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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, December 26th, 2013 - 7:00AM
Expires
Fri, December 27th, 2013 - 7:00AM
Forecaster
Kevin Wright
Avalanche risk The Bottom Line

The danger rating continues to be  MODERATE above treeline for the persistent weak layer around the December drizzle crust.  We’ve had a number of small, low volume avalanches on this layer in the last week.  As the weather has not contributed to instability in the last few days, the avalanche likelihood is slowly decreasing.  

Thu, December 26th, 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
  • 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 weak layer above the crust continues to show signs of poor strength and a tendency to propagate.  Somebody traveling in the wrong place may still pop a slab avalanche.  

The photo below illustrates the full depth (31 inches) of the snowpack at 2000 ft.  The most pertinent result from the test was the failure above the crust, which propagated in an extended column test.  The snowpack is showing moderate strength, with poor structure and a slick crust interface.

As recently as yesterday we got a report of whoomphing from the Tincan area.  This tells us that a person can still initiate a collapse.  A collapse occurring on a steep slope may trigger an avalanche.

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

Spontaneous, natural avalanches were observed during the windy conditions on Monday.  We know that areas near ridgetops, above treeline are still holding pockets of stiff wind slab from that wind event.  As is normal, a little more caution is warranted when approaching ridgelines where wind blown snow will be expected.  The denser and stiffer layers near the snow surface complete the recipe for the most probable way to find an avalanche today.  

Weather
Thu, December 26th, 2013

It has not snowed since Sunday.  Weather over the last few days has been clear and cold with light wind.  

A temperature inversion can be found in some areas.  It is quite a bit warmer than Anchorage this morning.  Sunburst sits at 23 degrees, Center ridge at 18, and Seattle ridge at 25 degrees.  Wind at the ridgetops is very light.  Mostly clear skies are expected today with patchy fog in the valleys.  

There is snow in the forecast for Friday night and Saturday!  Cross your fingers…

Observations
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Date Region Location
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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.