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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
Fri, February 2nd, 2018 - 7:00AM
Expires
Sat, February 3rd, 2018 - 7:00AM
Forecaster
Heather Thamm
The Bottom Line

The avalanche danger is  MODERATE  above 2,500′ for triggering a slab avalanche 1-3′ thick.  Slopes with signs of wind loading will be the most suspect for triggering an avalanche. Additionally, watch for easily initiated  loose snow avalanches (sluffs) on steep slopes.

Below 2500′ the danger is  LOW.   Remember low danger doesn’t mean  no  danger.  

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Fri, February 2nd, 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
  • Persistent Slabs
    Persistent Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    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

Roughly 1-3 feet of snow is covering a widespread layer of buried surface hoar across our region. In many places this snow is too loose to form a slab and the surface snow ’sluffs’ away with the weight of a person. In some areas of the alpine this week, where winds have created a denser more cohesive slab, the buried surface hoar has been reactive to both skiers and snowmachiners. Yesterday a group on Eddies triggered a slab remotely from the ridge above the steep Southwest face. This slab was 2 feet deep by 80 feet wide and ran to valley bottom. In general these slabs have been small in size, but large enough to take someone for an undesirable ride over rocks or possibly bury a person in a terrain trap. With that said a lot of people have been riding and skiing in steep terrain without incident where winds didn’t affect the snow or form a slab. In the periphery areas of Girdwood to Placer Valley where more sustained winds affected this zone, slabs may be larger and more connected. Basically this problem requires the ability to assess the snow and terrain as you venture into the mountains with an understanding that triggering even a small soft or hard slab could have consequences in the wrong place. Identify terrain traps like gullies, cliffs or rocks below before committing to a steep slope.  

Snowmachine triggered avalanche in God’s Country near Pyramid on a SE aspect that was triggered remotely on Sunday.

 

 Natural slab avalanche on Wolverine’s South face that occurred near the end of the storm last Friday. This is a good example of the potential size of a persistent slab and the kind of steep wind loaded terrain it could found in.

 

Hand pit on Sunburst at 3300′ on West aspect revealed the very large and well preserved buried surface hoar that is suspect in places with a consolidated slab. 

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

Expect loose dry snow to be fast moving in steep terrain and entrain more snow as it picks up speed. This quick moving surface snow can knock a person off their feet. Be aware of the consequences below you and manage your ‘sluff’ by slowing down and letting it pass you. 

Sluffing around ski tracks on SW face of Sunburst

Additional Concern
  • Deep Persistent Slabs
    Deep Persistent Slabs
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.
More info at Avalanche.org

Two and a half weeks have passed since a storm delivered heavy rain and strong wind to our region and caused a natural deep slab avalanche to release on the Southwest face of Sunburst. Immediately following this event the snowpack stabilized below 3000’ due to wet snow freezing into a solid strong layer. Observations over the last few weeks have found poor structure  (basal facets) near the ground in targeted thinner areas of the snowpack above 3000’. This leaves us with some degree of uncertainty in the upper most reaches of our forecast zone. Triggering a deep slab avalanche is becoming more unlikely with time, but is not impossible on a large steep slope combined with thin trigger spots to initiate such an avalanche. Should you find yourself approaching a 4000’ peak, remember this is a hard to trigger, low likelihood problem that deserves acknowledgment of it’s potential. 

Weather
Fri, February 2nd, 2018

Yesterday skies were clear and sunny with a temperature inversion, single digits (F) in valley bottoms and low teens (F) near ridge tops. Winds were light from the East in the morning becoming light and variable the rest of the day. No precipitation has occurred this week.  

Today looks similar with clear skies and an inversion causing temperatures in the single digits in valley bottoms. Temperatures may reach the low 20F’s in mid and upper elevations. Winds are expected to remain light and variable with no potential for precipitation today.  

A strong high pressure over mainland Alaska is expected to dominate our region through the weekend with continued clear skies and cold temperatures. Offshore winds may be present in Coastal areas and gap zones over the weekend. The first chance for this high pressure pattern to start shifting is Sunday into Monday, but confidence remains low at this point.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 14   0   0   64  
Summit Lake (1400′) -1   0    0 17  
Alyeska Mid (1700′) 10   0   0   51  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 11   Variable   5    20
Seattle Ridge (2400′) 12   Variable     4   16  
Observations
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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
Glacier District
Johnson Pass
Closed
Placer River
Closed
Skookum Drainage
Closed
Turnagain Pass
Closed
Closed as of May 1. Thanks for a fun, safe season!
Twentymile
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Carter Lake
Closed
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Closed
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Closed
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Closed
Snug Harbor
Closed
South Fork Snow River Corridor
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Closed

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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.