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Sat, February 16th, 2013 - 7:00AM
Sun, February 17th, 2013 - 7:00AM
Kevin Wright
Avalanche risk The Bottom Line

Colder temperatures and calm weather are trending the snowpack toward good stability.   Some problems still exist however, illustrated by large explosive triggered avalanches at the Seward Highway yesterday.   Above treeline a MODERATE danger exists for recent storm slab resting on various weak interfaces.  

Today looks like it will be a stellar day in the backcountry.   We are still within 24 hours of the end of the last storm, meaning that the snowpack is still adjusting to that load.   Some degree of terrain management and avoidance of high consequence steep terrain is advisable.  

Sat, February 16th, 2013
Above 2,500'
2 - Moderate
Avalanche risk
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. 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
  • Storm Slabs
    Storm Slabs
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.
More info at Avalanche.org

The storm ended yesterday, and left a blanket of fresh snow across all elevations and aspects of the mountains.  This is good news for backcountry quality, but makes it difficult to identify the areas of greatest concern.  Wind during the storm was trending from the east, so westerly aspects may be harboring wind pockets and larger cornices. 

A big piece of information came in yesterday from Seward highway/AKRR crews when they triggered a large avalanche near Kern creek.  The avalanche below was larger than anything we’ve seen in quite some time.  This begs the question – Is it still possible for a person to trigger a larger avalanche today?  It seems unlikely, but perhaps not impossible.  We think that areas closer to Girdwood may be somewhat less stable than Turnagain Pass.  This is due to greater snowfall in the recent storm cycles, and a more prominent crust/weak layer combination that has shown it can cause big avalanches.

Other storm related issues to think about –

1.  Wind loading up high – few people have ventured above 2500 feet in the last week due to poor visibility and stormy weather.  Watch for wind slabs in steep terrain.  This is a wild card, for which we currently don’t have a lot of information.

2.  Cornices – are likely to be large and unstable.  We found very small cornices to be easily triggered on Thursday.

3.  Loose sluffs – Steep terrain may have enough loose powder to entrain and pick up volume.

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
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.
More info at Avalanche.org

In Turnagain Pass, we have seen occasional pits that indicate avalanche propagation is possible on a melt/freeze crust that typically varies from 1-3 feet in depth.  This problem is not evident everywhere…  The most reactive elevation seems to be between ~1900 and ~2900 feet.  It has shown itself to be worse in Girdwood than Turnagain Pass.

The video in this link illustrates the nature of the crust problem.  It isn’t easy to initiate, but when it does it may propagate into deeper layers.

Sat, February 16th, 2013

The last burst of snowfall which ended yesterday was the highest daily snowfall we’ve had in a month at Turnagain Pass.   Over the last week the snow has been piling up, slowly but surely.   Above treeline elevation is more than a foot of new snow in the last 48 hours with closer to 3 feet in the last week.   The storm had some strong wind over the last few days, which diminished Thursday evening before the snowfall ended.   Temperatures also dropped as the storm ended, placing light density, unconsolidated snow (powder) at the surface.   Currently temperatures are in the teens at low elevations and single digits at higher elevations.

Today, mostly sunny skies are expected.   Wind is moderate currently but will become light by late morning.   Snow and wind is back in the forecast tonight, leaving a short window of good weather for today.

Wendy will issue the next advisory on Sunday morning 2-17.

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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.