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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, December 6th, 2012 - 7:00AM
Expires
Fri, December 7th, 2012 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

A bump in ridgetop wind today along with the possibility of an inch of snow will change our avalanche problems slightly, but not the overall danger. We continue to have a MODERATE avalanche danger above treeline for slab and loose snow avalanches. Human triggered avalanches are possible on slopes steeper than 35 degrees and most pronounced in areas with fresh wind deposited snow. The avalanche danger will remain LOW below treeline.

Watch for a rapid rise in danger this weekend as snow and wind is finally in our forecast.

Thu, December 6th, 2012
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
  • 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

An increase in ridgetop wind today, into the 20-30mph range from the east, will be enough to form shallow fresh wind slabs on the lee sides of ridges and gullies. With plenty of loose existing snow, warmer temperatures and the possibility of an inch of new snow, drifts should be fairly cohesive. These will be sitting on loose, non-cohesive snow that will make them touchy. Keep an eye out today for areas where the wind is picking up and loading slopes. Stiffer supportable snow, with any cracking and collapsing, will be bulls eye clues you found a wind slab and if the slope is steep enough, could slide.

The persistent slab problem has not entirely gone away as weak snow still lurks near the ground. See observations from yesterday. With added weight of a wind slab and a person there is the possibility of a slide breaking to the ground.  In this case, the consequences are higher than with triggering a fresh wind slab. Expect any slide triggered, no matter how small, to run far.

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

Sluffing in the weak faceted (sugar) snow is becoming quite prevalent. This video from yesterday shows just how easy it is to initiate one of these sluffs. Even though these are easy to trigger they are also easy to avoid due to their predictable nature. Know where your sluff will go (as to not be overrun from behind) and also watch that you do not hit someone below as they are running to valley bottoms.

This is a good example of how weak our snowpack is right now. The snow has lost so much cohesion that if nudged down a slope it will gain momentum, entrain more snow and run till the slope angle lessens. Once a fresh layer of snow falls on this existing surface (Saturday through Sunday) the sugar snow wil have a hard time holding any new snow to the ground and avalanche conditions will rise rapidly.

Weather
Thu, December 6th, 2012

Clear skies yesterday were replaced with cloud cover overnight and increased the frigid temperatures at Portage (10ft above sea level) from -22 to +2F overnight. Temperatures at the mid elevations and on the ridgetops have increased as well are in the 15-20F range. Winds are still light blowing 5-10mph from the NE and gusting 10-15mph. Expect temperatures to rise another degree or two throughout the day and winds to increase into the teens gusting in the 20’s from the east. We are likely to see some flurries and possibly an inch of snow today.

The big news will come Saturday and Sunday when a change in the weather pattern really sets in. Models are suggesting around 4-7 € of snow on Saturday in the Eastern Turnagain Arm area (.5 water equivalent) and 12-22 € of snow for Saturday night and Sunday (between 1 €“ 2 € water equivalent). Stay tuned as we get closer to the weekend.


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).  

Graham will issue the next advisory Friday morning, December 7th.

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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.