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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Tue, December 15th, 2020 - 7:00AM
Expires
Wed, December 16th, 2020 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

Today’s avalanche danger is MODERATE above 2500’, where it will be possible to trigger a wind slab avalanche 1-2’ deep on recently wind loaded slopes. Be careful with your snowpack assessment before approaching bigger terrain today, and avoid steep slopes with fresh slabs of wind-loaded snow. The avalanche danger is LOW at elevations below 2500’.

SUMMIT LAKE: The snowpack in the Summit Lake area is generally thinner and weaker, making it easier to trigger a deep slab avalanche near the ground. Be cautious with your terrain choices, and avoid steep, rocky terrain where these avalanches will be most likely.

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Tue, December 15th, 2020
Alpine
Above 2,500'
Moderate (2)
Avalanche risk
Treeline
1,000'-2,500'
Low (1)
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
Low (1)
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

Wind slabs that have formed over the past two days are becoming more stubborn and difficult to trigger, but today it will still be possible to trigger an avalanche 1-2’ deep on a wind-loaded slope, especially near ridgetops and in cross-loaded gullies. Winds are expected to shift and start blowing out of the west during the day today, and although we are not expecting to see any remarkable speeds, this change in direction may redistribute pockets of snow that have previously been sheltered from the generally easterly winds for the past 48 hours. A few inches of light snow this morning is not expected to increase the danger significantly. While the avalanche danger is low below 2500’, there is still an unlikely chance that you could trigger a small wind slab in isolated pockets at the mid-elevations (1000’-2500’). Be wary of indicators of instability such as shooting cracks and whumpfing, and seek out slopes that have not been hit by the wind. They will have the best riding conditions anyway.

Cornices: Yesterday we noted debris from a cornice fall near Magnum, and similar activity will be possible today as the cornices adjust to the recent wind events. If you are traveling along a ridge, be sure to give them a wide berth. It is equally important to minimize the time you spend below them, as they can release naturally and unexpectedly.

Cornbiscuit. Chunks of wind slab breaking on a small test slope. 12.14.2020

 

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

The weak snow at the ground continues to be a concern at elevations above 2500’, particularly in areas with shallower snowpack. Yesterday we got unstable test results in this layer (more info), and it is still on our radar for now. This is a tricky problem, because although it is unlikely you will trigger an avalanche like this, it could be very large if you do. Avoid steep and rocky terrain, and be extra cautious as you head farther south from Turnagain pass, where the snowpack is shallower and these avalanches will be easier to trigger.

If you find this type of problem particularly challenging and full of uncertainty, you are not alone! Tune in to our latest Forecaster Chat tonight to hear Aleph share some insight about deep persistent slab avalanches.

Weak facets like these at the bottom of the snowpack continue to give us cause for concern. Photo: Eric Roberts. 12.14.2020

Weather
Tue, December 15th, 2020

Yesterday: Easterly ridgetop winds were blowing 20-30 mph until they calmed down to 5-10 mph around 1 p.m. The mountains near Girdwood received 1-3” snow, while Turnagain Pass got a trace. Rain line rose to about 700 ft yesterday afternoon, with snow to sea level this morning. Temperatures reached the upper 20’s F at high elevations, and upper 30’s F at lower elevations.

Today: We will see periods of light snow throughout the day, with 1-2” expected by the end of the day. The rain line is expected to rise to 300 ft. Light westerly winds are expected, with speeds around 5 mph at the ridgetops. Mountain temperatures will reach into the upper 20’s F, with highs expected in the mid 30’s F at lower elevations.

Tomorrow: Light snowfall is expected to taper off through tonight, with only a trace expected by tomorrow morning. Temperatures will be in the upper teens F tonight at upper elevations, and in the low- to mid- 20’s F at middle and lower elevations, and will drop to the low teens to low 20’s F tomorrow. Light westerly ridgetop winds around 5-10 mph are expected tonight and tomorrow, with decreasing clouds through the day.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 32 2 0.1 59
Summit Lake (1400′) 29 0 0 24
Alyeska Mid (1700′) 32 3 0.4 61

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23 E 12 49
Seattle Ridge (2400′) 27 E 7 22
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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
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Open
No parking in turnaround at end of the road near the outhouse.
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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.