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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, December 28th, 2023 - 7:00AM
Expires
Fri, December 29th, 2023 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is MODERATE on all aspects above 1,000′. Lingering wind slabs around a foot deep could be triggered on steep wind loaded slopes and in cross-loaded gullies. Watch out for glide cracks. These cracks have been releasing into avalanches in popular areas, one of them occurred on Cornbiscuit yesterday. Last, there is a chance a person could trigger a slab 2-3′ deep on a layer of buried surface hoar; despite a high degree of uncertainty with this, it’s prudent to keep in mind if headed to bigger terrain.

The danger is LOW below 1,000′.

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!

Hatcher Pass:  HPAC forecasts on Thursday’s and Saturday’s, check out this morning’s avalanche forecast at hpavalanche.org.

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
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
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
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

Three new glide avalanches were seen yesterday at Turnagain Pass. One was confirmed to have released midday, pictured below on Cornbiscuit, and the other two on Seattle Ridge and in the Seattle Ck drainage. Otherwise, we heard of no other avalanche activity. The last known avalanches, other than glides, were from the end of the Holiday storm on Sunday when a widespread storm snow avalanche cycle occurred.

Glide avalanche that occurred yesterday between 11:30am and 2:30pm on the south face of Cornbiscuit. Note the tracks above and to the left. Seen and photoed by Allen Dahl, 12.27.23. 

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

Cold and clear weather is over us again today along with valley fog in places near Turnagain Arm. The two players in the avalanche game also remain similar. These are the winds and a questionable layer of buried surface hoar around 2 feet deep, give or take.

Wind Slab Avalanches:  The snow surface has seen a fair amount of wind effect over the past several days and finding a lingering wind slab should be on our radar. Ridgetop winds are blowing from the northwest 5 to 15mph the morning and look to remain in this range through the day. The only places new wind slabs might develop are the high peaks and ridgelines where the strongest wind should be. Be on the lookout for both old and new slabs, stiff snow over softer snow, and cracks that shoot out from you.

Persistent Slab Avalanches:  Despite several days with no human triggered avalanches and generally stable pit results, we are still thinking about that layer of buried surface hoar that sits under the Holiday storm snow (anywhere from 1.5-3′ deep). This layer could be tricky to find and may have only survived in some places. But at the end of the day, we know it was there before the storm and there could be some booby traps out there. If you are thinking about heading into the bigger terrain and steep slopes, we hope everyone knows this layer might be lingering and triggering a scary avalanche around 2 feet deep could happen.

A brand new crop of surface hoar is developing to ridgetops. This one is on a harder wind affected surface and could be more concerning than the one we keep talking about above. Time will tell once it gets buried. Thanks to Allen Dahl for the photo, 12.27.23. 

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

It seems this cold weather is inducing a glide avalanche cycle. Meaning many of the “brown frowns”, full depth cracks in the snowpack, are releasing into avalanches. The short story is avoid being under glide cracks. They exist in well traveled zones, such as along Seattle Ridge and above the motorized up-track, as well as on Cornbiscuit and Magnum.

Recent glide avalanche on southeast facing lower shoulder of Big Chief photographed by Kellie Okonek, 12.27.23. This is across from Seattle Ridge and the avalanche spills into Seattle Ck drainage.

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 old Thanksgiving crust/facet combo is now sitting under 4-6 feet or more of snow. We have not seen avalanches releasing on this layer for around two weeks and no known naturals failed on this layer during the Holiday storm last weekend. Those are great signs this issue is dormant. Meaning the layer is not producing avalanches even though it still exists deep in the pack. The one exception is the Summit Lake area where the snowpack is thinner and an outlier avalanche may not be out of the question. That said, we are not forgetting about it and will continue to look for any evidence that it could be reactive.

Weather
Thu, December 28th, 2023

Yesterday:  Partly sunny skies with valley fog was seen over the region. Snowing fog in Girdwood added about an inch of low density snow in the valley bottom. Ridgetop winds were 5-10mph from the east. Temperatures were civil again, in the teens to 20’sF at most locations.

Today:  Another mostly sunny day with possible valley fog is forecast. Ridgetop winds are currently 5-10mph with gusts in the teens where they should remain. Temperatures are chilly, in the teens to single digits. No precipitation is expected.

Tomorrow:  The clear and cold weather should continue through tomorrow. Models are showing winds decreasing to light and variable before turning easterly on Saturday. The next chance for weather and precipitation could come as early as Saturday night. Stay tuned.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 12 0 0 79
Summit Lake (1400′) 7 0 0 N/A
Alyeska Mid (1700′) 15 0 0 71
Bear Valley – Portage (132′) 17 0 0
Grouse Ck – Seward (700′) 18 0 0 50

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 15 var 4 16
Seattle Ridge (2400′) 16 N/A N/A N/A
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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.