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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
Thu, December 19th, 2019 - 7:00AM
Expires
Fri, December 20th, 2019 - 7:00AM
Forecaster
Aleph Johnston-Bloom
The Bottom Line

MODERATE avalanche danger exists above 2,500′ for wind slab avalanches and cornice falls. Triggering a lingering wind slab 1-3′ thick is possible in steep terrain just off ridgelines and in cross-loaded gullies. Give cornices a wide berth. They have grown and may break off easily.

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Thu, December 19th, 2019
Alpine
Above 2,500'
Moderate (2)
Avalanche risk
Treeline
1,000'-2,500'
Low (1)
Avalanche risk
Below Treeline
Below 1,000'
No Rating (0)
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
No Rating (0)
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

If traveling in the Alpine today, be on the lookout for wind effect from the almost two weeks of moderate to strong winds redistributing the snow that fell over the same time period. Windward areas are scoured to the tundra and catchment zones have drifts over 10 feet deep. After a day of benign weather yesterday, wind slabs may be more stubborn to trigger but should not be ruled out in steep wind-loaded terrain. Be especially cautious on unsupported slopes and in cross-loaded gullies. A wind slab avalanche could take you for a bad ride if triggered in high consequence terrain. You may be able to get well out onto the slope before it releases. Listen for hollow sounding snow, feel for stiffer snow over softer snow and pay close attention to loading patterns.

Cornices: The recent snow and wind have also created larger cornices that may be triggered along ridgelines. Give them a wide berth as they often fail farther back than expected. Be aware of other parties traveling above or below you in relation to the cornices.

Cornice development and wind effect along the Sunburst ridgeline, 12.17.19.

Looking at cross-loading on Tincan Proper, 12.17.19

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

With sunshine, good visibility and improved snow cover at upper elevations, travel further into the Alpine is more appealing. Keep in mind overall there is limited snowpack data from above 3000′. Various layers of facets and crusts could still remain intact at the high elevations, especially in shallow snow cover areas like the southern end of Turnagain pass and Summit Lake. Where this is the case, the potential exists for a larger avalanche to be triggered that breaks deeper in the snowpack.

Observers on Tuesday dug a pit at 3,500′ on Sunburst and found the old November Veteran’s Day facets over the hard basal melt-freeze crust. The layer didn’t show signs of reactivity here, which is good news. However, it is still something we are keeping on our radar.

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

Keep your eyes peeled for glide cracks in the terrain that you are traveling in. Steer clear and avoid lingering in the runout of this unpredictable hazard. There is a chance we may see some more glide cracks open and release as temperatures drop or the glide activity may completely stop.

Weather
Thu, December 19th, 2019

Yesterday: Skies were partly cloudy and there was some patchy valley fog. Temperatures were in the low 30°Fs at sea level and in the mid 20°Fs at the ridgetops. Winds were mostly light and easterly. Overnight temperatures slowly cooled to the high 20°Fs at sea level and low 20°Fs at upper elevations. Winds were light and variable.

Today: Skies will be partly cloudy with a chance of isolated snow showers. Temperatures will continue to slowly drop with ridgetops in the low 20°Fs to high teens and low elevations in the 20°Fs. Winds will be light from the northwest.  Tonight temperatures will get down into the teens and single digits.

Tomorrow: Sunshine and colder temperatures are on tap as high pressure moves over the region. The forecast is for teens and single digits. There is the potential for increasing northerly outflow winds as this weather pattern sets in. The sunny skies and cold temperatures look to continue through the weekend. The outlook for the early next week remains uncertain.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 28 0 0 30
Summit Lake (1400′) 27 0 0 7
Alyeska Mid (1700′) 29 0 0 15

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23 E 6 28
Seattle Ridge (2400′) NA* NA* NA* NA*

*Seattle Ridge weather station is not reporting.

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
Closed
Seward District
Carter Lake
Closed
Lost Lake Trail
Closed
Primrose Trail
Closed
Resurrection Pass Trail
Closed
Snug Harbor
Closed
South Fork Snow River Corridor
Closed
Summit Lake
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.