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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, March 27th, 2013 - 7:00AM
Expires
Thu, March 28th, 2013 - 7:00AM
Forecaster
Kevin Wright
Avalanche risk The Bottom Line

The recent snow storm on Monday gave us enough snow to worry about backcountry stability.  After a look around yesterday, we found mostly stable snow with pockets of stiff slab with a reactive character.  Today’s danger rating is  MODERATE for specific areas of stiffer wind slab which sits on top of loose powder.  South facing slopes are also a greater concern with buried sun crusts that have shown to be reactive over the last 5 days.  

Wed, March 27th, 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. 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
  • 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

The storm on Monday dropped 12-15 inches of snow in some parts of Anchorage.  You can expect to find similar amounts in the Turnagain forecast region, with Girdwood getting a little more in the areas we’ve surveyed.  A notable feature of this storm was the cold temperatures.  The snow fell cold and dry, making for a light density powder with low water content.  Despite only moderate wind during the storm, the light powder was easily moved around by wind.  There are a lot of wind affected slopes, varying from scoured surfaces on one aspect, to stiff wind slab, to sheltered pockets of loose powder.  

Today, the wind slab is my greatest concern.  Yesterday we found a couple pockets of stiff supportable wind slab that collapsed with a “whoomph” when we skied across (see pit profile and description here).  Given a steep enough slope, this would probably initiate a small avalanche.  Furthermore, the problem can be even worse if it sits on top of a sun crust.  The two things I would actively try to avoid today – stiff wind slab on steep slopes, and southerly aspects where reactive sun crusts may be found.  

If you can avoid these two issues, there are still pockets of good quality loose powder to be found.

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

The recent stretch of sunny weather produced 2 specific weak layer concerns.  

1.  Sun crusts on south aspects.  We found at least 3 distinct buried sun crusts yesterday, with the top one being moderately weak with a tendency to propagate.  A number of smaller skier triggered avalanches have been reported in the last week, and the majority seem to be on southerly slopes, probably because of the sun crust layers.

2.  Non sun affected slopes simply got weaker in the surface snow during the 2 weeks of clear weather.  Weak snow by itself isn’t necessarily a problem, until you get stronger or stiffer snow on top of the weak snow.  

Weather
Wed, March 27th, 2013

Following the snow storm two days ago, the weather turned cold and sunny.  Last night it got even colder, with temperatures dipping into the negatives.  There is a mild temperature inversion, making the mountains slightly warmer than the valleys.  

Temperatures at 6am

Portage valley -15 degrees

Turnagain Pass Center ridge 2.7 degrees

Summit Lake -7.3 degrees

Alyeska, top chair 6 is 4 degrees

We had no measureable precipitation yesterday and wind was light from the north at most of the weather stations.  
Today’s weather looks fairly tame if you can deal with the cold.  Incoming in the next few days is a larger storm system with warmer temperatures and more precipitation.


Graham will issue the next advisory tomorrow morning, March 28th.

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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.