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Issued
Tue, December 12th, 2023 - 7:00AM
Expires
Wed, December 13th, 2023 - 7:00AM
Forecaster
Daniel Krueger
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE above 1000′ today.  Human triggered wind slabs 1-3’ deep will be the most likely avalanche to encounter on steeper wind exposed slopes. We recommend carefully evaluating wind loaded features before committing to steeper terrain. Additionally, it is possible to trigger a larger avalanche on a weak layer buried 3-4′ deep which could run into low angle terrain. Being aware of overhead exposure and easing into terrain today will increase your margin of safety. 

Below 1000′ the avalanche danger is MODERATE. Below treeline wind slabs are less likely and the mix of rain and snow during the past few storm cycles makes larger avalanches on deeper weak layers less likely as well.

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.

Highway Avalanche Hazard Reduction Work: There will be intermittent traffic delays Tuesday December 12, 2023 on the Seward Highway for Avalanche Hazard Reduction work. From mileposts 44 to 46 on the Seward Highway, at Summit Lake. Motorists should expect delays of up to 45 minutes between 10:00am and 11:00am.

Tue, December 12th, 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
Wed, December 13th, 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
Wed, December 13th, 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
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

We were surprised not to find much widespread natural avalanche activity yesterday after the storm Sunday night into Monday morning. There were several small to medium size (D1 – D2) avalanches observed across the forecast area that likely failed at the interface with the old snow surface, but we did not observe any avalanches that released on on the facets above the Thanksgiving crust. However, visibility was not great so we still have some uncertainty about the extent of natural avalanche activity.

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

It has been 24 hours since a storm dropped 1-1.5’ of new snow in Girdwood and Turnagain Pass, while Summit and Seward received 4-6”. This storm came in with strong easterly winds averaging 50-75 mph, with gusts over 100 mph. Winds continued to transport the new snow throughout the day yesterday, with easterly winds averaging 10–25 mph and gusts to 50 mph. The combination of lots of soft new snow on the surface and strong winds has created widespread fresh wind slabs 1-3’ deep in exposed areas, like along ridgelines and convex features at and above treeline. 

To identify areas harboring wind slabs look for stiff or hollow feeling snow on the surface and use small, steep test slopes to check for shooting cracks or small avalanches. Even small wind slabs in steep terrain could turn into larger avalanches as they flow downslope due to the amount of soft new snow on the surface. Since the winds have died down today natural avalanches are not very likely, but human triggered avalanches in recently wind loaded areas are likely.

Storm Slabs: In the field yesterday we did not see the new snow causing avalanches in wind sheltered areas. However, we have limited information and still recommend evaluating how well the new snow is bonding to the old snow surface before committing to steep terrain. Dry loose avalanches are also likely on steeper terrain features.

Cornices: The ridgelines are likely to look a bit different after the storm with some impressive cornices forming.  These cornices could be tender when traveling on ridgelines usually breaking much further back from the ridge than expected. They can be large triggers if they fail so limit exposure under them.

High winds transporting snow on Seattle Ridge. 12.11.2023 

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

With limited visibility yesterday we are still uncertain if the new storm snow added enough stress to cause natural avalanches on a weak layer of facets above the Thanksgiving crust. Prior to this storm, there was a remotely triggered avalanche on Eddies at 2200’ that failed on this layer. This crust/facet layer is now buried 3-4’ deep and capable of producing a massive avalanche.  This weak layer has shown the ability to be triggered remotely, meaning you could trigger a large avalanche on a steeper slope next to or above you even if you are on flatter terrain. To avoid this problem stick to lower angle slopes and be aware of runout zones below steep terrain. Also, if you see evidence of avalanche activity or are getting out, snap a photo and send us an observation!

Screen shot of our pit in the Tincan Trees at 2200′. 12.11.2023

Weather
Tue, December 12th, 2023

Yesterday: Heavy snowfall tapered off early yesterday morning, with some areas receiving an additional 1-2″ of new snow in the morning. Rain line was approximately 500-700′ during the day. Strong winds continued to transport snow at upper elevations with averages of 10-25 mph during the day and gusts up to 50 mph. Cloud cover was mostly overcast and obscured across the forecast region with occasional periods of broken cloud cover that provided temporarily improved visibility. Temperatures were in the mid to low 30s F at sea level and stayed in the 20s F above 1000′.

Today: There will be a brief break in snowfall across the forecast area today, with a chance of snow showers but no significant accumulation expected. Cloud cover will remain mostly overcast with a low elevation cloud ceiling and the potential for occasional breaks to provide intermittent visibility. Winds are expected to die down today with averages of 0-10 mph and gusts to 15 mph out of the east during daylight hours. Temperatures have dropped into the teens F at mid and upper elevations and 20s F closer to sea level.

Tomorrow: Light snowfall should return to the forecast area tomorrow, with 2-5″ of new snow expected during the day. Temperatures will remain in the teens to 20s F, which will allow snow to fall down to sea level. Winds will pick up slightly during the day tomorrow with averages of 10-20 mph and gusts to 30 mph at upper elevations.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 27 1 0.1 62
Summit Lake (1400′) 25 2 0.2 31
Alyeska Mid (1700′) 29 1 0.15 49
Bear Valley – Portage (132′) 37 0 0.44
Grouse Ck – Seward (700′) 31 2 0.2 32

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 20 ESE 11 47
Seattle Ridge (2400′) 23 SE 7 19
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.