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Issued
Sat, November 17th, 2012 - 7:00AM
Expires
Sun, November 18th, 2012 - 7:00AM
Forecaster
Kevin Wright
Avalanche risk The Bottom Line

Good morning. This is Kevin Wright with the Chugach National Forest Avalanche Information Center on Saturday, November 17th at 7am. This will serve as 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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Bottom Line

Early season conditions are creating an unstable snowpack. Recent skier triggered avalanches and a weak snow structure means that more avalanches are likely if people travel in more agressive terrain.  Conservative terrain management is the key to staying safe this weekend.

Sat, November 17th, 2012
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
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
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

The early season thin snowpack is giving us a predictable unstable backcountry.  Weak, collapsable facets can be found at the ground in nearly every pit you dig right now.  Pit data is hardly relevant though, because we have better information in the form of 3 recent skier triggered avalanches.  The most recent happened yesterday, November 16th on the south face of Sunburst (see picture below).  Another one on the north face of Tincan on Monday, and a third near Raven glacier a week ago.  Check the observations for specific details on those avalanches.  All three were above 3000 feet, but happened on both south and north aspects.  While I don’t think the backcountry is going to be hair trigger everywhere, the prevalence of skier triggered avalanches (including the one yesterday which appears to be remotely triggered) gives us cause for concern.  The Considerable danger rating means that terrain management and conservative decisions are essential to avoid avalanches.  When choosing an area to ride today, look for moderate slope angles less than 35 degrees, and if you can find it, areas without a windslab on top of the weak facets.  

Tip for interpreting the danger ratings – Considerable above treeline today doesn’t mean that every slope is likely to avalanche when skied.  In this case it means that human triggered avalanches are likely in specific areas including steeper wind loaded slopes at higher elevation.  If we see a lot of tracks exploring farther back each ridge at Turnagain Pass, we will be seeing more avalanches.  

 

Avalanche South face Sunburst

Remember, the shallow snowpack means that avalanches are likely to rip all the way to the ground, like in the photo above.  Think about the cheese grater consequences of taking a ride through that terrain.  Exposed rocks will cause significant injuries in even a small avalanche. 

All of our pits yesterday showed the same structural problems, with well developed facets on the ground.  In some places buried surface hoar can be found on top of the facets, then the newer semi-supportable snow on top.  Boot penetration is consistently straight through to the ground through the 2 foot deep snowpack.  We did three extended column tests yesterday, and all three had full propagation at the top of the facet layer.  Near treeline showed easy failure, which corresponded to occasional subtle whoomphing when you venture off the set skin track.  Higher up we got moderate failure, but still full propagation with the test.  These tests corroborate the likelihood of avalanches like the one on Sunburst yesterday.  

Below treeline the danger should be less than up high.  Two reasons seem to be helping lower elevations – 1.  Warmer temperatures have started strengthening the facets at the ground.  They have a moist feel and the crystals are gaining strength.  2.  The sheltered terrain below treeline means less wind affected snow and less slab on top of the weak layers.  You basically have weak snow over weak snow, which is missing the critical slab component to create an avalanche.  

Weather
Sat, November 17th, 2012

The most recent snowfall happened early in the week, bringing 3-5 inches to Turnagain Pass.  Overnight temperatures dropped significantly, with single digit temps at the ridge tops, and negative temperatures in depressions like Granite Creek and Summit Lake (expect cold at the trailhead today!).  A slight chance of snow showers are in the forecast today, but we don’t expect significant precipitation.  Wind was negligible yesterday and overnight, but strong gap winds are predicted to increase in areas such as Turnagain Arm this afternoon.  

Observations
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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.