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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, February 11th, 2013 - 7:00AM
Expires
Tue, February 12th, 2013 - 7:00AM
Forecaster
John Fitzgerald
Avalanche risk The Bottom Line

The avalanche hazard is MODERATE above treeline and in open areas below treeline that were affected by the high winds from Saturday.   Lingering pockets of dense wind slab 1-2 feet thick are resting on a layer of lighter density snow above treeline and a slick crust in open areas below treeline.   The avalanche hazard is generally LOW below treeline except for in isolated pockets that received wind loading over the weekend.

Mon, February 11th, 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. 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

Sustained 40-60 mph winds on Saturday (two days ago) helped to form large dense wind slabs in many areas.  Most of the new snow that fell over the weekend is well bonded to the old snow surfaces.  In areas that received wind loading this bonding is not as good and it is easy to find “upside down” snow on ridgelines, in starting zones and above cross loaded gullies.  While these slabs have been reluctant to move under the weight of a snowmachine or skier, they still warrant respect, as they are large in size and able to do damage to a person once released.  Avoiding starting zones above 35 degrees, wind affected snow above terrain traps and isolated areas below treeline that have these slabs will be your best bet today.

Avalanche Problem 2
  • Cornice
    Cornice
Cornice
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
More info at Avalanche.org

The winds and snow that formed wind slabs over the weekend also helped to increase the size of cornices.  Always give cornices a very wide berth.  When approaching from below try to envision what would happen if that cornice you’re looking up at came towards you.  When traveling above cornices in the higher elevations, always avoid the edge.  If you can’t see what is below you or a cornice is looming above you it’s probably time to back off.  Try to view cornices from the side to see where the ground is and where the snow is overhanging & unsupported.  Heating from the sun combined with an absence of wind could increase the likelihood of cornices releasing today.

 

 

Additional Concerns

Loose Snow Avalanches

Several inches of light density snow will be found on the surface today.  The potential for relatively small shallow loose snow avalanches exists.  These will become a greater problem in steeper terrain.  Avoiding these surface slides in steep terrain above cliff bands and gullies will help to minimize your exposure to this hazard.

Deep Slabs

We are still monitoring the weak snow at the base of the snowpack that formed in the first part of the season.  Above 2,000 feet and in areas with less overall snow this problem is more pronounced.  While it is unlikely to trigger a deep slab today, it can’t be ruled out.  Assess the terrain you are on and ask yourself some questions.  What is the overall snow depth?  Are there potential trigger points where the snow is shallow?  Continuing to follow good travel habits, i.e. only putting one person at a time on a slope and using true islands of safety will help to mimimize your exposure to this problem.

Weather
Mon, February 11th, 2013

The mountains around Turnagain Pass have picked up 2-4″ of new snow with .2-.3″ of water in the past 24 hours.   The Girdwood Valley has had up to 6″ of new snow with .3-.6″ of water.   Winds at 3,800′ have averaged in the high teens out of the E and SE and temperatures have been in the low 20s F.

Veering winds have slowed down this morning and temps are on the decline.   Light snow is falling at sea level.  

Snowfall will taper off this morning.   Ridgetop winds will be light, blowing 10-15 mph out of the W and NW.   Temperatures at 1,000′ will be around 30 degrees F and skies will be clearing through the morning hours.

The next chance for snow will come tonight and into tomorrow morning, with generally low amounts expected.

________________________________________________________

Graham will issue the next advisory tomorrow morning February 12th.

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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.
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