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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sun, January 21st, 2018 - 7:00AM
Expires
Mon, January 22nd, 2018 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line

A  MODERATE avalanche danger exists on all aspects above 2,500′ for triggering a very large slab avalanche between 3 and 8+’ thick. These dangerous avalanches are more likely to be found at the high elevations, above 3,000′ and though tough to trigger, have very high consequences. Additionally, loose snow sluffs are getting faster and larger by the day and be aware of cornice falls along ridgelines. There is a LOW danger below 2,500′ where triggering an avalanche is unlikely.

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Sun, January 21st, 2018
Alpine
Above 2,500'
Moderate (2)
Avalanche risk
Treeline
1,000'-2,500'
Low (1)
Avalanche risk
Below Treeline
Below 1,000'
Low (1)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
Moderate (2)
Treeline
1,000'-2,500'
Avalanche risk
Low (1)
Below Treeline
Below 1,000'
Avalanche risk
Low (1)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Avalanche Problem 1
  • 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

It has been 4 days since a very warm storm slammed into our area dropping inches of rain up to 2,500′-3,000′ and several feet of snow above this. Since then, cold temperatures have cemented the wet snow into place and locked up any slab avalanche problem below 2,500′ and 3,000′ in areas. Therefore, our main concern lies at elevations above this where a ‘deep persistent slab’ problem exists. On these upper elevation slopes, a dense hard slab is sitting on a variety of weak layers in the mid pack (buried surface hoar) and old November facets near the ground. Triggering a deep slab will be more and more difficult as the days pass, but still possible. The most likely spots to trigger them are thin areas in the snow cover, often near rocks, or where the slope rolls over. High peaks, that see a lot of wind, can also be thinner and more likely to find a deep slab.

If you are headed out today, and the skies stay clear enough for easy travel above treeline, keep these points in mind:

–  This is ‘low probability, high consequence’ situation – often refered to a ‘Scary Moderate’ avalanche danger
–  Several tracks may be on a slope before someone finds a trigger spot
–  
Obvious signs of instability are not likely to been seen (such as whumpfing and cracking)
–  Remote triggering is possible 
–  This issue can simply be avoided by sticking to terrain below 3000’ (which is a good portion of terrain at Turnagain) or choosing low-consequence terrain in the alpine.

 Photo: Old cornice fall and avalanche from 1/16 near the Seattle Headwall region at 3,300′. Steep upper elevation slopes that have not slid are the ones most suspect for triggering a large slab avalanche.


Avalanche Problem 2
  • Dry Loose
    Dry Loose
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Dry Loose
Dry Loose avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.

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

Loose snow sluffs are rocketing down steep slopes as this cold weather continues to loosen and facet the surface snow. If choosing large steep terrain, watch your sluff. At mid elevations where 4-8″ of loose snow sits over a hard crust, sluffs are still likely and although not deep, will run quite far. 

Additional Concern
  • Cornice
    Cornice
Cornice
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
More info at Avalanche.org

Cornices have grown with the last storms, many have fallen, yet many have not. As always, give these features a wide berth and remember they can break further back than expected. A cornice fall at the high elevations could trigger a large avalanche on the slope below.

Weather
Sun, January 21st, 2018

Mostly sunny skies with thick mid-elevation and valley fog filled the area yesterday. Temperatures were chilly, generally in the teens at all elevations. Overnight temperatures have dropped into the single digits in many valley bottoms and mid-elevation locations. Winds over the past 24-hours have been light and variable.

Today, we should see a slow climb in temperatures as light Easterly flow brings clouds and a chance for snowfall. By late today, an inch of snow is possible at all elevations (including sea level). Overnight, another 2-4″ is possible, again at all elevations. Temperatures are slated to climb into the upper teens at most locations. Winds are expected to be 5-10mph from the Northeast.

Tomorrow, Monday, light snowfall is possible, but little accumulation is expected. Skies also may clear a bit. Winds look to remain light and temperatures will be on a downward trend.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 12   0   0   54  
Summit Lake (1400′) 10   0   0   15  
Alyeska Mid (1700′) 15   0   0    43

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 14   NE   3   12  
Seattle Ridge (2400′) 10    N calm   16  
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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.