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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, February 26th, 2019 - 7:00AM
Expires
Wed, February 27th, 2019 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is  MODERATE  on slopes over 35 degrees above 2,000′. Triggering a slab resting on weak faceted snow is still possible. Slabs could be anywhere from 6″ to 2′ thick depending on prior wind-loading.  Additionally, pay attention to changing conditions with warm temperatures in the Alpine, especially on steep slopes in direct sunshine. As always, give cornices a wide berth, limit exposure under glide cracks and watch your sluff.  

SUMMIT LAKE / JOHNSON PASS:    Triggering a larger, more dangerous slab remains a concern due to  variety of old weak layers in the mid and base of the snowpack. Extra caution is advised. Evaluate snow and terrain carefully.  

SEWARD / LOST LAKE:   Slabs up to 2′ in depth could be found and triggered in this area on steep wind-loaded slopes.

 

Special Announcements

Take a minute out of your day to help the Community Snow Observations crew with gathering snow depth information in Alaska!! Website is  communitysnowobs.org. It’s easy to do, you just need a smart phone, 30 seconds and your probe to measure the snow depth.

Tue, February 26th, 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

Yesterday Alpine weather stations recorded above freezing temperatures for most of the day.  The weather station on Max’s Mountain (3200′) above Girdwood recorded a high of 42F at 5 pm. Some small wet loose avalanche activity was observed as well as some “sticky” surface conditions on slopes getting direct sunshine. No new slab avalanche activity was reported. After a night with temperatures dipping below freezing and clear skies expect crust in the morning on surfaces influenced by yesterday’s warming. Watch for changing conditions again during the heat of the day. The Valentine’s near surface facets sitting around a foot below the surface is the main persistent weak layer of concern. As the days pass since our last loading event the likelihood of triggering an avalanche has decreased. However warming can sometimes make slabs resting on persistent weak layers more easy to trigger and stability tests are still showing that this layer has the potential to avalanche. 

What to keep in mind today:
    –  Wind-loaded, steep, unsupported slopes are the most suspect for popping out a slab that may be sitting on weak snow.
    –  Larger and more dangerous avalanches are possible in the Summit Lake and Johnson/Bench peak area where a thinner snowpack exists with multiple weak layers. 
    –  SUN EFFECT and moist/wet sluffs on or under steep rocky southerly terrain. On wind protected, shaded, steep slopes watch for facet sluffing.
    –  Cornices. Warmer temperatures at higher elevations can help loosen these monsters and with good weather and ridgeline travel, don’t forget to give cornices a wide berth.
 And remember to practice safe travel protocol

 

Slab over the Valentine’s facets, 2.22.19. This set-up has been noted across the advisory area.

 

Wet loose avalanches on Goat, 2.25.19

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

Glide cracks are unpredictable, not associated with human triggers, and can release without warning at any time.  The best way to manage this problem is to avoid traveling on slopes directly below glide cracks. A short list of known cracks in popular zones:  Magnum, Lipps, Seattle Ridge, Eddies, Lynx Ck. See a new glide crack or one that appears to be opening up? Please snap a photo and send us a quick ob!

Glide cracks on Magnum, 2.25.19. Photo: Nikki Champion

Weather
Tue, February 26th, 2019

Yesterday:  Clear and sunny! Temperatures were in the 30Fs to low 40Fs in the Alpine and in the teens to mid 20Fs at lower elevations. Winds were very light and easterly. Overnight temperatures in the Alpine were in the low 30Fs to high 20Fs. Valley bottoms were in the single digits to low teens, skies were clear and winds were calm.

Today:  Another day of sunshine and a temperature inversion. Expect 30Fs-40Fs at upper elevations and teens to 20Fs at lower elevations. Winds remain light. Temperatures will drop again overnight with clear skies and the inversion will continue.  

Tomorrow: The ridge of high pressure is still very much in place and the current weather pattern is expected to continue into the weekend. We may see a few more clouds later in the week but overall more of the same. Seems like a good time for a snow dance! Marvelous March powder???

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′)  25   0    0     60  
Summit Lake (1400′)  13   0     0     29  
Alyeska Mid (1700′)  23 0   0   55  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 31    SE  3 11  
Seattle Ridge (2400′)  30      E  1   6    
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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.