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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Wed, January 3rd, 2018 - 7:00AM
Expires
Thu, January 4th, 2018 - 7:00AM
Forecaster
Aleph Johnston-Bloom
The Bottom Line

The avalanche danger is  CONSIDERABLE  on all aspects and elevations above 1000′. A weak layer of snow has been overloaded by heavy snow, rain and strong winds over the past few days. Natural avalanches are still possible and human triggered slab avalanches are likely on slopes steeper than 30 degrees.  Remote triggered avalanches from below slopes, next to and above are also possible.  Deep slab avalanches remain a concern above 3000′.  Cautious route-finding and conservative decision-making are essential.  

Below 1000′, where little snow exists, the avalanche danger is  MODERATE  where an avalanche releasing from above could send debris through steep channeled terrain into this zone.

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Wed, January 3rd, 2018
Alpine
Above 2,500'
Considerable (3)
Avalanche risk
Treeline
1,000'-2,500'
Considerable (3)
Avalanche risk
Below Treeline
Below 1,000'
Moderate (2)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
Considerable (3)
Treeline
1,000'-2,500'
Avalanche risk
Considerable (3)
Below Treeline
Below 1,000'
Avalanche risk
Moderate (2)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Avalanche Problem 1
  • Storm Slabs
    Storm Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    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.

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

Yesterday there was enough visibility to look around and see the evidence of a natural avalanche cycle in the advisory area. This confirmed that the new snow, strong winds and rain overloaded the weak layer of buried surface hoar and near surface facets. The connected slab avalanches on Eddies were the most notable in Turnagain Pass. The crowns extended along terrain features. Today any additional snow will add weight to the storm slab over the weak layer. Surface hoar and near surface facets are buried now by 1-3′ of snow. Slopes that didn’t avalanche naturally could now be teetering on the brink of failure just waiting for a human trigger. Conservative decision-making is essential today. Slopes harboring surface hoar and near surface facets may also be triggered remotely. 

There really is a smorgasbord or buffet of issues to consider if you decide to venture out today. Strong winds have loaded leeward aspects and potentially created winds slabs as well as growing cornices. At lower elevations if there is still saturated snow wet avalanches are possible in steep terrain. As temperatures cool these will be less likely and the crust that was noted to 2300′ may have grown stouter and more supportable. The “railroad skiing”/ breakable crust will hopefully improve. In addition, pay attention to whether or not the new snow that falls to today bonds to the crust. 

Recent natural avalanches on Eddies. 

The buried surface hoar is the suspected weak layer and our layer of concern. 

 

Avalanche Problem 2
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
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.

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

In the alpine, above 3,000’, the storms over the past few days have added additional load to slopes that already have a hard slab, 3-5+ feet thick, is sitting on top of weak sugary snow (basal facets) near the ground.  We have been talking about this for weeks now.  At these elevations, human triggered, large and dangerous deep slab avalanches ARE still possible. This is a high consequence avalanche problem that is impossible to outsmart and can take a long time to heal. Keep this in mind as improving visibility in the next few days may allow for travel to the Alpine. It is really important to remember that triggering an avalanche in the upper layers of the snowpack on may then initiate a deep slab avalanche. Cautious route-finding is essential. This includes thinking about the remote trigger potential from below. 

Weather
Wed, January 3rd, 2018

Yesterday was mostly overcast with a few breaks in the cloud cover. There were rain and snow showers on and off throughout the day. Rain/snowline went as high as 2300′ in the morning and then crept back down to around 1500′. Temperatures were in the mid to high 20Fs in the alpine, 30Fs at mid-elevations and 40Fs at sea level. Overnight the temperatures cooled a bit. Winds were easterly 15-25 mph gusting into the 50s, slowing down in the evening.  

Today will be mostly cloudy with rain and snow showers. Rain/snowline is forecasted to be around 800′. 1-5″ of snow, .25 water is possible throughout the day.  Temperatures will be in the mid 30Fs to mid 20Fs. Winds will be light and variable. There is a chance of continued snow showers into the evening as temperatures cool into the teens. Skies will clear overnight.  

Thursday and Friday look to be clear and sunny with calm winds and temperatures in the 20Fs.  There is snow in the forecast for the weekend but the timing, amount and temperatures are still TBD. Stay tuned and keep thinking cold powder thoughts!  

*Seattle Ridge anemometer is rimed.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′)  35  rain .2  46
Summit Lake (1400′)  33 rain .2 14  
Alyeska Mid (1700′)  34 .5  1.5 36  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 25   ENE   25   62  
Seattle Ridge (2400′) 28    *n/a  *n/a  *n/a
Observations
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Date Region Location
05/06/20 Turnagain Avalanche: Pastoral Peak, north face
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04/04/20 Turnagain Observation: Pete’s North
03/26/20 Turnagain Avalanche: Tincan – Proper (SW facing)
03/26/20 Turnagain Avalanche: Seattle Ridge
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Riding Areas
Updated Fri, May 01st, 2020

Status of riding areas across the Chugach NF is managed by the Glacier and Seward Ranger Districts, not avalanche center staff. Riding area information is posted as a public service to our users and updated based on snow depth and snow density to prevent resource damage at trailhead locations. Riding area questions contact: mailroom_r10_chugach@fs.fed.us

Area Status Weather & Riding Conditions
Glacier District
Johnson Pass
Closed
Placer River
Closed
Skookum Drainage
Closed
Turnagain Pass
Closed
Closed as of May 1. Thanks for a fun, safe season!
Twentymile
Closed
Seward District
Carter Lake
Closed
Lost Lake Trail
Closed
Primrose Trail
Closed
Resurrection Pass Trail
Closed
Snug Harbor
Closed
South Fork Snow River Corridor
Closed
Summit Lake
Closed

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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.