Turnagain Pass Avalanche Forecast RSS

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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Wed, December 22nd, 2021 - 7:00AM
Expires
Thu, December 23rd, 2021 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is MODERATE on all aspects above 1,000′. Triggering a wind slab avalanche, up to 2′ thick, that formed during yesterday’s northwest winds will be possible on slopes with recent wind deposited snow. Additionally, there is still a chance a person (or smaller wind slab avalanche) could trigger a larger slab, 2 to 5+ feet deep, that breaks in old buried weak layers.

Below 1,000, the danger is LOW where triggering an avalanche is unlikely.

SUMMIT LAKE:  Strong northwest winds impacted this area and caused a natural avalanche cycle yesterday morning. Wind slabs were able to overload buried weak layers and create larger avalanches. Cautious route-finding is recommended for this zone as human triggered wind slabs, or deeper slabs, may be likely.

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Wed, December 22nd, 2021
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)
Recent Avalanches

The Solstice wind event, though pretty short lived, created a natural wind slab avalanche cycle early yesterday morning. These northwest winds peaked just over 24 hours ago and have diminished since then. Crowns of several small to large wind slabs were visible, especially on the southern end of the forecast zone and in Summit Lake. Some of these wind slabs stepped down to the weak layers buried in the snowpack, causing a much larger avalanche than your typical wind slab.

We also had a report of one human triggered avalanche in the Girdwood Valley, west of the Crow Pass trailhead. This was a remotely triggered wind slab on a steeper rollover.

 

Natural avalanche triggered by the strong Solstice winds early on Tuesday 12.21.21, photo taken later that day. The wide propagation of this avalanche is indicative of buried weak layers. 

 

Two natural slabs triggered by the Solstice wind event on easterly facing slopes in Lynx Creek as well as on Silvertip. 12.21.21.

 

Natural wind slab avalanches that stepped down to buried weak layers in the Summit Lake area. These are on the east facing gullies of Summit Peak. Photo: Paul Wunnicke, 12.21.21.

 

Large natural wind slab from the Solstice wind event, NE shoulder of Summit Peak in the Summit Lake region. Photo: Paul Wunnicke, 12.21.21.

 

Skier remotely triggered this wind slab on a steeper rollover in the upper Girdwood Valley. Photo: Ryan Mclaughlin, 12.21.21.

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

Triggering a wind slab formed by yesterday’s winds will be the main concern today. These new wind slabs are likely to be quite hard and up to 2′ thick. They could lure us out on the slab before releasing and alternatively, be triggered remotely (from the side, below, or the top of a slope). Although the northwest winds are still breezy (~10mph), they are not forecast to be strong enough to move much snow until late tonight and into tomorrow when they are expected to ramp back up.

Reports from yesterday are the winds made it to most open areas at and above the trees. That means we could find wind slabs lower on slopes than we may expect. Keeping a close eye out for any signs of wind loading will be key. A thin wind crust can transition into a wind slab quickly, especially near the tops of ridges and on rollovers. Watching for changes in the snow surface, a stiffer layer of snow over softer snow and cracks that shoot out from you, are great ways to assess a wind slab.

Keep in mind that even a small wind slab that is triggered could ‘step down’ and trigger a larger slab that breaks in buried weak layers. This was the case with many natural avalanches yesterday in areas with a shallower snowpack (south end of Turnagain Pass and Summit Lake).

 

Wind transport along the Magnum Ridge yesterday. This was after the brunt of the winds moved through. Photo: Scott Johnson, 12.21.21.

 

Winds were able to blow in tracks through the day on the Southern end of Turnagain Pass. This is from Pete’s North yesterday, 12.21.21. 

Avalanche Problem 2
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
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.

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

With the evidence from yesterday’s wind event that overloaded buried weak layers, we definitely have to remember that triggering a deeper slab is still a concern. These faceted weak layers are from early November and are slowly getting squished under a harder slab of snow anywhere from 2 to 5+ feet thick. The likelihood for one of us to trigger a deep slab (2 to 5ish feet deep) is becoming less as the days go by, but still something we all should keep in mind. The most concerning areas are those with a more shallow snowpack. This would be areas on the south end of Turnagain Pass, Johnson Pass, and further inland on the Kenai (i.e., Summit Lake).

It is also good to remember that unlike wind slabs, deep persistent slabs rarely give us any clues they are close to avalanching until they do. Snow pit tests are unreliable and signs of instability are often nonexistent. The snowpack can ‘feel fine’ but it might not be. Now that the winds have redistributed the surface snow to some degree, there could be more thin spots and trigger points as well. All reasons to keep our guard up coming into the Holiday weekend!

Weather
Wed, December 22nd, 2021

Yesterday:  Sunny skies were over the region yesterday along with chilly NW winds. These ridgetop winds were 10-20mph through the day after they diminished from peaking two nights ago. Temperatures were in the teens to 20F and have dropped several degrees in valley bottoms as an inversion has set up overnight.

Today:  Some high clouds have moved in this morning and we should see partly cloudy skies before transitioning to mostly cloudy later in the day. The NW ridgetop winds should remain in the 10-15mph range, yet are slated to ramp back up tonight to 20-25mph. We also have a shot of an inch or two of snow tonight as there is moisture associated with this westerly flow over Southcentral.

Tomorrow:  Skies look to begin to clear up again on Thursday and through Christmas Day. The NW winds however, look to remain at this point and blow in the 20-30mph range with strong gusts on Thursday before letting up on Friday. Temperatures will continue to be chilly, in the teens and single digits.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 23 0 0 70
Summit Lake (1400′) 15 0 0 25
Alyeska Mid (1700′) 19 0 0 44

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 16 NW 10 27
Seattle Ridge (2400′) 20 NW 11 25
Observations
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Riding Areas
Updated Fri, May 13th, 2022

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
Closed as of May 1 per Forest Plan.
Placer River
Closed
Closed as of April 25th due to insufficient snow coverage.
Skookum Drainage
Closed
Closed as of April 1st per Forest Plan.
Turnagain Pass
Open
Open. Extended opening through May 31.
Twentymile
Closed
Closed as of April 6th due to insufficient snow coverage.
Seward District
Carter Lake
Closed
Closed as of May 1 per Forest Plan.
Lost Lake Trail
Closed
Closed as of May 1 per Forest Plan.
Primrose Trail
Closed
Closed as of May 1 per Forest Plan.
Resurrection Pass Trail
Closed
Closed as of May 1 per Forest Plan.
Snug Harbor
Closed
Closed as of May 1 per Forest Plan.
South Fork Snow River Corridor
Closed
Closed as of May 1 per Forest Plan.
Summit Lake
Closed
Closed as of May 1 per Forest Plan.

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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.