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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, February 17th, 2018 - 7:00AM
Expires
Sun, February 18th, 2018 - 7:00AM
Forecaster
Heather Thamm
Avalanche risk The Bottom Line

The avalanche danger is  MODERATE  above 1,000′ on all aspects where triggering an isolated wind slab or larger slab 2 feet deep remains possible. Be aware of radiation from the sun adding additional stress to solar aspects. Additionally, weak layers deeper in the snowpack may still be triggered, creating a larger avalanche.   Evaluate snow and terrain carefully.

The avalanche danger  below 1,000′ is LOW where triggering an avalanche is unlikely, but not out of the question. Avoid terrain traps where a pocket of unstable could have high consequences.  

The Summit Lake area has seen recent natural activity. Read the Summit Summary HERE.  

Special Announcements

Be aware of dangerous natural and human triggered avalanche activity in Hatcher Pass this week. Yesterday three people were caught and one person buried in a large human triggered avalanche. Luckily everyone is okay. If you’re headed North check out the Hatcher Pass avalanche forecast HERE.

TODAY, Saturday, Feb 17th, from 11am-12:30pm  – Free Avalanche Beacon Practice with CNFAIC! We will be hosting a short workshop on how to effectively perform a companion rescue at the motorized parking lot. Open to all users and skill levels. Alaska Mining and Diving Supply is providing a free  lunch and the event is also a chance to pay tribute to Christoph Von Alvensleben and Jeremy Stark, who lost their lives in an avalanche, 10 years ago, on Feb. 15th, 2008.

Sat, February 17th, 2018
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. 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
  • 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

Triggering a persistent slab avalanche up to 2′ deep remains possible across the region, and a sunny Saturday may increase the odds. Several weak layers including widespread buried surface hoar (from Jan. 21) sits roughly 1-2′ below the surface. New snow combined with periods of strong ridgetop winds have created both soft and hard slabs on a variety of aspects. Be extra cautious on solar aspects and if you see moist surface snow or point releases near rocks, these are obvious clues the sun is adding stress to these aspects. Unfortunately obvious signs of instability may not be present today and all aspects are suspect due to poor structure. Assessing the terrain and the potential outcome of an avalanche breaking deeper in the pack is key. If you find yourself in a crowded area, consider changing your objective or waiting for people to clear a slope before continuing. 

 

Jan. 21st buried surface hoar  is ~2′ below the surface and continues to show propagation potential in stability tests. Snow pit at 2700′ on Tincan on a West aspect.  

 

A photo of some of the wind affected terrain near Squirrel Flats. Little snowpack information exists in Placer Valley/Skookum area. If you see or experience any avalanche activity please take a picture and send us an observation HERE

 

Avalanche Problem 2
  • 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

Triggering an isolated wind slab is possible on all aspects and will be more likely in steep terrain. Earlier in the week moderate to strong winds (from a variety of directions) loaded some slopes and scoured ridgelines. Pay attention to where the snow feels stiff, looks pillowed, sounds or feels hollow and watch for shooting cracks. Hard wind slabs tend to break when you are out onto the slope and often fracture above you.  Wind loaded convexities and cross-loaded gullies facing the sun are most suspect. 

Recent natural wind slab from earlier in the week in Zero Bowl, NW aspect of Seattle Ridge. 

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

Above 3,000′ in the Alpine, several old weak layers of facets and buried surface hoar sit near the ground and in the mid-pack. This structure is most pronounced in places with a thin overall snow cover, such as the South end of Turnagain Pass and the Summit Lake area.  As you plan your day, keep in mind that there are deeper persistent layers that could ‘wake up’ if you find a trigger spot in a shallow area. 

Weather
Sat, February 17th, 2018

Yesterday was mostly cloudy with light Westerly ridgetop winds and a brief period of moderate Westerly winds this morning. Temperatures in the upper elevations were in the high teens/low 20F’s and sea level temps bumped into the high 20F’s during the day and teens F’s overnight.  

Sunny clear skies and light Westerly winds are expected today.   Daily temperature swings should average in the upper 20F’s today and dip down into the teens (F) overnight. No precipitation is expected.  

Sunday will range from party to mostly cloudy with daily temperature swings in the low 20F’s to low 30F’s. Light winds will shift from the West to an Easterly direction. A similar pattern is expected through Monday followed by the possibility of warming trend starting Tuesday. However at this point low confidence remains in the long term forecast.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 22   0   0   64  
Summit Lake (1400′) 13   0   0   24  
Alyeska Mid (1700′) 20   0   0   57  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 19   W    7 25  
Seattle Ridge (2400′) 22   NW   5    17
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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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