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Issued
Sun, December 27th, 2020 - 7:00AM
Expires
Mon, December 28th, 2020 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

Today’s avalanche danger will rise to CONSIDERABLE above 2500’, where it will become likely a person could trigger a wind slab avalanche 1-2’ deep as winds pick up throughout the day. Watch for active wind loading near ridgetops, in gullies, and below convex rollovers. Pay attention to changing conditions and increasing avalanche danger as the next storm system moves in. Avalanche danger will be MODERATE between 1000-2500′, where lower wind speeds will make fewer and smaller wind slabs, and will remain LOW below 1000′, where a lack of soft snow will make wind slab avalanches unlikely.

SUMMIT LAKE: With a thinner and weaker snowpack, there is still a small possibility of triggering an avalanche on a weak layer buried  mid-pack or near the ground.

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Sun, December 27th, 2020
Alpine
Above 2,500'
3 - Considerable
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

Skiers triggered multiple wind slabs on test slopes in Pete’s North yesterday. While they were not deep (~6”), they did propagate fairly wide (50-150’). We can expect more of the same today.

Wind slab that fractured with a ski-cut. The slope was just not quite steep enough for the slab to run. 12.26.2020

 

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

Winds are ramping up today ahead of a storm that will last through tonight and taper off tomorrow. As increasing winds through the day continue to drift snow into sensitive slabs, it will become likely that a person could trigger a wind slab avalanche 1-2’ deep. These avalanches will be the largest and most sensitive at elevations above 2500’, where wind speeds will be the highest and slopes the least protected. The most suspect slopes will be near ridges, below convex rollovers mid-slope, or in cross-loaded gullies (picture those facing the highway on Seattle Ridge, or on the steeper south-facing shots below Hippy Bowl). Be on the lookout for clear signs of decreasing stability, such as shooting cracks, collapsing, and recent avalanches. Pay attention to changing conditions as this next system moves in– with close to a foot of low-density snow accumulating over the past few days and more on the way, there is plenty of ammo for slab-building as winds pick up throughout the day.

Shooting cracks on a freshly wind-loaded slope. This crack propagated roughly 150′. Photo: Andy Moderow. 12.26.2020

Additional Concern
  • Deep Persistent Slabs
    Deep Persistent Slabs
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.
More info at Avalanche.org

We have been tracking the crust that was formed on 12/1, after the rain level made it up to 2500’. At this point, the 12/1 crust is buried about 2-3′ deep. While we have yet to see any avalanches failing at this layer, we have noticed some faceting in the middle of the crust, which has been showing mixed results in stability tests. Yesterday, a group of skiers in Pete’s North reported a large collapse in this layer while skinning up through the trees. This observation was unexpected and it is the only red flag reported on this layer thus far. For now, we are keeping an eye on this layer as a potential problem if it continues to show signs of decreasing strength.

The Summit Lake region to the south of our forecast area has a thinner snowpack with faceted snow at the ground. This layer is gaining strength and it is becoming less and less likely to trigger an avalanche deep in the snowpack, However, we cannot entirely rule out the possibility of a person triggering a deep slab avalanche in, to quote Wendy, ‘just the wrong thin spot’. This is still a layer worth keeping in mind before trying to push into bigger terrain in the Summit Lake area.

Weather
Sun, December 27th, 2020

Yesterday: Skies cleared in the afternoon after a quick round of snow left 2-4”. Winds were light to calm for most of the day, with the exception of the south end of Turnagain Pass, where easterly winds blew 15-20 mph for a few hours mid-day. Easterly ridgetop winds picked across the advisory area yesterday evening, blowing 15-20 mph with gusts to 44 mph.

Today: Our next round of snow and wind moves in this afternoon, with 2-4” snow expected by this evening and another 6-12” by tomorrow morning. Easterly winds will continue to pick up ahead of and during the storm, with ridgetop speeds expected between 20-30 mph today and 30-40 mph tonight. Temperatures are expected to rise gradually through today and tonight, reaching the low 20’s F at upper elevations and low to mid- 30’s F at lower elevations. It looks like the rain level will stay down to 500 feet.

Tomorrow: Light snowfall will continue, with another 1-2″ expected during the day. High temperatures will be in the mid-20’s to upper 30’s F, with easterly ridgetop winds blowing 20-25 mph and gusts around 30 mph.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 27 2 0.2 77
Summit Lake (1400′) 22 1 0.1 31
Alyeska Mid (1700′) 28 2 0.2 78

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 20 NE 11 44
Seattle Ridge (2400′) 24 SE 9 18
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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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