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 10th, 2020 - 7:00AM
Expires
Fri, December 11th, 2020 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is MODERATE above 1000′. Human triggered wind slabs 1-3′ thick and cornice falls are possible on steep, leeward slopes just off of ridge-lines and in cross-loaded gullies. In addition, there is uncertainty about a new persistent slab issue. Look for signs of instability, obvious wind effect and choose terrain carefully.

Below 1000′ the avalanche danger is LOW where a new rain crust exists on the surface.

SUMMIT LAKE: The snowpack is generally thinner and weaker in the Summit Lake region. In steep Alpine terrain it may be easier to trigger an avalanche on a mid-pack buried weak layer or near the ground. Extra caution is advised.

 

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Thu, December 10th, 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)
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

With decent visibility yesterday it was evident that winds during the storm moved snow around. There were small winds slabs in many of the front side gullies on Seattle Ridge, and lots of cross-loading and scouring. Cornices had grown and there was stiff snow in wind exposed terrain. Today lingering wind slabs are still a concern.  It will be important to pay attention to terrain to with stiff hollow snow, especially steep unsupported slopes and gully features, and look for shooting cracks.

There is a bit of question mark today as well. Are the storm slabs from the Sunday/Monday storm becoming a new persistent slab issue or not? There are signs pointing to a stabilizing snowpack but there is still some uncertainty and with that potential for large human triggered avalanches. As you head out here are few things to keep in mind about this question:

  • Much of the snowpack in the forecast area remains untested after this last storm.
  • The new snow fell on surface snow that had weakened during our last clear period and below 2500′ this set-up sits on a rain crust.
  • During the storm on Monday observers found storm slabs easily sliding on the weak snow on the crust. Tuesday, two parties of skiers, one on Sunburst and one on Magnum, reported turning around due to large whumpfs in the mid elevation band where the old rain crust exists.

On the flip side, the Tuesday observers on Seattle Ridge found the new snow bonding well to the old snow and crust and no signs of instability. This was similar for observers on Sunburst yesterday and skiers and snowmachiners ventured out a bit more with no reported incidents.  This seems promising but is still somewhat of an unknown as folks spread out testing more terrain. As we move into the next couple days of clear skies it is very important to ease into terrain, use good travel protocol, think about consequences and look for signs of instability.

Cornices:  We know cornices have grown. Give them a wide berth as they can now easily break further back than we might expect. As seen last week, they can also trigger an avalanche below, creating a bigger problem if a person is involved.

Wind textured surface snow on Sunburst. 12.9.20. Pay attention to stiff hollow snow and look for shooting cracks. Note the scouring and cross loading on the north side of Magnum as well. 

Will the new storm slab stick to the weak snow on top of 12.1 rain crust? That is still a bit of a question mark. Sunburst snowpack at 2200′, 12.9.20.

 

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

In the Alpine, hitting a thin area in the snowpack or traveling in areas with an overall shallower snowpack (around 3-4′deep) is still concerning for triggering a large slab that fails in the old October snow. The southern end of Turnagain Pass through Johnson Pass, Silvertip and Summit Lake all have slopes without as much snow as northern end of Turnagain or Girdwood.

Weak snow at the base of the snowpack on Sunburst at 3200′. 12.9.20.

Weather
Thu, December 10th, 2020

Yesterday: Skies were overcast to partly cloudy. Winds were light and easterly and temperatures were in the teens to mid 20°Fs. Overnight skies were partly cloudy, winds remained light and temperatures stayed in the teens to mid 20°Fs.

Today: Skies will be mostly sunny. Winds will be light and northwesterly and temperatures will be in the teens to mid 20°Fs. Overnight skies will mostly clear, winds will be light and variable and temperatures will be in the teens and single digits.

Tomorrow: Mostly sunny skies, calm winds and temperatures in the teens and low 20°Fs. The mostly clear skies look to continue through Saturday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 26 0 0 65
Summit Lake (1400′) 15 0 0 25
Alyeska Mid (1700′) 27 0 0 59

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 17 NE 8 29
Seattle Ridge (2400′) 22 E 4 14
Observations
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Riding Areas
Updated Tue, January 12th, 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
Early season conditions exist, including thin ice on rivers, swamps and lakes. Please do not ride along Railroad tracks. Cross tracks at 90 degree angle and clear the right of way.
Skookum Drainage
Open
Early season conditions exist, including thin ice on rivers, swamps and lakes. Please do not ride along Railroad tracks. Cross tracks at 90 degree angle and clear the right of way.
Turnagain Pass
Open
Twentymile
Closed
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.