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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, February 13th, 2013 - 7:00AM
Expires
Thu, February 14th, 2013 - 7:00AM
Forecaster
Kevin Wright
Avalanche risk The Bottom Line

Continued snowfall in small to moderate amounts, combined with a mid elevation crust keeps us at MODERATE above treeline.   The avalanche problem ranges from small wind slabs, to larger and more dangerous but less likely persistent slabs.   The crust problem seems to be at a specific elevation band, but limited information is keeping us from being confident about where to avoid that problem.  

Wed, February 13th, 2013
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
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. 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

On Monday we got a report of a disconcertingly large skier triggered avalanche in the Girdwood valley.  It appears to have been remotely triggered near the 2700 foot elevation and slid on or under a 2 week old crust which is now buried 2-3 feet deep.  Yesterday we confirmed the reactive nature of that weak layer.  See the full test results on our observations page.  Our testing found a specific elevation band between 2000 and 3000 feet contains a series of crusts and facets that can collapse under the weight of a person and propagates large distances.  Snow pit testing shows only moderate strength, and failure happens in a clean and fast nature. 

We don’t know how widespread this problem is.  It appears to be common in the Girdwood valley, but so far we have few reports across Turnagain Pass that correlate to a reactive ice crust.  However, it should be assumed to be common at the mid-elevation band regionwide until we prove otherwise.  This problem is not likely to go away quickly, and may get worse before it gets better. 

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 slabs, large cornices, and building storm snow will be a problem above treeline.  Over the last week, no single day has brought a large amount of snow, but the combined 7 days equals close to 20 inches of new snow at Turnagain Pass and perhaps double that amount in Girdwood.  Wind has been consistently strong.  We can expect to find areas of stiffer and deeper wind deposited snow which may be triggerable by a person.  Any steep wind loaded terrain above treeline should be approached with caution corresponding to the terrain consequences. 

Weather
Wed, February 13th, 2013

Over the last week, each day has brought 1-4 inches of snow to Turnagain Pass.   The last 24 hours was similar with about 3 inches new snow.   Temperatures have been in the mid 20s with a moderate east wind reaching into the 30s and 40s yesterday.

Today looks like more of the same.   An inch of snow is expected this morning, with snowfall tapering off by noon.   Tonight 4-8 inches is expected and increasing wind 40-60mph as the next frontal system moves across southcentral Alaska.  


Wendy will issue the next advisory tomorrow morning, February 14.

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.