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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, March 19th, 2017 - 7:00AM
Expires
Mon, March 20th, 2017 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger could rise to MODERATE today on all aspects and elevations. New snow is falling on very weak snow. Triggering storm slabs and loose snow avalanches will be possible in steep terrain. Watch for changing conditions. Avoid travel on or under cornices and give glide cracks a wide berth.  

Summit Lake, Johnson Pass, Lynx Creek and the Northern portion of Girdwood Valley: There is still a chance for triggering a deeper more dangerous avalanche in these areas. Read the  Saturday Summit Summary  HERE.

Sun, March 19th, 2017
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
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
  • Storm Slabs
    Storm Slabs
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.
More info at Avalanche.org

After 25 days of no snow in the core advisory area the spell has finally been broken and light snow started falling last night. We received 1-2″ overnight and 4-9″ is forecasted to fall today. This combined with ENE winds gusting up into the 30s may create storm slab conditions on steep slopes as it accumulates on very weak surface snow.  The likelihood of triggering a loose snow avalanche (sluff) will also increase with the new snow. An observer in the upper Skookum Valley (which received 5-9″ on Thursday morning in a sneaky storm) reported natural sluffs as this current system moved in. This zone with more new snow warrants extra caution as slabs/sluffs will be larger. Today’s storm will again favor this area. 

Today it will be important to pay attention to how the new snow is bonding to old snow surfaces. The snow is landing on near surface facets, surface hoar, sun crusts, and wind crusts. Watch for changing conditions, cracking, collapsing and blowing snow. 

Keep in mind these additional concerns if venturing out into the mountains today and remember to practice safe travel techniques. 

Wind Slabs:  Small fresh wind slabs are also possible near ridgetops on leeward terrain. Old stiff wind slabs, such as the one triggered on Seattle ridge last weekend, will be stubborn to trigger. Smooth pillowed snow on steep unsupported features or in rocky areas will be the most suspect places to initiate an old wind slab.

Glide AvalanchesGlide cracks continue to slowly open above popular terrain on Seattle Ridge and in other zones across the advisory area. These may release at any time. Minimize exposure time spent under visible cracks.

Cornices: Cornices should always be given an extra wide berth if traveling along a corniced ridge.  Like glide cracks, minimize your exposure time spent under these backcountry bombs. 

Persistent Slabs and Deep Slabs: There are various weak layers in our thin snowpack. Buried surface hoar sits 1-3+’ below the surface and faceted snow sits in the mid and base of the pack. Due to the lack of changing weather over the past three weeks, these weak layers (with varying degrees of strength) are in a ‘dormant stage’.  Although unlikely, an avalanche breaking deeper in the pack isn’t completely out of the question in areas such as Johnson Pass, Lynx Creek and on the Northern side of Girdwood Valley (near Crow Pass). Observers continue to find poor structure. The buried weak layers will require monitoring as the load on top increases incrementally or if we get a significant warm-up. 

An example of surface conditions in the Girdwood Valley. Near surface facets on sun crust. 

 

Weather
Sun, March 19th, 2017

Yesterday started out clear and sunny. Clouds rolled in throughout the day becoming mostly cloudy by the evening. Temperatures rose into the teens at ridgetops and low 20Fs in the valleys. Light snow stared falling in the evening. 1-2″ fell overnight. ENE winds increased gusting into the 30s.  

Today will be mostly cloudy with temperatures in the 20Fs. ENE winds will continue blowing 10-20 mph with gusting again into the 30s. 4-9″ of snow is forecasted to fall throughout the day with a chance of snow overnight. Tomorrow will be partly sunny with warmer temperatures. There is another chance of snow showers on Tuesday.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 15    .1  1  60
Summit Lake (1400′)  15  .05  2  57
Alyeska Mid (1700′)  13 .1    1  29

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  12  ENE 10    32
Seattle Ridge (2400′)  13  ENE  5 16  
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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.