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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
Tue, December 24th, 2019 - 7:00AM
Expires
Wed, December 25th, 2019 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line

The avalanche danger remains MODERATE above 1,000′. Human triggered wind slab avalanches are possible on slopes over 30 degrees with wind deposited snow. These are likely to be 4-12″ thick and could be more touchy than expected. Additionally, very shallow soft storm slabs (4-8″ thick) may be found in areas out of the wind and later in the day as a few inches of snow accumulates. As always, watch your sluff and avoid being under glide cracks.

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Tue, December 24th, 2019
Alpine
Above 2,500'
Moderate (2)
Avalanche risk
Treeline
1,000'-2,500'
Moderate (2)
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
Moderate (2)
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
  • Wind Slabs
    Wind Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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

Another round of snow is expected today, Christmas Eve- and this time, should hit most of Southcentral. Light snowfall should begin late morning in the Girdwood/Turnagain Pass area. Between 2-4″ is forecast to accumulate by the evening with 3-6″ falling in the higher terrain. Easterly ridgetop winds are slated to pick up into the teens and possibly up to 25mph. These are perfect wind slab development speeds in areas the winds are transporting snow.

Image from our all-star weather forecasters at the National Weather Service!


The primary avalanche concern will be wind slab avalanches.
These are expected to be touchy as they are not likely to bond well to the surface underneath. Remember the clear weather last weekend? This old snow surface, blanked in surface hoar and pictured below, is now buried under Sunday’s snow and will become buried a bit more with today’s snow. What to watch for if headed out:

  • How much snow has fallen since the weekend?
  • Have the winds moved the snow around and loaded the slope you intend to ride?
  • If skies break for a moment, are there any recent avalanches?
  • Is the snow stiffer over softer snow? Does it crack or collapse under you?

These are all signs the new snow is unstable and could avalanche on slopes steep enough to slide. Due to the smaller amounts of new snow, slabs should be on the smaller side- 4-12″ thick. The size of a potential avalanche is completely dependent upon the new snow amounts and size of the terrain, or slope in question. Quick hand pits are also good ways to test if the recent layer of snow is bonding to the old surface.

Storm slabs:  In the treeline band and areas out of the wind with over 6″ of new snow since the weekend, shallow soft slabs may be found.

Sluffs:  With the new low density snow, triggering sluffs on steep slopes should be fairly easy.

 



 



  


Solstice Surface Hoar photos: Left, Allen Dahl and right, Troy Tempel. 

Avalanche Problem 2
  • Glide Avalanches
    Glide Avalanches
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
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.

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

Glide cracks are likely obscured and difficult to see with the poor visibility and light snow covering cracks. They are still there however, hanging out under rocky cliffs and rolling slopes. There are two in the Tincan Trees we know of and many in the Corn Biscuit and Magnum area. As always, limit/avoid time under these as they can release spontaneously at any time.

Glide cracks in the Gold Pan region of Turnagain Pass. This area is in the upper north zone of Bertha Creek, behind the Corn Biscuit/Magnum ridges. Thank you to Eric Parsons for the photo!

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

In the high Alpine, above 3,000′, we have been tracking a weak layer of snow sitting near the base of the snowpack.  This weak layer is buried anywhere from 1-6+ feet deep due to the variable wind loading during the December storm cycles. There has been a lingering concern a person could trigger a large avalanche if they hit the wrong spot. Data is pointing to this layer being dormant as this point. However, we will keep it on the radar especially in areas where the overall snowpack is shallower – towards the southern end of Turnagain Pass, in Summit Lake and in the Crow Pass terrain north of Girdwood.

Weather
Tue, December 24th, 2019

Yesterday:  Partly cloudy skies were over the region with a few snow flurries here and there. Only a trace of snow is being reported at the snow stations. Ridgetop winds have been light from the east, blowing in the 5-10mph range. Temperatures remained in the upper teens along ridgetops and low teens at 1,000′.

Today:  Cloudy skies are forecast with snowfall beginning in the late morning. Between 2-4″ of new low density snow is expected, to sea level (.2-.3″ of water equivalent). Double these amounts are possible at the high elevations and areas closer to Portage Valley and Turnagain Arm. Ridgetop winds should start rising mid-day to the 15-20mph range from the East. Temperatures should stay cool, low 20’sF at 1,000′ and near 20F along ridgetops.

Tomorrow:  Another front moves over the region Christmas Day and should bring another chance for a few inches of snow, warming temperatures (yet still snow to sea level) and easterly ridgetop winds. An unsettled weather pattern bringing periods of snow showers will be over Southcentral for the remainder of the week.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 21 trace trace 30
Summit Lake (1400′) 12 0 0 7
Alyeska Mid (1700′) 19 trace trace 18

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 18 E 5 14
Seattle Ridge (2400′) N/A* N/A* N/A* N/A*

*Rime has covered the wind sensor on Seattle Ridge once again.

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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
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Closed
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Closed
Skookum Drainage
Closed
Turnagain Pass
Closed
Closed as of May 1. Thanks for a fun, safe season!
Twentymile
Closed
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Carter Lake
Closed
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Closed
Primrose Trail
Closed
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Closed
Snug Harbor
Closed
South Fork Snow River Corridor
Closed
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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.