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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, January 18th, 2019 - 7:00AM
Expires
Sat, January 19th, 2019 - 7:00AM
Forecaster
Heather Thamm
Avalanche risk The Bottom Line

The avalanche danger is MODERATE above 2500′ for triggering a slab 1-2′ thick. Slabs may be small and isolated or large enough to bury a person. Cornices have been recently loaded and could be triggered by the weight of a person or snowmachine. Identify glide cracks and avoid/limit your exposure time under this unpredictable hazard.

SUMMIT LAKE / JOHNSON PASS:  Keep in mind buried weak layers exist in the middle and base of the snowpack. More potential for triggering a large slab avalanche exists in this zone, especially in terrain that has seen recent wind-loading.  

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Fri, January 18th, 2019
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
  • Persistent Slabs
    Persistent Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.
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

Across our region the snowpack is quite variable in the Alpine. Two storms this week left varying amounts of snow across our region favoring coastal areas like Portage, Girdwood, and the Northern end of Turnagain Pass. These new slabs were formed by strong winds and may range from 1-2’ thick. This new snow may be resting on several persistent weak layers (buried surface hoar or near surface facets) formed during the cold clear period last week and between the two storms on Tuesday. Yesterday we received a report of a skier triggered slab near Placer Valley. The slab was around 18” deep, and triggered by a ski cut on a steep terrain feature. No one was caught, but the slab did have two separate layers and was large enough to get someone in trouble in high consequence terrain. Keep in mind there is potential for a slab to be larger and more connected in bigger terrain. Shooting cracks and whumpfing may not be present until a slab releases suddenly. Identify wind-loaded features, evaluate the terrain for consequences, and always us safe travel protocols.

Further away from coastal areas where less snow fell, strong winds also impacted the upper elevations. Triggering a smaller slab in steep terrain remains possible. Be weary of hard supportable snow especially if it feels hollow or drum-like. 

South of Turnagain – Johnson Pass/Summit Lake zone: A very poor snowpack structure exists in these areas.  Multiple mid-pack weak layers of facets and buried surface hoar have been found as well as a facet/crust combination in the bottom of the snowpack. No recent avalanche activity has been observed since the New Year’s storm, but more uncertainty exists for triggering a deeper more dangerous avalanche. If you’re headed this way, it will be important to evaluate terrain and snowpack as you travel. Be on the lookout for signs of instability and maintain extra cautious around wind-loaded slopes.

Skier triggered slab on a West aspect in the Placer Valley. Also notice the large cornices. 

 

Avalanche Problem 2
  • Cornice
    Cornice
Cornice
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
More info at Avalanche.org

Cornices:  Recent strong winds and new snow have added stress to cornices. Not only can they break farther back than expected, they may be more tender than usual. Give these features lots of space.

Loose Snow: Yesterday several sun triggered wet-loose avalanches were observed on South facing slopes where rocks and vegetation were heating up the new surface snow. This harzard is unlikely today, but something to keep in mind with inverted temps in the upper elevations. It’s still pretty early in the season for any significant radiation affect at our latitude. 

Several sun triggered wet-loose avalanches observed on South facing terrain of Goat Mt near Crow Creek. Photo by James Lyons.

Additional Concern
  • Glide Avalanches
    Glide Avalanches
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
More info at Avalanche.org

Be on the lookout for glide cracks opening up. They may be hard to see and filled in by recent snow and wind this week. Remember the known areas with cracks are Eddies, Tincan, Sunburst, Magnum, Cornbiscuit, Lipps, Seattle Ridge, Johnson Pass, Lynx Creek, Summit Lake, Petersen Creek, and Girdwood. This avalanche hazard is unpredictable and can release without warning. Identifying cracks and avoiding exposure time under them is the best way to manage this problem.

Weather
Fri, January 18th, 2019

Yesterday: Skies were clear with valley fog in some areas. Warm air from the day before was mixing in places and slightly inverted. Temps near sea level dropped into the low 20F’s and upper elevations temps were in the low 30F’s upper 20F’s. No precipitation fell yesterday and winds were calm.

Today: Expect clear skies with valley fog. Temperatures will be inverted with valley bottoms dropping from the low 20F’s into the teens and upper elevations will be in the upper 20F’s. Winds will be light and variable. No precipitation is in the forecast.

Tomorrow: Clear skies are expected through the weekend. Temperatures will continue to fall.   Expect upper elevations in the teens and valley bottoms may see low’s in the single digits F. Winds are expected to be light and variable.

 *Seattle Ridge weather station was heavily rimed and the anemometer (wind sensor) was destroyed. We are currently working to replace it.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 29   0   0   51  
Summit Lake (1400′) 17   0   0   20  
Alyeska Mid (1700′) 29   0   0   39  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 28   Variable    4  10
Seattle Ridge (2400′) 32   *N/A   *N/A     *N/A    
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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