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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, December 4th, 2020 - 7:00AM
Expires
Sat, December 5th, 2020 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is MODERATE on all aspects and slopes above 1,000′. At elevations above the surface crusts, watch for lingering wind slabs that could still be triggered on steep wind loaded slopes. Additionally, there remains a chance that a larger avalanche, breaking near the ground, could be triggered. Choose your steep terrain carefully and watch your partners.

Below 1000′ the avalanche danger is LOW.

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.

Fri, December 4th, 2020
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
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.
Recent Avalanches

We had a few second hand reports of a skier triggered slab avalanche in the Tincan Proper area yesterday. The slide was triggered by the 2nd skier on the slope, who took a similar line to the first skier. It broke 3-6′ deep, a little bit below the ridge and in steep 45 degree terrain (seen in the photo below that was taken from the Sunburst ridge). The skier was able to escape being caught. It appears this avalanche could have broken in weak snow near the ground, making it more complicated than just a wind slab. We hope to gather more information today.

Skier triggered slab avalanche on the western rib of Tincan Proper. 12.3.20. Photo: Thomas Bailly

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

Although much of the snowpack is gaining strength with the cool temperatures after Wednesday’s warm and wet storm, there are still areas that are concerning for triggering an avalanche. These are steep wind loaded slopes and gullies in more complex terrain. Even more concerning are those places that are unsupported, meaning slopes that rollover into steeper terrain or cliff features. The avalanche that was triggered yesterday is an example of this type of terrain.

If you’re headed out today, at elevations above the ‘dust on crust’, be suspect of these steep wind loaded features, and cornices for that matter. As usual, watch for cracking and collapsing in the snow around you. However, not many signs of instability are likely to be present. In fact, it may not be the wind slab itself that is unstable anymore, but the weak older snow below it, creating for a bit of a tricky situation discussed below.

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

Unfortunately, we can’t quite forget about that old weak snow near the ground and the chance a person could trigger a large avalanche. The pummeling of the recent storms helped us out a lot, but there are still areas to be suspect of. These are not only wind loaded steep slopes, but thin areas in the snowpack and especially shallow snowpack zones, such as the south end of Turnagain Pass through Summit Lake. In these shallower snowpacks, it’s technically easier to trigger an avalanche breaking near the ground because our weight can better impact the weak layer.

Things to keep in mind:

  • The snowpack can feel ‘stable’ and no signs of instability may be present before a deep slab releases.
  • The likelihood of triggering a large slab increases if you find shallower spots in the snowpack and near rocky areas.
  • Slopes may already have tracks on them and then still release.
  • These slabs can be triggered remotely, from the bottom, top or side.

This is a good time for us to remember our safe travel practices. One person on a slope at a time, really watch our partners, have escape zones planed along with safe zones to watch from.

Weather
Fri, December 4th, 2020

Yesterday:  Clearing skies and patches of valley fog covered the region yesterday. Winds were light from an easterly direction (0-10mph). Temperatures dropped off significantly during the day to the teens at the parking lots and single digits at 4,000′.

Today:  Mostly clear skies with some patchy valley fog is expected again today. Winds are forecast to be light (5-10mph) from the east. Temperatures are chilly, in the teens at 1,000′ and single digits along ridgetops.

Tomorrow:  One last day of clear skies, light winds and cool temps will be over us tomorrow before another round of storms and precipitation moves in for Sunday. This next storm cycle is looking to remain cold enough for snow to sea level, at this point anyhow. We could see another foot or more by Tuesday. Stay tuned!

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 17 0 0 57
Summit Lake (1400′) 9 0 0 24
Alyeska Mid (1700′) 18 0 0 58

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 9 E 4 12
Seattle Ridge (2400′) 13 SE 3 10
Observations
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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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