Turnagain Pass RSS

Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, February 3rd, 2018 - 7:00AM
Expires
Sun, February 4th, 2018 - 7:00AM
Forecaster
Heather Thamm
Avalanche risk The Bottom Line

A generally LOW avalanche danger exists in the mountains surrounding Turnagain Pass. Although triggering an avalanche large enough to bury a person is unlikely, isolated slabs 1-2′ deep can still be found in very steep or wind loaded terrain. LOW danger does not mean No Danger and evaluating the consequences of the terrain will be important before committing to a slope. Additionally watch for cornices or triggering loose surface snow that can be fast moving and knock a person over.  

In the periphery zones of Girdwood to Portage Valley, and Johnson Pass more caution is advised where a slab could be larger and more connected.  

Check out the Summit Lake Summary HERE.  

Sat, February 3rd, 2018
Alpine
Above 2,500'
1 - Low
Avalanche risk
Treeline
1,000'-2,500'
1 - Low
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
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.
Avalanche Problem 1
  • Persistent Slabs
    Persistent Slabs
  • 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.
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 like to think of the danger scale on a continuum and we have inched our way towards LOW in Turnagain Pass. Slope testers (skiers and riders) have been in full force over the last week pushing further into the mountains. There have been a handful of slabs triggered without incident this week and it’s not impossible to find unstable snow. Keep the following problems in mind as you travel today. 

Persistent Slab: These slabs have been in places where the snow has been stiffed by wind and are failing on buried surface hoar about 1-2’ deep. Most of the activity has been relatively small, but big enough to take someone for an undesirable ride over a cliff or sweep them into a terrain trap. These slabs could be either soft or supportable to the weight a person and will be located on slopes that are more wind loaded. In fact they may resemble a windslab-type look: smooth pillow-shaped and isolated to a specific terrain feature. In the periphery areas of Girdwood, Placer Valley and Johnson Pass more caution is advised due to slightly deeper snow depths, more sustained winds, and the possibility of a larger slab. Identify terrain traps like gullies, cliffs or rocks below before committing to a steep slope.  

 *Triggering a Deep Persistent Slab is unlikely at this time and has become an outlier.  Due to poor structure (basal facets) that can still be found near the ground in the upper reaches of our terrain, 3000’ – 5000’ zone, it is worth keeping in the back of your mind. At this time cold temperatures and benign weather are helping the stability of our snowpack. 

Loose Snow: In many places the snow is too loose to form a slab and the surface snow ’sluffs’ away with the weight of a person. Expect loose dry snow to be fast moving in steep terrain and don’t be surprised if it knocks you over as it picks up speed. Manage your ‘sluff’ and be aware of the consequences below you.

Cornices: Cornices are unpredictable and can break further back along a ridge than expected. Give these features plenty of space. 

Soft slab triggered remotely on Wednesday along the upper ridge of Eddies. This slab was about 80′ wide and slid into very steep Southwest facing terrain below. Photo by George Creighton

 

This is an older wind slab on Raggedtop, Girdwood, and is a good example of where a slab could be more supportable due to stronger winds. Photo by Mike Ausman

 

Loose surface snow and cornices on the Southwest face of Magnum. 

 

Weather
Sat, February 3rd, 2018

Yesterday skies were clear and sunny with a temperature inversion, single digits (F) in valley bottoms and low teens (F) near ridge tops. Ridge top winds averaged around 10 mph with some gusts in the 20’s. No precipitation has occurred in over a week.  

Winds are expected to be light from the East (5-15mph) and skies will be clear and sunny. Temperatures look similar to yesterday with slightly warmer ridge top temps, low 20F’s and precipitation is not likely.    

A blocking high pressure system over mainland Alaska is expected to persist through the weekend with continued clear skies, cold temperatures and light winds. The long term forecast is calling for similar weather this week with a possibility of a pattern change mid to late week, but a lot of uncertainty still remains.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 17   0   0   63  
Summit Lake (1400′) -1   0   0   17
Alyeska Mid (1700′) 16   0   0   50  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 14   ENE   7   23  
Seattle Ridge (2400′) 16   SE   10   25  
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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.
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