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Issued
Sat, December 16th, 2023 - 7:00AM
Expires
Sun, December 17th, 2023 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE at all elevations today. Strong winds and snowfall will impact the forecast area, with the stormy weather becoming most intense in the afternoon and evening. Natural and human triggered wind slab avalanches 1-3′ deep are likely on exposed features. In addition, a layer of weak snow buried 3-5′ deep could produce very large avalanches. At low elevations the precipitation will likely fall as rain later in the day, which may cause wet avalanches on steep slopes.

Roof Avalanches: With temperatures increasing today and rain likely to fall near sea level, we could see another round of roof avalanches. Be extra cautious around loaded roof lines and be sure to keep kids, cars, and canines out from underneath eaves.

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.

Headed to Seward, Summit Lake, or the Chugach Front Range today? Check out our new weekend outlook pages under the ‘Forecast’ tab for more information on these areas.

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'
3 - Considerable
Avalanche risk
Sun, December 17th, 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
Sun, December 17th, 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

During the last major dump of snow on Wednesday (~18″ in Turnagain Pass) there was widespread avalanche activity within the new storm snow in the forecast area (see obs from Tincan and Sunburst). We have not heard any reports of new avalanche activity on the facets above the Thanksgiving crust this week.

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

Another storm front is approaching the forecast area today, bringing strong winds averaging 30-45 mph with gusts to 60+ mph and 4-8″ of new snowfall. In more coastal areas, like Portage and Placer, we could see closer to 8-14″ of new snow today. Unfortunately, this storm front will also bring warming temperatures which are expected to reach the low 30s F at lower elevations and bring rain line up to 600-900′.

In the majority of the forecast area the most concerning avalanche problem today will be wind slabs 1-3′ deep which can form rapidly due to strong winds. These most commonly form along ridgelines and convex features at and above treeline. We recommend using small, steep test slopes to check whether wind slabs are reactive in the area you are travelling before committing to bigger terrain.

Given the low visibility and potential for rain at low elevations today it seems unlikely that many folks will venture into higher elevations in coastal areas, like Placer Valley. However, it is worth noting that due to the higher rates of snowfall expected there it is possible that avalanches could release within the new snow as storm slabs. Since this area just opened for motorized use yesterday we have very limited information and recommend approaching avalanche terrain cautiously.

Wet Loose: There is a lot of light dry snow on the surface at lower elevations right now, which could become unstable rapidly if rain starts to fall on dry snow. Wet loose avalanches are typically not large enough to bury a person but they can push you around if you get caught in one.

 

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

We are still concerned about a weak layer of facets above the Thanksgiving crust that are now buried 3-5′ deep. Due to the potential for very large avalanches releasing on this weak layer we are continuing to monitor and test it throughout the forecast area. In the past week we have not seen any observations of natural or human triggered avalanches releasing on this layer, but it often takes awhile for stress to build on top of a layer like this before we see widespread avalanches. Digging a snowpit and checking out the snowpack structure in the area you are travelling can provide some information, but with the layer now buried roughly 3+’ deep our standard stability tests (Extended Column Test, Compression Test) struggle to provide consistent and accurate results. The only way to know for sure that you will not trigger a large avalanche on this weak layer is to be patient and stick to low angle terrain.

Snowpack structure near treeline in Lynx Creek from earlier this week. Photo 12.13.23

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

Glide avalanches continue to release in the forecast area, with the most recent being on Thursday on the southern end of Seattle Ridge. Staying out from underneath glide cracks is the only way to avoid these unpredictable avalanches.

Weather
Sat, December 16th, 2023

Yesterday: Cloudy skies and moderate to light snowfall with 1-4″ of new snow falling down to sea level. Winds picked up during the middle of the day with averages of 15-20 mph and gusts up to 36 mph. Temperatures stayed just cold enough to keep snow falling down to sea level.

Today: A storm front is approaching the forecast area today, bringing strong winds and snowfall starting in the late morning and becoming most intense in the afternoon and evening. Winds are expected to average 30-45 mph with gusts to 60 mph possible this afternoon. For snowfall Girdwood and Turnagain Pass should receive 4-8″ of new snow today, with Portage and Placer receiving closer to a 12″. Along with the strong winds and precipitation will come rising temperatures into the low 30s F at lower elevations. This will move rain line up to 600-900′ this afternoon.

Tomorrow: Light snowfall is expected to continue throughout the day on Sunday with 1-3″ of new snow and cloudy skies expected for most of the day. Temperatures will drop back down into the 20s F on Sunday, which will bring snowfall down to sea level again. Winds are expected to shift to the SW and remain elevated with averages of 20-30 mph and gusts of 40 mph.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 21 1 0.1 73
Summit Lake (1400′) 15 2 0.2 34
Alyeska Mid (1700′) 22 4 0.32 63
Bear Valley – Portage (132′) 26 3 0.28
Grouse Ck – Seward (700′) 21 4 0.3 36

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 15 ENE 9 36
Seattle Ridge (2400′) 18 SSE* 4* 18*

*Seattle Ridge weather station is rimed over with several hours overnight missing from wind data.

Observations
Recent Observations for Turnagain Pass
Date Region Location
02/24/24 Turnagain Observation: TinCan Backdoor/ Center Ridge
02/22/24 Turnagain Avalanche: Lynx Creek
02/22/24 Turnagain Observation: Turnagain, Seattle, Mt Ascension
02/21/24 Turnagain Observation: Tincan Trees
02/21/24 Turnagain Observation: Sunburst
02/20/24 Turnagain Avalanche: Tincan
02/20/24 Turnagain Observation: Seward Highway across from Johnson Pass TH
02/19/24 Turnagain Avalanche: Base of Seattle Ridge
02/18/24 Turnagain Observation: Lynx creek
02/18/24 Turnagain Observation: Tincan Trees
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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.