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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, December 15th, 2023 - 7:00AM
Expires
Sat, December 16th, 2023 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

Today’s avalanche danger is CONSIDERABLE above 2500′. Increasing winds will form fresh wind slabs 1-3′ deep that could cause natural and human triggered avalanches. In addition, a deeply buried weak layer (3-5′ deep) could cause very large avalanches in isolated areas. We recommend evaluating the snowpack conditions before committing to steep terrain.

Below 2500′ the avalanche danger is MODERATE. Winds will probably not impact these elevations as strongly, but keep an eye out for active wind transport. The deeper avalanche concern is also an issue at lower elevations.

Special Announcements

Become a Member in December! The Friends of the Chugach Avalanche Center is a non-profit, which means we need your help to keep our avalanche center running. Everyone who donates during the month of December will be entered to win some awesome prizes at our Girdwood Brewery Forecaster Chat on January 19.

Fri, December 15th, 2023
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
Sat, December 16th, 2023
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
Sat, December 16th, 2023
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
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

The consistent snowy weather recently has made it difficult to assess how much avalanche activity has been associated with the recent snowfall events.  A brief period of clearing yesterday showed some avalanche activity that occurred during the most recent storm, likely Wednesday afternoon or evening. Glide avalanche activity also continues, with a release observed along Seattle Ridge during the day yesterday. No new activity has been reported on the layer of facets above the Thanksgiving crust.

Silhouette of an avalanche that likely released during the storm on Wednesday and was subsequently buried by new snowfall. Photo Peter Wadsworth 12.14.23 

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

Stormy weather resumes today, with 2-6″ of snow and cloudy skies expected throughout the day. The sneaky 18″ of low density snow that fell with very light winds in the forecast area on Wednesday will start to be redistributed by increasing SE winds today, averaging 15-25 mph with gusts to 35 mph. Wind slabs 1-3′ deep could form rapidly given the amount of soft snow on the surface available for transport.

Keep an eye out for active wind loading and hollow or stiff feeling snow on the surface as a sign of areas that are being wind affected. Jumping on small, steep test slopes can be a good way to evaluate how reactive wind slabs are before entering more consequential terrain. To avoid this issue look for wind sheltered areas where you can take advantage of the soft snow on the surface.

Dry Loose Avalanches are very likely in steep terrain that is sheltered from the winds. Be mindful of managing your sluff if you are travelling on steep slopes, especially if there is exposed terrain underneath you.

The blanket of soft snow on the mountains from Wednesday’s storm will be redistributed today with increasing wind speeds. Photo Peter Wadsworth 12.14.23

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
  • Aspect/Elevation
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

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

The layer of facets above the Thanksgiving crust has not shown any obvious signs of being active during the past couple of storm systems. However, we have limited information from the forecast area. An avalanche triggered on this 3-5′ deep weak layer could be very large and destructive so we are keeping it in the forecast as a reminder to check the snowpack in the areas you are travelling. If you find a layer of weak, faceted snow on top of the crust or get a concerning result in a stability test on this layer we recommend sticking to lower angle slopes in order to minimize your exposure to a potentially high consequence avalanche. This type of weak layer becomes more difficult to evaluate the deeper it gets buried so we will be keeping track of it to try and determine when it could become more likely for people to trigger.

Basic snowpack strucutre on Sunburst at ~3000′. Photo Peter Wadsworth 12.14.23

Additional Concern
  • Glide Avalanches
    Glide Avalanches
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.
More info at Avalanche.org

A glide avalanche released during the day yesterday on Seattle Ridge. These avalanches are very unpredictable and can release spontaneously regardless of weather conditions. Try to avoid spending time underneath a glide crack to reduce your exposure.

Glide release on the S end of Seattle Ridge on Wednesday 12.14.23. Photo Peter Wadsworth

Weather
Fri, December 15th, 2023

Yesterday: There was a brief break in the cloudy skies and stormy weather yesterday, with predominantly clear skies except for a layer of valley fog building in the afternoon. Temperatures were chilly, in the low to mid teens F. Winds averaged 5-10 mph in the alpine with gusts up to 20 mph picking up overnight. No significant new snowfall was recorded.

Today: Light snowfall is expected to resume again today, starting in the early morning hours and accumulating roughly 2-6″ during the day. Winds will shift to the SE as the snowfall starts and increase to averages of 15-25 mph and gusts to 35 mph at upper elevations. Temperatures will be in the low to mid 20s F, which will keep the precipitation falling as snow down to sea level. Cloud cover is expected to be overcast and obscured during the storm, with a possible period of clearing Friday night into Saturday.

Tomorrow: Saturday should be similar to Friday in regards to snowfall, with light snow throughout the day resulting in 2-6″ of additional accumulation. Temperatures will increase which could cause rain line to move up to 500-700′. Wind speeds are expected to increase on Saturday, but the timing and intensity of the winds are a question mark. Current weather models show winds up to averages of 45-65 mph starting around noon on Saturday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 11 0 0 76
Summit Lake (1400′) 3 0 0 32
Alyeska Mid (1700′) 15 0 0.02 61
Bear Valley – Portage (132′) 14 1 0.12
Grouse Ck – Seward (700′) 14 0 0 32

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 12 W 5 20
Seattle Ridge (2400′) 13 SE* -* 10*

*Seattle Ridge weather station is rimed over with only a few hours or data reporting for wind.

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.