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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, March 29th, 2018 - 7:00AM
Expires
Fri, March 30th, 2018 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger continues to be  MODERATE  above 1500′ on all aspects. Triggering a hard slab avalanche 2-4 feet thick remains a possibility and may be triggered remotely. Pay attention to afternoon warming and give cornices a wide berth.  

Similar avalanche concerns exist in the  Summit Lake area    and other zones on the Kenai.

 

Thu, March 29th, 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
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.

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

With little change in the weather and the days since avalanche activity increasing, the chances of triggering an avalanche are decreasing. The core Turnagain terrain is trending towards LOW hazard. However, we are still concerned about someone triggering a hard slab avalanche 2-4’ deep, especially in periphery areas like Girdwood and the South end of the pass towards Johnson and Lynx. The weak layers in the snowpack that formed in January will not just go away but over time can become “dormant”. The question is… are they? Without any big changes in the weather, these older layers (facets and buried surface hoar) are becoming more and more difficult to trigger and we are in a low probability, high consequence situation. The tricky part about this is, the pack appears to be stable but the chance remains for an unmanageable and destructive avalanche if a person hits just the wrong spot. No obvious signs of instability may be present before a slope releases and it may be the 10th skier or snowmachiner onto a slope that finds a trigger point. These trigger spots will be in thinner areas of the snowpack near rocks or in scoured areas along ridges. Although triggering a slab remotely is also becoming less likely with time, it is not out of the question with this snowpack structure. 

If the sun does shine today be aware of warming later in the day on Southerly slopes. 

Poor snowpack structure in Summit Lake. The concern is that the snowpack in the Southern end of the pass is very similar to this and a trigger spot may still be found.

 

It can be hard to remember that under all the hard wind affected snow and crusts there is a slab over buried weak 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

Many cornices are quite large. Temperatures have been cool lately but as we head into warmer, sunnier weather remember this can help de-stabilize them. As always, give cornices plenty of space and limit exposure underneath them.

Weather
Thu, March 29th, 2018

Yesterday was partly to mostly cloudy in the morning becoming overcast in the afternoon. There were a few snow showers. Temperatures were in the mid 20Fs to mid 30Fs. Winds were light and variable. Overnight skies were mostly cloudy and temperatures were in the 20Fs and winds remained light.

Today will be mostly to partly cloudy with a chance of snow showers. Temperatures will be in the 20Fs to low 30Fs. Northwest winds will be light. Tonight will be mostly cloudy with a continued chance of snow showers, temperatures in the 20Fs and light winds.

Tommorrow clouds decrease and skies clear for the weekend. As high pressure sets up there is an increased chance of outflow winds impacting the area. Clear and sunny looks to be the pattern into next week. Time to do some spring snow dances!  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′)  29  0  0  76
Summit Lake (1400′)  31  0  0  30
Alyeska Mid (1700′)  30  0  0  72

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  22  variable 5    15
Seattle Ridge (2400′)  27   variable  

 2    

 14
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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.