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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, February 12th, 2015 - 7:00AM
Expires
Fri, February 13th, 2015 - 7:00AM
Forecaster
John Fitzgerald
Avalanche risk The Bottom Line

The avalanche danger is MODERATE in the Alpine and Treeline elevations.   Slabs up to 10 € in depth could be triggered in steep terrain.   These slabs have the potential to run far on steep sustained slopes.   Wet loose avalanches will also be easy to trigger on steep slopes at all elevations today.

Thu, February 12th, 2015
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
0 - No Rating
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

This week has brought much welcomed new snow to the area.  Snowfall amounts have been variable, with greatest amounts near Turnagain Arm and lesser amounts in the Girdwood Valley and as you head South along Turnagain Pass.  Winds have been in the moderate range.  What this leaves behind is a high degree of variability in the newest slab.

A layer of very weak snow below this slab is much more uniform throughout the forecast area.  A 3 week dry spell created this layer of mostly facets.  This layer is sitting on a stout crust in the lower and mid elevations.  

On Turnagain Pass you will encounter soft slabs 6-10” in depth depending on location.   Yesterday skiers remotely triggered slabs from up to 100 yards away.  Tests yesterday showed the potential for avalanches to propagate across slopes (see video below).

In the Girdwood Valley older wind slabs (4-8” thick) which formed almost a week ago sit on this weak layer.  These older wind slabs are concealed by 6” of storm snow.  This makes it more difficult to detect these slabs.

While slabs are not very thick, they do have the ability to connect across terrain features and produce enough volume to injure or bury a person.  This issue becomes more pronounced on broad, open and complex terrain.

What does this all boil down to?  Staying off of terrain over 35 degrees including slopes connected to steeper terrain will be the best way to avoid triggering slabs today.

 

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

Warm temperatures, occasional sunshine and loose surface snow will combine to make wet loose avalanches another snowpack issue to manage today.  Avoiding steep sunlit slopes will be the best way to manage this concern.  While volume may be low initially, steep sustained slopes hold the potential to produce higher volume far running wet loose avalanches capable of carrying or injuring skiers and riders.  This issue becomes even more pronounced when terrain traps such as cliffs, gullies, glide cracks or trees are below.

Rollerballs and shallow wet loose avalanche activity on Lipps yesterday (Feb 11, 2015).  Expect more of the same today.

rollerballs on Lipps

Weather
Thu, February 12th, 2015

Over the past 24 hours we have seen temperatures on the rise with ridgetop stations averaging in the mid to high 20s F.   Winds have kicked up on Seattle ridge into the 15 mph range in the early morning hours with other stations showing light winds.   A trace to an inch of snow fell on Turnagain Pass.

Today expect showery conditions, with periods of flurries and occasional clearing.   Snow accumulations will be light, in the 1-2 € range.   Rain/snow line should hover around the 1000′ level.   Temps will remain mild as warm air pushes up from the South, keeping ridgetop temps in the high 20s to low 30s F range.   Winds will be out of the East at 10-15 mph.

The extended outlook is calling for continued unsettled weather.   A broad area of circulation around the Gulf of Alaska will deliver generally light precipitation and warm temps through the weekend.

PRECIPITATION 24-hour data (6am – 6am)

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 30 1 .1 36
Summit Lake (1400′) 29 1 .1 9
Alyeska Mid (1700′) 31 0 0 26

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 25 E 3 27
Seattle Ridge (2400′) 26 N/A 11 30
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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.