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Turnagain Pass Avalanche Forecast RSS

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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Tue, January 23rd, 2018 - 7:00AM
Expires
Wed, January 24th, 2018 - 7:00AM
Forecaster
Aleph Johnston-Bloom
The Bottom Line

The avalanche danger remains  MODERATE  on all aspects above 2,500′. Triggering a very large slab avalanche between 3 and 8+ feet thick is possible. These deep slab avalanches are more likely to be found above 3,000′. Additionally, loose snow avalanches (sluffs) should be expected on steep slopes and be aware of cornice falls along ridgelines. There is a  LOW  danger below 2,500′ where triggering a slab avalanche is unlikely but sluffs are possible.

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Tue, January 23rd, 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

Deep persistent slab avalanches remain the primary concern in the advisory area. We know the snowpack structure above 3000′ is poor, there is a dense hard slab (3-8+ feet thick) sitting on a variety of weak layers in the mid pack (including buried surface hoar) and old November facets near the ground. Triggering a deep slab is becoming more and more difficult, but is still possible. The most likely trigger spots are thin areas in the snow cover, often near rocks, or where the slope rolls over. High peaks, that see wind, can also be thinner and it is more likely to find the trigger point for a deep slab where some of the slab has been stripped away. 

Key points that keep in mind about our current snowpack:

–  We have a ‘low probability, high consequence’ situation at the upper elevations for deep slab avalanches. If you do trigger an avalanche it could be huge and unsurvivable.
–  Obvious signs of instability are not likely to been seen before a deep slab is triggered (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

Snow pit on Sunburst that illustrates the buried weak layers in the snowpack above 3000′.

Avalanche on Magnum that ran during the storm last week. Slopes that did not slide at upper elevations are suspect. Photo: Andy Moderow

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

The additional snow we picked up over the past two nights, 5-7″ in Girdwood and 1-2″ at Turnagain landed on already loose surface snow. The surface snow has been quick to sluff on steep slopes and the new snow will just add to the volume of the sluffs (loose snow avalanches). As the sluffs grow larger they could definitely catch a person by surprise, especially on steep, committing terrain.  

On the flip side if you encounter stiff snow along ridgelines be on the look out for small pockets of wind slab. The wind has been pretty light but loose snow blows around easily. Pay attention to changing conditions.

The loose surface snow, comprised of surface hoar and near surface facets, which is now buried under the new snow.  Photo Allen Dahl

 

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
Tue, January 23rd, 2018

Yesterday skies were broken with patches of blue and some valley fog rolling in. Temperatures were in the teens to low 20Fs. Winds were light and easterly. Overnight the region picked up 1-2″ of snow.  

Today will be mostly to partly cloudy with snow showers in the morning, 1-2″ of snow possible. Temperatures will be in the teens and low 20Fs. Winds will be light and shift to the NW. Winds will pick up overnight and skies will clear. Temperatures are forecasted to start falling overnight.

Wednesday looks to be clear, sunny and cold with highs in the single digits. Outflow winds could increase during the day. Overnight temperatures will drop below 0F. Thursday day may even be a bit colder and then temperatures should rise Thursday night as a low moves into the gulf and brings a chance of snow showers for the weekend.  

*Seattle Ridge anemometer is rimed over and not able to collect wind data.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′)  21  1 0.1 57  
Summit Lake (1400′) 14   1   0.1 16  
Alyeska Mid (1700′) 15   2 0.1   47  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  20  ENE  5  15
Seattle Ridge (2400′)  24 *n/a   *n/a   *n/a  
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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
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Skookum Drainage
Closed
Turnagain Pass
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Closed as of May 1. Thanks for a fun, safe season!
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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.