and have been most prominent in the 1500 – 3000′ elevation band. In areas south of Turnagain pass (southern end of Seattle Ridge, areas closer to Johnson Pass) the weak layers are not buried as deeply due to an overall shallower snowpack which could make them easier to trigger.
Luckily, the low angle powder is still quite appealing in areas that have not seen too much wind since the last storm. To avoid the risk of triggering a large deep slab avalanche we recommend keeping your slope angles below 30° and being aware of the terrain above you.
If you decide to venture into avalanche terrain you can reduce your exposure by using safe travel protocols, such as one skier/rider on the slope at a time and always spotting your partners. Remember that it is often not the first skier on slope to trigger a deep slab. Initiating a failure in deeply buried weak layers tends to be easier in thin areas of the snowpack and near rocks or trees.
Snowpack summary from Cornbiscuit yesterday, sorry for the vertical video orientation. We saw propagation in the faceted weak layer for the first time in awhile but it took a lot of force to initiate the failure in the extended column test. 12.15.21
Weather
Thu, December 16th, 2021
Yesterday: Skies started off largely clear but clouds moved into the area throughout the day. Temperatures also climbed throughout the day and leveled off in the mid-teens overnight. Winds were light during the day but picked up overnight, with averages around 10 – 15 mph and gusts to from 20 – 25 mph since 2 am. No new snow accumulation.
Today: Skies will remain cloudy today with snowfall starting this afternoon and about 1-4″ possible for Turnagain Pass. Greater snowfall is likely near Portage and Placer with up to 4-10″. Winds should be out of the southeast at 10-20 mph with gusts into the 30s. Temperatures should remain in the teens throughout the day.
Tomorrow: The majority of the snow is forecast to fall Thursday afternoon through overnight Friday, but some lingering snowfall could persist through Friday morning. Winds should shift back to the northwest and calm down as the snowfall ends early Friday morning. Skies will remain partly cloudy on Friday with temperatures in the teens.
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.
Archives 
The Bottom Line
Wind affected snow along ridgeline of Sunburst from 12.12.21
