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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, December 1st, 2012 - 7:00AM
Expires
Sun, December 2nd, 2012 - 7:00AM
Forecaster
Kevin Wright
Avalanche risk The Bottom Line

ANNOUNCEMENT

Due to inadequate snow conditions and to prevent resource damage, operating or possessing a snowmobile on or within the Seward and Glacier Ranger Districts of the Chugach National Forest is prohibited until further notice.  You can view the official Forest Order for this closure HERE.


BOTTOM LINE

The entire shallow snowpack now consists of weak faceted forms, with surface hoar on top.  Stiffer slabs are becoming harder to find, and the avalanche danger is slowly diminishing.  Continue to be wary of pockets of stiffer snow in steep terrain above treeline where moderate avalanche danger can still be found.  

Sat, December 1st, 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
  • 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

Pockets of unstable snow may still be present in wind loaded areas at high elevation.  Most slopes where people are skiing have weakened to the point where the entire snowpack is one big weak faceted layer.  This actually makes the backcountry safer now than it was a couple weeks ago, except that it’s easier to tag rocks on the way down…  

The analogy I’ve come up with compares the snowpack to engineering a building.  We all know that we can construct a tall stable skyscraper that can withstand significant stresses from wind and earthquakes.  The strength of that building relies fundamentally on the strength of the foundation.  If you build a skyscraper on top of a weak foundation, failure and catastrophic collapse is likely.  If you make a smaller and lighter building, the foundation doesn’t need to be as strong to support the structure above.  Our snowpack right now is a tent, without any foundation.  The tent itself is inherently weak, but collapse of that tent isn’t nearly as catastrophic as a collapse of the skyscraper.  In fact, the tent will bend when stressed before it breaks and collapses.  The big problem will happen when the next snow storm (new construction) gets placed on top of the tent.  How much weight can that tent support?

Why did the snowpack turn into one big weak layer?  Check out Wendy’s temperature profile graph HERE.  It’s all about weeks of high temperature gradients caused by shallow snow (only 22 inches in November) and cold air temperatures.

temperature gradient

Weather
Sat, December 1st, 2012

The forecasted high wind over the last couple days has not affected the Turnagain Pass region.  Clear, sunny, and cold weather will continue today and through the weekend.  A temperature inversion is still present, making higher elevations warmer than the valleys below.  Portage this morning is reading -18 degrees, while Sunburst at 3812 ft is reading 15 above.  

A pattern change is beginning to show up in the extended forecast.  A chance of snow is predicted for next Wednesday!  


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).  

Wendy will issue the next advisory Sunday morning, December 2nd.

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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.