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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, February 18th, 2013 - 7:00AM
Expires
Tue, February 19th, 2013 - 7:00AM
Forecaster
John Fitzgerald
Avalanche risk The Bottom Line

The avalanche hazard is MODERATE today above treeline where loose snow avalanches and older isolated pockets of wind slab will be the main concerns.   The mid elevations, between ~1,500-2,800′ continue to have a MODERATE hazard where a weak layer sits on a crust buried 2-3′ deep.   The avalanche hazard is LOW below treeline today, where recent snow has adhered well to older snow surfaces and avalanches are unlikely.

Mon, February 18th, 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
  • 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

Loose Snow Avalanches

New snow in the past 24 hours fell in many places without any associated winds.  Up to a foot of new snow can be found in the upper elevations with half that amount in the lower elevations.  This snow is “right side up”, as temperatures overnight have gradually cooled.  Loose snow avalanches will be a concern on steep upper elevation slopes today.  Periods of sunshine will increase the likelihood of triggering these slides.  These avalanches have the potential to knock people off of their feet or snowmachines.  As such it will be best to avoid steep slopes above terrain traps where sloughing could pull people into gullies, over cliff bands and into trees.

Wind Slabs
In the upper elevations isolated pockets of older wind slab may be triggered by a skier or rider today.  These pockets will be harder to detect, as a blanket of lighter density new snow sits on top of these slabs.  Staying off of steep previously wind loaded terrain features will help in avoiding this problem today.
With light to moderate winds forecasted for today, expect to see shallow new wind slabs forming near ridgetops through the day. With ample light density snow available for transport, expect these slabs to form easily and be most sensitive as they are forming.

Avalanche Problem 2
  • 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 mid elevations (between 1,500-2,800′) have a snowpack structure that has some potential to produce larger more dangerous avalanches.  A crust formed in late January has a very thin layer of weak snow above it in many areas.  Above that weak layer are slabs ranging in depth from 1-3 feet.  We have been investigating the likelihood of triggering an avalanche on this interface and have found mixed results within our forecast zone.  In some areas snowpit tests have shown this layer to be reactive and in other areas it has been a non issue.  While we’ve only had one report of a human triggered avalanche on this layer in the last week, the potential still exists.  The best way to avoid this problem is by managing your terrain and staying away from potential trigger points (e.g., steep rollovers, areas of thin snow) within this elevation band.

Weather
Mon, February 18th, 2013

Snowfall that began in the early morning hours on Sunday have accumulated up to a foot of new snow in the higher elevations.   Water amounts across the area range from .75″ in the Girdwood Valley, .5″ on Turnagain Pass and .3″ at Summit Lake.   Winds have been very calm out of the North.   Temperatures have been on a steady decline this morning with ridgetop stations reporting an average of 10 degrees F.

Today we should expect to see snow showers tapering off in the morning giving way to clear skies by this afternoon as a brief pause between storms takes hold.   This break will be short lived, as the next low pressure system moves into the area from the Southwest by tomorrow afternoon.

______________________________________________

Wendy will issue the next advisory tomorrow morning, February 19th.

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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.