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Thu, November 22nd, 2012 - 7:00AM
Fri, November 23rd, 2012 - 7:00AM
Kevin Wright
Avalanche risk The Bottom Line

We are issuing advisories 5 days a week through November on Sat, Sun, Tue, Thur, Fri.

Bottom Line

Stronger wind overnight, combined with a serious weak layer and recent natural avalanche activity, will keep us at a CONSIDERABLE danger rating above treeline.  We are still finding frequent collapsing in undisturbed snow, which tells us the facets are still weak and ready to fail when stressed.  Any fresh windslab from overnight wind should be approached with a strong sense of caution.

Thu, November 22nd, 2012
Above 2,500'
3 - Considerable
Avalanche risk
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

We found a big piece of the puzzle yesterday in the form of recent natural avalanches.  The presence of naturals underscores the seriousness of the current snowpack.  Sugary faceted snow crystals form the foundation of our snowpack everywhere right now.  They are showing a tendency to collapse when stressed and will avalanche on steeper slopes.  

Turnagain Pass got a small amount of snow on Sunday, which probably added enough stress to cause the avalanche in the picture below.  This is a steep north facing slope on Seattle ridge, and is representative of most of the region.  Similar problems have been found near Portage, Summit, and Eagle River.  

Natural avalanche in Warmup Bowl, north face.

The cold weather combined with such a shallow snowpack is transforming all the snow into weaker faceted crystals.  We are finding a strong temperature gradient everywhere which is made worse by sub-zero temperatures.  In most places you won’t find much of a stiffer slab on top of the weaker snow.  This lack of a slab is preventing a lot of slopes from avalanching for the time being.  When we do get stiffer snow and more stress on top of the delicate foundation, avalanches will be much more likely and dangerous.  

Avalanche Problem 2
  • Wind Slabs
    Wind Slabs
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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

Wind overnight hit gusts to 30mph at ridge tops.  This is the strongest wind we’ve had in over a week.  If the wind was strong enough to transport the loose surface snow, we could have enough slab to cause problems.  Any stronger or denser snow on top of the super weak facets near the ground is likely to exacerbate our problems.  

Thu, November 22nd, 2012

Wind overnight is the big news since yesterday.  The last snowfall is now several days in the past, but was enough to cause some snow stability problems in the form of natural avalanches.  The storm hitting Southeast Alaska today is not expected to have a significant effect on Southcentral.  A slight chance of snow showers is possible today.  Wind is expected to increase in some areas this afternoon.  

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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.