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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, December 14th, 2013 - 7:00AM
Expires
Sun, December 15th, 2013 - 7:00AM
Forecaster
Kevin Wright
Avalanche risk The Bottom Line

New storm snow falling on a slick rain crust is increasing the avalanche danger.  With only a moderate amount of snow from this storm, I expect only  MODERATE avalanche danger.  However, confidence is low in this forecast until we are able to get into the field to verify the assessment.  

The interface of new snow on the ubiquitous crust is expected to provide poor bonding and low friction, meaning the new snow will not stick well and will slide off easily.  We also had sporadic surface hoar on top of the crust in some areas.  This unusual situation may make the avalanche conditions more dangerous than normal for a storm this size.

Natural and human triggered avalanches will be possible today in wind loaded areas, but are expected to be small in size and low volume.

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Sat, December 14th, 2013
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
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
  • 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

Given a relatively small amount of snow from this storm, I don’t think storm slabs will be widespread without a wind loading component.  Areas with deeper deposition due to wind – higher elevation, near ridgetops – will be the primary avalanche concern.  

Sunburst weather station recorded a gust to 43 mph this morning.  The light weight, low density snow will be easily and quickly moved around by wind.  

Avalanche Problem 2
  • Dry Loose
    Dry Loose
Dry Loose
Dry Loose avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.
More info at Avalanche.org

Areas unaffected by wind may have a tendency to form loose snow avalanches.  I expect any sluffs to be low volume and low force due to the cold, light density nature of this storm.

 

Weather
Sat, December 14th, 2013

Yesterday the snowfall started in the morning.  Temperatures have risen a little bit, but remain well below freezing in all locations.  Ridgetop wind has also been increasing since yesterday, with gusts reaching into the 30s and 40s from the East.

Snowfall accumulation up to this point is around 6-8 inches near treeline with more expected up higher.  Water content is low from the cold temperatures – this is light density powder.  

Today’s forecasted weather is continued snow showers, with less precipitation intensity than we had overnight.  A second round of snow will commence later this afternoon into the evening.  The storm is expected to taper off by late Sunday morning.  When it’s all said and done, storm totals may reach more than a foot of snow in some places.  

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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.