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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, February 7th, 2018 - 7:00AM
Expires
Thu, February 8th, 2018 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is  MODERATE  above 2,500′ for triggering a slab 1-2′ deep. Additionally, watch for cornices along ridgelines and sluffs in steep terrain.

*For the periphery zones, such as Girdwood to Portage Valley, and Johnson Pass to Summit Lake, more caution is advised where a slab could be larger and more connected.

Check out the Summit Lake Summary  HERE.

Special Announcements

For all the Hatcher Pass users out there – Hatcher Pass Avalanche Center is having their annual fundraiser this Saturday night, Feb 10th at the Palmer Moose Lodge!! HPAC is a growing avalanche center for a high use zone with a high number of avalanche accidents in Alaska, they need your support! Click  HERE  for details.

Wed, February 7th, 2018
Alpine
Above 2,500'
2 - Moderate
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

Yesterday’s storm brought a few inches of snow to Girdwood and Turnagain Pass. Portage Valley near the lake got 8-10″. The winds died down in the early morning but the strong easterly winds from Monday night should not be forgotten today. We have been talking about the January 21st buried surface hoar layer for a couple weeks now. This layer has been responsible for the scattered avalanche activity over the past week and a half (including a large avalanche that occurred on Saturday on Twin Peaks). Slopes that got more wind effect/loading may now have a stiffer and more reactive slab over the buried surface hoar. Leeward slopes in the Alpine are the most suspect. The new snow may be may have covered up the surface signs of wind effect. It will be important to pay attention to where the snow feels stiff, looks pillowed or sounds hollow and watch for shooting cracks. Before committing to steep terrain, identify terrain traps like gullies, cliffs or rocks below and consider the consequences if even a small slab is released.  The new snow also fell on a fresh crop of surface hoar that developed last week. In Portage due to more new snow, even on lower elevation slopes, this February 5th surface hoar layer could be the layer of concern.  

*Deep Persistent Slab:  Weak snow can still be found near the ground at the upper most elevations in our forecast area, 3,000′ – 5,000′. Although triggering a Deep Persistent Slab is very unlikely, it is worth keeping in mind that poor structure does exist at the high elevations. 

The January 21st buried surface hoar found in a snow pit yesterday under some stiffer wind-affected snow. Photo: Sam Galoob

 One of the lower piles of debris from the Twin Peaks avalanche that occurred on February 3rd.

The February 5th buried surface hoar layer under a couple of inches of new snow at Turnagain Pass. Photo: Sam Galoob

 

Additional Concern
  • Dry Loose
    Dry Loose
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.
More info at Avalanche.org

Loose Snow Avalanches, ‘sluffs’: In most places the snow is very loose and ’sluffs’ easily on the steeper slopes. The new snow will add volume to these.  Watch 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.

Weather
Wed, February 7th, 2018

Yesterday was mostly cloudy and there were snow showers throughout the day. Temperatures were in the 30Fs at sea level and 20Fs at upper elevations. Winds were easterly and became light in the afternoon. Snow stopped in the evening and temperatures were in the low 20Fs.  

Cloud cover should diminish this morning and skies will become clear and sunny. There is the potential for some valley fog. Temperatures will be in the 20Fs and winds will be northerly 5-15 mph. Tonight temperatures drop into the teens and winds remain light and northerly.

Tomorrow and Friday should be clear and sunny with temperatures in the low 20Fs and light variable winds.   There is snow in the forecast for the weekend but the details are pretty uncertain. Stay tuned!  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 28    2 0.1  64
Summit Lake (1400′) 24  2  0.2    20
Alyeska Mid (1700′) 25  2 0.09    52

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  21  NE 6   26  
Seattle Ridge (2400′) 22 ENE   2   12  
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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.