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Fri, December 28th, 2012 - 7:00AM
Sat, December 29th, 2012 - 7:00AM
John Fitzgerald
Avalanche risk The Bottom Line

The avalanche hazard remains at HIGH today, where natural avalanches are still likely and human triggered avalanches are very likely.   Below treeline the hazard is CONSIDERABLE, where the threat of natural avalanches remain and human triggered avalanches are likely.   Travel in avalanche terrain is not recommended today.

Fri, December 28th, 2012
Above 2,500'
4 - High
Avalanche risk
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
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
  • 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 potential for skiers, riders and snowmachiners to trigger avalanches within layers of the new snow is very high today.  Terrain above treeline harbors large areas of slab, including wind slabs, that will be very sensitive to human triggers.  The steady and often rapid loading of this week has created unstable conditions in the upper layers of the snowpack.  Those of you who have ventured out into the mountains this week have witnessed this firsthand, getting machines stuck in the weak snow and enduring arduous trail breaking.  This new snow is dense and unsupported in many areas, making travel slow and challenging.  Below 1000 feet we have wet snow due to rain falling over the past 24 hours.  It all boils down to the fact that we have a lot of new snow sitting on multiple weak layers.  While we have limited info from the upper elevations, we know that the snow is very unstable, as both AKDOT & Alyeska resort have been able to trigger avalanches on a variety of aspects and elevations.  Large natural avalanches as recent as yesterday morning have been observed in the region.  The recent observations from the backcountry that we do have paint the picture of instability well.

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

While deep slabs are a secondary concern to storm snow/wind slabs, do not underestimate the potential destructive force of this type of problem.  Large, dense slabs that have formed over the past four days sit on a base of weak snow.  Triggering avalanches that break into these deeper layers could have dire consequences.  If caught in a deep slab avalanche, the chances of surviving are low.  I like to think of this problem as one that is unmanageable.  As such, it is important to keep terrain choices very conservative.  An avalanche triggered even in seemingly “small” terrain could have serious consequences given the large volume of snow.  While storm snow instabilities will diminish within days of the end of a storm, this deep slab problem will be with us for an extended period of time.

Fri, December 28th, 2012

The Turnagain Pass area has received ~7 inches of new snow with .6 inches of water in the past 24 hrs., bringing our storm period totals to over 50 inches.   Yesterday morning winds diminished to an average of 20 mph out of the East.   Temperatures have been mild, hovering in the low 20s on ridgetops and low 30s at 1000 feet.   Rain was falling yesterday below 1,000 feet.

Today will bring lingering snow showers to the area with accumulations of up to 3 inches of snow during the day.   Temperatures will remain just below freezing at 1000 feet and winds will be out of the East and Southeast at 20-30 mph.

Snowfall and winds will increase in intensity this evening and into tomorrow.   We should expect to see this generally moist weather pattern to continue over the next several days.

Kevin will issue the next advisory Saturday morning December 29th.

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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.