Turnagain Pass RSS

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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, February 17th, 2019 - 7:00AM
Expires
Mon, February 18th, 2019 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is rising to  CONSIDERABLE  due to new snow and wind over the region. Naturally occurring storm snow avalanches are possible and human triggered avalanches are likely. Fresh wind slabs, 1-2′ thick, should be easily triggered on wind loaded slopes. Areas seeing over 10″ of new snow should expect slab avalanches in wind sheltered zones. Additionally, expect natural cornice falls, sluffs in steep terrain and wet loose avalanches below 500′.  Careful snowpack evaluation and conservative decision-making are essential if headed into avalanche terrain.  

SUMMIT LAKE / COOPER LANDING:   Central and Western Kenai mountains have seen 4-8″ of new snow with more on the way. This area has a very poor snowpack with multiple weak layers. New snow avalanches are a concern, but  triggering a slab 1-3′ thick, breaking deeper in the snowpack is a much more dangerous problem. Watch for whumpfing, shooting cracks and recent avalanches.

SEWARD/ LOST LAKE:   Avalanche danger has risen in this region as well and storm snow avalanches are likely today.  

BYRON GLACIER TRAIL Hikers:    Natural avalanches may send debris to valley floors. Recognize and avoid being in the runout paths. Also, the popular snow cave is very dangerous and unstable.  

Special Announcements

Avalanche danger is rising region-wide with new snow and wind. Hatcher Pass is calling for dangerous avalanche conditions – Please check their forecast HERE before heading this way.

GOOD LUCK to all the Iron Dog racers today!  Friends of the CNFAIC along with forecaster Aleph Johnston-Bloom connected with riders Friday night at the ‘Iron Dog Expo’.

Sun, February 17th, 2019
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
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. Dangerous avalanche conditions. 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
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.
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

For anyone that was out yesterday, you may have noticed that the several inches of fluffy low-density snow greatly improved the surface conditions. Despite the good news, warmer temperatures have set in along with heavier and denser snowfall. The National Weather Service has continued their Winter Weather Advisory and is expecting an additional 7-10″ of snow today. This has set the stage for touchy new snow avalanche issues. The old hard surface will provide a slick bed surface and yesterday’s low-density snow will make it tough for the new heavier snow to stick right away. This type of storm, which begins cold and ends warm, is what we call in the avalanche world an ‘upside-down’ storm. 

If the forecast verifies and we really do see 7-10″ of new snow we can expect:

  • Wind slab avalanches to be likely triggered on wind loaded slopes over 35 degrees
  • Storm slab avalanches in areas out of the wind where enough snow piles up to create a ‘slab’, typically around 10″ of new snow or more
  • Sluffs on sleep slopes to be likely and should be occurring naturally as well
  • Cornices to grow and fall, likely triggering an avalanche on the slope below

All this said, the size of these avalanches is directly related to the amount of new snow. With somewhere between 10-18″ of storm snow total by this afternoon, wind slabs should be in the 1-2′ range and storm slabs in the 8-16″ range. 

Storm Totals at mid-elevations as of 6am Sunday morning:
Turnagain Pass:    4-6″  
Girdwood Valley:   7-10″ 
Summit Lake:       4-6″  
Bear Valley (Whittier Tunnel):  7-10″  
Moose Pass:  3-4″  
Seward (Exit Glacier):  4-6″ 

Avalanche Problem 2
  • 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

The load of the new snow today will again add stress to underlying weak layers. As we have been hammering home, roughly 1-3′ below the snow surface sits a layer of buried surface hoar and in some areas facets in Turnagain Pass proper. Periphery zones such as Summit Lake and Johnson Pass harbor a weaker snowpack with a variety of weak layers. Although these layera have not been reactive lately, additional load may start to tip the balance. It is good to keep in mind that avalanches occurring today have the ability to step down and trigger a larger more dangerous slide. This is a more likely case for the Summit Lake and central Kenai mountains. 

Additional Concern
  • Glide Avalanches
    Glide Avalanches
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
More info at Avalanche.org

Glide avalanches may be on the move again with the warm up seen over the past two days. Many cracks are present on heavily traveled slopes. The best way to manage this problem is to identify and avoid traveling directly below them.

Weather
Sun, February 17th, 2019

Yesterday:   Overcast skies and light snowfall covered the region. Roughly 4-6″ of low-density snow has fallen over the past 24-hours at most mid-elevations with the exception of Girdwood Valley that recorded up to 8″. Ridgetop winds have been 15-25mph with gusts in the 40’s from the east. Temperatures remained in the teens to 20F yesterday along ridgelines before warming up overnight to the mid 20’sF and the low 30’sF at sea level.  

Today:    Moderate to heavy snowfall adding 7 to 10″ is expected over the course of the day with an additional 2-4″ overnight. Warm air is streaming in and pushing the rain/snow line up to ~500′. Ridgetop winds are rising and forecast to average in the 40-50’s mph from the east. Temperatures will sit in the mid 30’sF at sea level and in the mid 20’s F above treeline.  

Tomorrow:   Another frontal system moves in on President’s Day, which will continue our active weather pattern into the work week. Right now models are showing a cooling trend with this next system and snow to sea level.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 26   4-6   0.3   61  
Summit Lake (1400′) 25   4-6   0.3   27  
Alyeska Mid (1700′) 25   8   0.6   54  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 16   NE   13   47  
Seattle Ridge (2400′) 22   SE    15 36  
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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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