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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, February 9th, 2018 - 7:00AM
Expires
Sat, February 10th, 2018 - 7:00AM
Forecaster
Heather Thamm
Avalanche risk The Bottom Line

The avalanche danger is  MODERATE  in the Alpine and Treeline zones where winds may be forming shallow wind slabs and adding stress to persistent slabs 1-2′ deep. If you see blowing snow or shooting cracks avoid leeward terrain features.   Additionally watch for unstable cornices along ridge lines.

*For the periphery zones, such as Girdwood to Portage Valley, and Johnson Pass to Summit Lake, more caution is advised where a slab could be larger and more connected.

Check out the Summit Lake Summary  HERE.

Special Announcements

For all the Hatcher Pass users out there – Hatcher Pass Avalanche Center is having their annual fundraiser tomorrow, Saturday night, Feb.10th, at the Palmer Moose Lodge!! HPAC is a growing avalanche center for a high use zone with a high number of avalanche accidents in Alaska. They need your support! Click  HERE  for details.

Fri, February 9th, 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. 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

Today elevated Southeast winds, 10-30mph, may be moving snow around in the alpine depending on where you travel. As of early this morning Seattle Ridge has been recording wind speeds in the 10-20mph range while Sunburst had remained protected. Loose surface snow is available for transport and it will be important to monitor surface conditions and watch for blowing snow as you travel. Small pockets of newly formed wind slabs may be tender on leeward features, but more concerning will be drifting snow adding stress to a persistent slab.  Several weak layers are buried within the top 1-2’ of the snowpack, including buried surface hoar and near surface facets. Avoid smooth pillowed surfaces in steep terrain or slopes with active wind loading. Shooting cracks or ‘whumpfing’ sounds will be obvious clues that the snow is unstable.    

*Deep Persistent Slab:  Weak snow can still be found near the ground in the upper elevations of our forecast area, 3,000′ – 5,000′. Although triggering a Deep Persistent Slab is unlikely, it is worth keeping in mind that poor structure does exist in places with a generally thinner snowpack. The Southern end of Turnagain Pass and Summit Lake zone are more suspect, and it wouldn’t be impossible to trigger in another part of our forecast zone with shallow snow cover. On Saturday (Feb.3) a large human triggered avalanche occurred on the East face of Twin Peaks and failed on weak snow (buried surface hoar and facets) sitting on a hard bed surface. Although the size of this avalanche is shy of 1 meter deep (a deep slab criteria,) its behavior is pretty much the same. This was a high consequence, hard to trigger avalanche where the 9th and10th skiers on a skin track found just the right trigger spot in a very shallow part of the snowpack. Luckily no one was caught or injured in this avalanche. A conservative approach would be to avoid large steep terrain with a shallow snowpack and exposed rocks. 

Earlier in the week wind affected snow was observed on the Northern end of Turnagain Pass on Eddies to Wolverine. This is a good example of where a shallow wind slab or a persistent slab may be lurking.  

 

 An avalanche triggered on the skin track at 3400′ on the East face of Twin Peaks on Saturday, Feb. 3, 2018.   

 

Crown profile about 30′ away from the trigger spot shows several weak layers including older weak snow near the ground where this avalanche failed, and recently buried surface hoar in upper layers. 

Additional Concern
  • 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: In windy areas cornices may be seeing additional stress today. Cornices are unpredictable and can break further back along a ridge than expected. Give these features plenty of space and avoid traveling underneath them. 

Loose snow avalanches ’sluffs’: In places protected from winds, expect loose surface snow to be small in volume, but could slide easily in steeper terrain with the weight of a person. Watch your ‘sluff’ and be aware of the consequences below you.

Weather
Fri, February 9th, 2018

Yesterday skies were clear with some patches of valley fog. Easterly ridge top winds were light at Sunburst weather station, but increased to 10-20 mph on Seattle Ridge. An inversion has kept valley temps in the single digits, but upper elevation temps bumped up into the upper 20F’s during the heat of the day in the sun.  No precipitation was recorded.  

Today clear skies will become cloudy by the evening. Southeast ridge top winds are expected to range from 10 to 30mph. Temperatures will be in the teens to mid 20F’s. No precipitation is expected today.  

Overcast skies and scattered snow showers are possible Saturday with gradually warming temperatures. Sunday into Monday temperatures are expected to be in the upper 20F’s in the mountains and low 30F’s near sea level. Although much uncertainty exists in the extended forecast, rain and snow showers are possible Sunday and Monday.   Easterly winds may range from Light to Moderate.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 20    0 0   63  
Summit Lake (1400′) 5   0   0   19  
Alyeska Mid (1700′) 21   0   0   51  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23   E   5   18  
Seattle Ridge (2400′) 21   SE   12   27  
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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.