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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
Sat, March 10th, 2018 - 7:00AM
Expires
Sun, March 11th, 2018 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line

A CONSIDERABLE avalanche danger exists at all elevations and on all aspects in the mountains from Girdwood to Seward. Large slab avalanches composed of 2-4′ of yesterday’s storm snow are likely to be triggered on slopes steeper than 30 degrees. These may be triggered from a distance or from the bottom of a slope. Larger slabs are possible that break in overloaded weak layers in the snowpack. Cautious route finding and conservative decision-making will be essential for safe backcountry travel this weekend.

Summit Lake area – see today’s  snowpack and avalanche summary.

*A  Special Avalanche Bulletin  has been issued through the National Weather Service.  This will remain in effect through 6pm Sunday.  

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Sat, March 10th, 2018
Alpine
Above 2,500'
Considerable (3)
Avalanche risk
Treeline
1,000'-2,500'
Considerable (3)
Avalanche risk
Below Treeline
Below 1,000'
Considerable (3)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
Considerable (3)
Treeline
1,000'-2,500'
Avalanche risk
Considerable (3)
Below Treeline
Below 1,000'
Avalanche risk
Considerable (3)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Avalanche Problem 1
  • Storm Slabs
    Storm Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    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

A combination of several feet of new snow, powder starved people and clearing skies this weekend all equal a chance for an avalanche incident. Between 2-4′ of new snow fell over the region Thursday night through yesterday morning. Many natural avalanches were seen, yet at the same time, many popular areas were obscured in clouds and little information exists. Therefore, we know the new snow that rapidly loaded slopes avalanched immediately in some areas, but we are suspect that more slopes remain intact and could be lying in wait for a human to tip the balance. How quickly the storm snow will bond to the old surface is uncertain and today (and tomorrow) are days to let the slopes adjust. 

Mid-elevation storm totals from Thursday through 6am Saturday:

  • Turnagain Pass: 26-36 inches 
  • Girdwood: 32-42 inches 
  • Portage: 26-36 inches 
  • Summit Lake: 12-16 inches

If choosing to head to the backcountry, keep in mind that triggering large slabs is likely and remote triggering from a ridge or below a slope is also likely. These are unmanageable and dangerous avalanches. Whumpfing (collapsing) and shooting cracks will be keys the snowpack is very unstable. We need to watch our terrain (keeping to the flats and low angle slopes) and watch the other groups around us. Types of storm snow avalanches that could be triggered today:

–  Wind slabs:  Slabs up to 2-4+ feet thick could be found and triggered on slopes that saw wind loading during the storm. They are most likely to be triggered along ridgelines and in cross-loaded gullies.

–  Storm slabs:  Soft slabs (2-3 feet deep) are expected in areas out of the wind, similar to the photos below. With various weak layers under the new snow, this can cause a storm slab that ‘seems’ to be bonding with the old snow to remain unstable as the weakness may lurk just below the old/new interface.

–  Cornice falls:  Cornices have grown and cornice falls are likely. They are a concern in themselves, but could also trigger a wind slab or storm slab below.

Clearing skies? Sunshine can quickly de-stabilize slopes with enough solar warming. This could occur today if skies open up, but is more likely to be an issue tomorrow. Be suspect of slopes that are ‘baking’ in the sun, including being under these as they could release naturally.

Natural storm slab avalanches that released yesterday morning due to the rapid loading of 2-3′ of new snow. These are small slopes along the cut bank of Granite Creek on the Southern end of Turnagain Pass. Larger terrain out of view in the clouds likely experienced similar avalanche activity.


Avalanche Problem 2
  • 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

Several widespread persistent weak layers exist within our snowpack region-wide, including buried surface hoar and facets 1-2′ below the old surface. It’s been over 6 weeks since a significant storm has added weight to these layers and a lot of uncertainty exists around how well they will adjust to the new load. Avalanches failing in these layers are possible, along with a storm related avalanche ’stepping down’ into these layers. In this case a much larger avalanche is possible. Areas on the South end of Turnagain Pass and the Summit Lake area are most suspect for having deeper weak layers release.

The slabs that released in the lower elevations yesterday, pictured above, were technically failing in facets below the storm snow (pictured below). The snowpack structure changes with elevation and upper slopes could be overloading other weak layers. Nonetheless, weak layers exist at all elevations.


Weather
Sat, March 10th, 2018

Snowfall continued through yesterday morning before giving way to broken skies and intermittent snow showers. Between 5-10″ of snow has been recorded over the past 24-hours with only an inch or so in the Summit Lake area. Ridgetop winds were Easterly in the 15-25mph range with stronger gusts and temperatures were in the teens along ridgelines and the mid 30’sF at sea level.  

For today, Saturday, we are expecting mostly cloudy skies with light snow showers to add 3-5″ of snow (.3″ water) and another 2-3 tonight (.2 water). Ridgetop winds should remain from the East in the 10-20mph range. Temperatures will stay in the teens along ridgetops and near 30F at sea level.  

An active weather pattern will continue to bring periods of precipitation (mostly in the form of snow) with periods of clear skies this week. Sunday there looks to be a big enough break in clouds to get longer periods of sunshine.  

*Snowfall numbers are estimated  
** Seattle Ridge anemometer (wind sensor) is rimed and not reporting

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 25   *5-6″     0.5     93  
Summit Lake (1400′) 27   *1″ 0.1 36  
Alyeska Mid (1700′) 26   *10″   1   85  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 16   NE   22   66  
Seattle Ridge (2400′) 23   **n/a   **n/a     **n/a    
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Riding Areas
Updated Mon, October 26th, 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

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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.