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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, December 17th, 2020 - 7:00AM
Expires
Fri, December 18th, 2020 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is MODERATE on slopes over 35 degrees above 2,500′. In steep rocky terrain, there is still a chance a person could hit a thin spot in the snowpack and trigger a large avalanche, 4-6′ deep or deeper, failing in weak snow at the ground.

The avalanche danger is LOW at elevations below 2500’.

PLACER VALLEY/SKOOKUM: With this zone opening to snowmachines today for the first time this season, be mindful that the snowpack is untested. Extra caution is advised. Ease into big terrain, look for signs of instability and have escape routes planned.

SUMMIT LAKE: The snowpack in the Summit Lake area is generally thinner and weaker above 2500′, potentially making it easier to trigger a slab avalanche failing near the ground. Avoid steep, rocky terrain where these avalanches will be most likely.

As always, be sure to carry your rescue gear and practice safe travel protocol!

Special Announcements
  • Heading to Hatcher Pass? Remember to check the Thursday Morning Conditions Summary at hpavalanche.org.
  • Member Gear Giveaway: To show appreciation for current members and new members that sign up by January 15, the Friends of the CNFAIC will give away three pairs of skis in a drawing on January 16. Please encourage friends and family to go to our website’s Sponsors & Members page and sign up. For as little as $20 your name will be added to the members list, and you’ll be eligible for the ski drawing! Thanks to Ski AK for donating the skis, and to all of you for supporting your local avalanche center.
Thu, December 17th, 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. Dangerous avalanche conditions. 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
  • 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

With benign weather today, as we trend towards overall LOW danger in the Turnagain pass area, only very specific terrain still warrants extra caution at this point. Steep (above 35°), rocky, unsupported slopes with variable snowpack coverage remain suspect for triggering a deep persistent slab avalanche. If hitting a thin spot does initiate an avalanche it could be deep, running on weak faceted snow at the ground. We are in the phase where there is a low probability of this but due to the depth, the consequences remain high. There are many pieces of data that are in our favor. It has been two weeks since the last human triggered avalanche on this layer and neither the last storm snow load or the last wind loading event were known to have triggered avalanches to the ground. Many skiers and snowmachiners have tested steeper slopes over the past two weeks. This all points to the snowpack adjusting and stabilizing. However, we still do have the poor snowpack structure above 2500′. We know the weak snow is there under a very hard slab of snow and if this happens to fail it could be dangerous. It’s this remaining, ‘What if?’ that sucks. The most likely places to trigger a large avalanche failing near the ground, will be in the areas with the thinnest snowpack in steep and rocky terrain. The total snow depth gets thinner as you head south in Turnagain Pass and to the north around Crow Pass. In the upper elevation terrain in Placer Valley and Skookum we have no data on the snowpack and have to assume guilty until proven innocent. 

As Andrew said in the forecast yesterday, ‘At some point– hopefully sometime soon– we will be able to tuck this problem away as an additional concern. For now, we are still finding weak snow at the ground, and we are actually getting some stability tests to fail in this layer, making this our primary concern for today.’

Besides this lingering deep slab issue, normal caution is warranted in avalanche terrain today. Steer clear of cornices, watch for lingering wind slab pockets just off of ridgelines and for sluffing in steep terrain, and pay attention to changing conditions if winds ramp up more than forecasted today.

This is an avalanche that occurred two weeks ago in Gold Pan that was triggered by a cornice fall. It is this type of terrain that is still suspect. Steep, rocky, unsupported slopes in the Alpine. Photo: Billy Finley.

Snowpack structure in a snow pit at 3000′ on Cornbiscuit, 12.14.20. Note the hard slab over the soft weak layer.

 

 

 

Weather
Thu, December 17th, 2020

Yesterday: Skies were mostly cloudy, unless you happened to be above the low stratus layer. There were light snow showers near the end of Turnagain Arm in the morning. Winds were calm and temperatures were in the low 30°Fs at sea level and mid to high teens in the Alpine. Overnight skies were partly cloudy, winds were calm and temperatures were in the high teens to low 20°Fs.

Today: Skies are forecast to be mostly cloudy with a chance of snow showers. Easterly winds bumped up a bit early this morning at ridgetops, blowing 10-15 mph with gusts into the 20s. These should ease by mid afternoon. Temperatures will be in the 20°Fs. Overnight skies will be mostly cloudy with a continued chance of snow showers. Winds will be light and northeasterly and temperatures will be in the teens to mid 20°Fs.

Tomorrow: Cloudy skies and a chance of snow with light northerly winds and temperatures in the 20°Fs. Looking ahead there is some uncertainty about the next storm developing for the weekend and how it will impact this region. Stay tuned! Think cold thoughts.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 22 0 0 59
Summit Lake (1400′) 20 0 0 25
Alyeska Mid (1700′) 23 0 0 59

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 14 E 3 27
Seattle Ridge (2400′) 18 E 2 12
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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.