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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, December 23rd, 2023 - 7:00AM
Expires
Sun, December 24th, 2023 - 7:00AM
Forecaster
John Sykes
Avalanche Warning
Issued: December 23, 2023 6:00 am
Travel in avalanche terrain is not recommended. Avoid being on or beneath all steep slopes.
Avalanche risk The Bottom Line

The avalanche danger is HIGH at all elevations today. A strong storm system is currently impacting the area with heavy snowfall and extreme winds, creating dangerous avalanche conditions. Both natural and human triggered avalanches are very likely and we recommend avoiding avalanche terrain. Avalanches have the potential to run down to valley bottoms, so it is important to avoid being underneath large avalanche paths.

Roof Avalanches: With the temperatures increasing and rain expected to fall at sea level today roof avalanches will be a significant hazard. Be aware of any roof line loaded with snow and try to avoid letting kids play, dogs wonder, and vehicles park underneath.

Special Announcements

The National Weather Service has issued a variety of winter storm warnings / blizzard warnings / high wind warnings / winter storm watches throughout Southcentral Alaska.

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.

Sat, December 23rd, 2023
Alpine
Above 2,500'
4 - High
Avalanche risk
Treeline
1,000'-2,500'
4 - High
Avalanche risk
Below Treeline
Below 1,000'
4 - High
Avalanche risk
Sun, December 24th, 2023
Alpine
Above 2,500'
4 - High
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
Avalanche risk
Sun, December 24th, 2023
Alpine
Above 2,500'
4 - High
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
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

Folks who ventured into the backcountry yesterday to try and beat the storm got a front row seat to see the increasing winds transform the soft surface snow into widespread wind slabs. The visibility decreased as the snowfall began so we have limited observations other than the small wind slabs that individuals were triggering in Turnagain Pass. Glide avalanches continue to be very active, with three new releases on the east face of Seattle ridge yesterday.

Fresh glide avalanche releases along Seattle Ridge. Photo Peter Wadsworth 12.22.23 

Avalanche Problem 1
  • Storm Slabs
    Storm Slabs
  • Aspect/Elevation
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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

The rapid accumulation of up to 2′ of new snow from Friday afternoon through Saturday evening combined with sustained wind speeds of 50-75 mph and gusts over 100 mph will cause widespread avalanches. We recommend avoiding avalanche terrain, including runout zones from overhead avalanche paths. Prior to the latest storm we observed widespread surface hoar on the surface throughout the region. This can cause avalanches to release on lower angle slopes than typical and cause wider propagation, creating larger and more destructive avalanches. Due to the rate of new snow accumulation avalanches are likely even in areas protected from winds.

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 added stress of the new storm snow could awaken our buried weak layer, the facets over the Thanksgiving Crust. This layer is buried 4-6′ deep now, so any avalanche releasing on this layer would be very large and destructive. During periods of increased stress on the snowpack, like the current storm, it is unwise to poke the dragon.

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

Glide avalanche activity has increased significantly over the past week, with avalanches observed in Girdwood, Turnagain Pass, and Summit Lake. It is important to avoid spending time underneath glide cracks to avoid being caught in an unpredictable and very destructive avalanche.

Weather
Sat, December 23rd, 2023

Yesterday:  Cloud cover and increasing winds started to impact the forecast area yesterday morning. Winds speeds started off averaging 10-20 mph in the morning and gradually increased to averages of 50-75 mph overnight with gusts over 100 mph! Snowfall also started in the afternoon, and as of 4 am on Saturday roughly 1″ of water and 1′ of snow has fallen in Turnagain Pass and Girdwood. Temperatures climbed rapidly as the storm approached from the teens F in the morning to the mid 20s to low 30s F by the afternoon.

Today:  The storm is upon us! The highest snowfall rates are expected to continue through 9am, with 1-2″ of snowfall per hour. Snow will continue at a lower rate throughout the day with a total of roughly 2′ expected from Friday afternoon through Saturday night. Temperatures are expected to increase today, with rain line moving up to 900-1000′. Winds will also stay sustained out of the east with averages of 40-60 mph and gusts of 75 mph + through the afternoon. Winds will start to decrease and shift to the south overnight as the snowfall tapers off.

Tomorrow:  Sunday will be significantly less stormy than Saturday, but light snowfall is expected to linger throughout the day. We could see periods of broken cloud cover allowing some visibility. Temperatures will drop back down into the 20s F and continue to decrease into the teens and even single digits F overnight on Sunday. Winds will shift to the south but remain moderate with averages of 20-35 mph and stronger gusts.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 23 12 1.0 78
Summit Lake (1400′) 20 1 0.1 -*
Alyeska Mid (1700′) 24 9 0.8 72
Bear Valley – Portage (132′) 28 -* -* -*
Grouse Ck – Seward (700′) 24 5 0.5 45

* Snow depth sensor not currently working at Summit Lake, and precipitation gauge not working at Bear Valley.

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 17 ENE 38 108
Seattle Ridge (2400′) 20 SE 16 91

 

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.