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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, December 29th, 2023 - 7:00AM
Expires
Sat, December 30th, 2023 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger is MODERATE above 1000′. Wind slab avalanches 1-2′ deep could be human triggered at upper elevations, especially in steep terrain. Glide avalanches have been releasing at an alarming rate over the past week and the widespread distribution of glide cracks in popular areas is concerning. These avalanches release spontaneously and are very large and destructive. Avoiding spending time underneath glide cracks is the only way to mitigate the risk.

Below 1000′ the avalanche danger is LOW.

Special Announcements

Become a Member in December! The Friends of the Chugach Avalanche Center, our non-profit arm, needs your help to keep our avalanche center running. Note, everyone who donates will be entered to win some awesome prizes at Andrew’s Girdwood Brewery Forecaster Chat on January 19!

Fri, December 29th, 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
Sat, December 30th, 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
Sat, December 30th, 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

A few new observations of glide avalanches came in from Seattle Creek yesterday, but the exact timing of when they released is uncertain. Otherwise no new avalanche activity was reported.

Two recently released glide avalanches on the far side of Seattle Creek. Photo from Troy Tempel 12.28.2023

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

Today promises to bring another day of cold and clear weather with very light winds. Yesterday, we observed some active wind transport on higher elevation ridgelines which could form small wind slabs 1-2′ deep in exposed areas. These fresh wind slabs are mixed in with widespread wind affected snow along upper elevation ridgelines from the stormy weather over the weekend, which could make them difficult to identify. Keep an eye out for shooting cracks in small pockets of wind transported snow that could be sensitive to the weight of a skier or rider. These avalanches are probably too small to bury a person but could easily knock you off balance in steep terrain.

Prior to the storm last weekend there was a widespread layer of surface hoar on the surface that we have been searching for in the snowpack because of the potential to cause avalanches at the storm snow interface, which is roughly 2′ deep. So far our field observations have not turned up any concerning results at this interface, but it is worth keeping in mind if you are venturing into new terrain today. Surface hoar can be difficult to identify visually in a snow pit, but tends to reveal itself if you use a compression test or extended column test to check the stability of the snowpack in the area you are travelling.

Dry loose avalanches are very likely in steep wind sheltered terrain. We recommend making a plan to manage your sluff if you are travelling in steep and complex terrain.

Wind affected snow surfaces along upper elevation ridgelines could produce a surprise avalanche in steep terrain. Photo 12.28.2023

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

Over the past week or 10 days there has been a notable spike in the number of glide avalanche releases across the region. This type of avalanche is not well understood. All we really know is that they can spontaneously release under any conditions and when they release they take the entire snowpack and are highly destructive. The glide avalanche on Cornbiscuit on Wednesday was roughly 5-6′ deep and ran almost all the way to Bertha Creek. The short story is that you definitely do not want to mess with this type of avalanche and we strongly recommend giving glide cracks a wide berth and minimizing the amount of time you spend underneath them. Take a careful look at the slopes above you and consider setting a new skin track or riding off the beaten path if you can find a way to avoid travelling underneath glide cracks.

It was sobering to peer into this recent glide release and see the destruction it caused in the runout zone. Photo 12.28.2023

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 facets over the Thanksgiving crust are still lingering 4-6′ deep in the snowpack. The lack of recent avalanche activity on this layer despite many storm systems adding stress over the past few weeks is a good indicator that the layer is dormant, but we are continuing to monitor it due to the history of crusts producing very large avalanches in our region. It is possible that layers like this can re-activate later in the season if the snowpack structure becomes weaker or if melt water pools on top of the crust. Hopefully continued snowfall will bury this crust so deeply that we can soon remove it from the forecast.

Weather
Fri, December 29th, 2023

Yesterday: Clear skies and cold temperatures in the low teens to single digits, with a temperature inversion causing colder temperatures near the valley bottoms. Valley fog was hanging out around 1000-1500′. Winds were light averaging 0-5 mph out of the NW with gusts up to 20 mph at upper elevations.

Today: Another clear and cold day is on tap with temperatures ranging from negative to positive single digits, colder at lower elevations. Winds are expected to remain light at 0-5 mph.

Tomorrow: Cloud cover is expected to increase on Saturday as a storm approaches the forecast area in the evening. Winds are expected to remain light at 0-10 mph, but shift to the south. Light snowfall is possible during the day Saturday, but no significant accumulation is expected.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 2 0 0 77
Summit Lake (1400′) -10 0 0
Alyeska Mid (1700′) 7 trace 0.02 70
Bear Valley – Portage (132′) 4 0 0

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 9 W 6 20
Seattle Ridge (2400′) 8 N 1 7
Observations
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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.