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 31st, 2020 - 7:00AM
Expires
Fri, January 1st, 2021 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is MODERATE above 1,000′ today. Triggering a wind slab remains possible on slopes steeper than 35°. These slabs could be 1-2′ deep and will be found on wind-loaded slopes and in cross-loaded gullies. Additionally, between 1300′-2500′, there is a small chance of triggering a deeper avalanche (3-5′ deep) on weak snow near a crust that formed Dec 1st. Look for signs of wind effect and choose terrain carefully.

The avalanche danger is LOW below 1,000′.

SUMMIT LAKE: In addition to the wind slab potential, with a thinner and weaker snowpack, it is still possible to trigger an avalanche on a weak layer buried mid-pack below 2500′ or near the ground above 2500′.

 

 

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Thu, December 31st, 2020
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

A group of skiers skinning along the Cornbiscuit ridgline remote triggered a small avalanche on the north side. From the observation, ‘Debris, though, shallow, ran most the way to the valley floor.’

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

As skies clear and venturing into more terrain becomes easier, it is important to remember that yesterday the winds continued to move snow around. Observers reported gusty ridgetop winds and blowing snow flagging at times. The wind slab avalanche on Cornbiscuit was touchy enough that it was triggered remotely from above on the ridge. The winds eased off overnight. Today wind slabs remain the primary concern. These wind slabs could be 1-2′ deep and may be a bit more stubborn to trigger. These will found in wind exposed terrain on steep unsupported slopes and cross-loaded gullies.  You may be able to travel out onto the slab before it breaks above you as is often the nature of hard wind slabs. Looking for signs of wind effect and wind slab habitat will be key to safe travel today.

Classic wind slab signs:

  • Rounded and pillow-like surfaces (wind deposited snow)
  • Hollow feeling or drum-like feeling on the snow due to stiff snow over softer snow
  • Cracks in the snow that shoot out from your machine, skis or board
  • Any collapsing in the snow or ‘whumpfing’

In addition, there are couple of ‘Normal Caution’ avalanche issues to pay attention to today. On steep slopes out of the winds where loose surface snow is found, keep an eye on your sluff. And, as always, give cornices a wide berth and limit time under them.

Wind effect along the Tenderfoot ridgeline, 12.30.20

Scoured ridges and cross-loaded gullies on Magnum north side, 12.30.20. Photo: Emily Sullivan

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

A month ago it rained up to around 2500′ and then snow fell on top. This formed a crust that is now buried 3-5′ below the surface. The snow around the crust is showing signs of forming a reactive weak layer (facets) in some parts of the forecast area, but not all which is tricky. Triggering an avalanche on this layer is unlikely but could have bad consequences due to the depth of the layer and the connectivity of crust across terrain. Unfortunately signs of instability will not likely be present with this type of avalanche problem and stability tests could give unreliable data. Steep mid-elevation slopes (1,300′ – 2,500′) should be considered suspect and there may be multiple tracks on the slope before it fails. We are tracking the evolution of the snowpack structure around the crust and are concerned about the developing weak layer and deep slab potential.

The Summit Lake region to the south of our forecast area also has the December 1st crust/facet combo below 2500′. The overall snowpack is thinner and the weak snow and crust are only buried 1-2′ deep. Observers found this to be reactive in snowpack tests yesterday.  In addition, there is weak snow at the ground that is a concern above 2500′. Both these weak layers are worth keeping in mind before trying to push into bigger terrain in the Summit Lake area.

Extended column test that failed (propagated) with 13 taps at 1950′ on Tenderfoot. 12.30.20. Photo: Eric Roberts

Weather
Thu, December 31st, 2020

Yesterday: Skies were mostly cloudy in the morning and became partly cloudy in the afternoon. Temperatures were in the 30°Fs at sea level and 20°Fs at the ridge top weather stations. Winds were easterly 5-15 mph with gusts into the 20s and 30s. Overnight skies were partly cloudy with temperatures in the 20°Fs and light and easterly winds.

Today: Partly cloudy skies in the morning with a chance of scattered snow showers becoming mostly sunny by the afternoon. Light easterly winds and temperatures in the 20°Fs. Overnight partly cloudy skies becoming mostly clear with temperatures in the teens and 20°Fs and light variable winds.

New Year’s Day: Mostly sunny skies with calm winds and temperatures in the teens to mid 20°Fs.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 29 0 0 79
Summit Lake (1400′) 27 0 0 31
Alyeska Mid (1700′) 29 0 0 82

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 21 NE 12 40
Seattle Ridge (2400′) 25 E 7 17
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Riding Areas
Updated Thu, April 01st, 2021

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
Open
No parking in turnaround at end of the road near the outhouse.
Placer River
Open
Please do not ride along Railroad tracks. Cross tracks at 90 degree angle and clear the right of way.
Skookum Drainage
Closed
The Skookum Valley is closed to snowmachines. This closure occurs annually on April 1 as per the CNF Forest Plan.
Turnagain Pass
Open
Twentymile
Open
Please do not ride along Railroad tracks. Cross tracks at 90 degree angle and clear the right of way.
Seward District
Carter Lake
Open
Lost Lake Trail
Open
Primrose Trail
Open
Resurrection Pass Trail
Closed
Closed for the 2020/21 winter season.
Snug Harbor
Open
South Fork Snow River Corridor
Open
Summit Lake
Open

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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.