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ARCHIVED - Forecasts expire after 24 hours.
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.

Special Announcements
Tue, December 15th, 2020
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
1 - Low
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
Travel Advice Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern. Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches Natural and human-triggered avalanches unlikely. Natural avalanches unlikely; human-triggered avalanches possible. Natural avalanches possible; human-triggered avalanches likely. Natural avalanches likely; human-triggered avalanches very likely. Natural and human-triggered avalanches certain.
Avalanche Size and Distribution Small avalanches in isolated areas or extreme terrain. Small avalanches in specific areas; or large avalanches in isolated areas. Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas. Large avalanches in many areas; or very large avalanches in specific areas. Very large avalanches in many areas.
Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    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
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    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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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.
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