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Issued
Wed, December 27th, 2023 - 7:00AM
Expires
Thu, December 28th, 2023 - 7:00AM
Forecaster
Daniel Krueger
Avalanche risk The Bottom Line

The avalanche danger is MODERATE above 1,000’ and LOW below 1,000’.  Easterly winds picked up yesterday in the afternoon and continued overnight, likely creating fresh wind slabs on a variety of terrain features. Today wind slabs could be 1-2’ deep and large enough to bury someone. Additionally, there still is uncertainty as to how reactive a layer of buried surface hoar is that is roughly 2-3’ deep and underneath the Holiday storm snow.

A cautious mindset is recommended if headed into the bigger terrain. Be sure to keep in mind that the storm snow may still be unstable and be aware of red flags such as whumpfing in the snowpack.

 

Wed, December 27th, 2023
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
Thu, December 28th, 2023
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
Thu, December 28th, 2023
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. 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

Other than one fresh looking glide avalanche in the Seattle Creek drainage, no new natural or human triggered avalanches were reported. The last avalanches seen/reported were from Saturday and Sunday when the Holiday storm deposited 2-3 feet of snow accompanied by strong east winds.

Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • Aspect/Elevation
  • 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.

Aspect/Elevation of the Avalanche Problem
Specialists develop a graphic representation of the potential distribution of a particular avalanche problem across the topography. This aspect/elevation rose is used to indicate where the particular avalanche problem is thought to exist on all elevation aspects. Areas where the avalanche problem is thought to exist are colored grey, and it is less likely to be encountered in areas colored white.

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

Yesterday, gusty ridgetop winds were depositing snow on leeward features from Turnagain Pass to Girdwood. They began in the afternoon and ramped up in the evening, in the 25-50 mph range. These winds combined with all the snow available for transport likely drifted snow into 1-2’ wind slabs. The wind has died down this morning, which will allow the snow to adjust to this load. However, there is a chance that a person could trigger one of these wind slabs. As you are traveling, feel for stiffer snow over softer snow and look for shooting cracks under your machine, board, or, skis. These are great indicators telling you that there is a combination of a wind slab over softer snow. If triggered, these avalanches could become more dangerous if they step down into a layer of buried surface hoar 2-3’ deep.

Avalanches failing under the Holiday storm snow are still a concern. While the Holiday storm has had some time to settle, the surface hoar that it buried has been identified in snowpits between 2 and 3’ deep. This would have been on the surface before the Holiday storm started. First of all, we do not know how intact and widespread this layer is. It was buried between two very soft layers of snow and in many places could be hard to pick out and not an issue. However, this may not be the case everywhere. Therefore, we are still concerned that a person could trigger this layer and get caught up in a large avalanche. While we keep getting more intel, this is a heads-up situation where conditions can seem a lot safer than they might actually be. This a difficult problem to identify and why we recommend a cautious mindset. There is plenty of great skiing to be had in less consequential terrain until we know more.

Ridgetop winds transporting snow yesterday in the afternoon in the Portage area. 12.26.2023

 

Surface hoar identified 3′ deep but did not fail in stability tests on Tincan yesterday at 1,900′. 12.26.2023

Avalanche Problem 2
  • Glide Avalanches
    Glide Avalanches
  • Aspect/Elevation
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.

Aspect/Elevation of the Avalanche Problem
Specialists develop a graphic representation of the potential distribution of a particular avalanche problem across the topography. This aspect/elevation rose is used to indicate where the particular avalanche problem is thought to exist on all elevation aspects. Areas where the avalanche problem is thought to exist are colored grey, and it is less likely to be encountered in areas colored white.

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

Glide cracks and glide avalanches have been observed all along both sides of Turnagain Pass, in Girdwood Valley and south through Summit Lake. Two newer glide avalanches were seen on Seattle Ridge on Monday. It is important to look for and know how to identify what these cracks and avalanches look like. Because these avalanches release to the ground it can be easy to spot them as “brown frowns” as they open or “brown spots” when they release. It is important to minimize exposure under these glide cracks because they can spontaneously release and be large and dangerous.

Glide crack on Main Bowl in the Seattle Ridge area. Photo by Troy Tempel 12.26.2023 

 

Additional Concern
  • Deep Persistent Slabs
    Deep Persistent Slabs
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.
More info at Avalanche.org

The Thanksgiving crust/facet combo is now sitting under 4-6 feet or more of snow. We still have not seen avalanches releasing on this layer after the storm but that does not mean this layer is not capable of sliding. Only three days have passed since the storm. We are still gathering information on if there was avalanche activity on this layer.  This would have created a very large slide with a big crown face. Due to the depth, the layer is not only very difficult to assess at this point, but also would be very difficult to trigger. We are not forgetting about it however and will continue to look for any evidence that it could be reactive.

Weather
Wed, December 27th, 2023

Yesterday: Clear skies with a layer of low clouds approaching from the south in the afternoon. Ridgetop winds were light and variable, with a few 10-20 mph gusts from the east. East winds picked in the afternoon into the evening with gusts of 30-50 mph. Temperatures warmed throughout the day (-4 F to 15-20 F). Tuesday night saw a trace to a couple inches of snow.

Today: Partly cloudy skies will become mostly clear in the afternoon. Models are showing ridgetop winds light and variable before shifting out of the west in the afternoon in the 5-10 mph range throughout the day. Temperatures should be around the teens slowly dropping in the evening. There is no expected precipitation.

Tomorrow: Mostly clear skies are expected tomorrow, Thursday. Ridgetop winds are forecast to increase from the west and northwest into the 10-15mph range. Temperatures look cold, dropping from the teens to possibly single digits later in the day. No precipitation is expected.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 14 1 0.1 82
Summit Lake (1400′) 5 trace trace N/A
Alyeska Mid (1700′) 17 1 0.05 71
Bear Valley – Portage (132′) 12 2 0.15

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 11 NE 12 46
Seattle Ridge (2400′) 12 N/A N/A N/A
Observations
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Date Region Location
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02/21/24 Turnagain Observation: Tincan Trees
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02/20/24 Turnagain Observation: Seward Highway across from Johnson Pass TH
02/19/24 Turnagain Avalanche: Base of Seattle Ridge
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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.