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Issued
Mon, December 29th, 2014 - 7:00AM
Expires
Tue, December 30th, 2014 - 7:00AM
Forecaster
John Fitzgerald
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE in the alpine today.   Wind slabs building into the 12-16 € range could release naturally and will become increasingly sensitive to human triggers as the day progresses.   The trend for instability will be on the rise throughout the day.

It will be possible for avalanches originating in the higher elevations to run into the lower elevations today.   Because of this the avalanche danger is MODERATE at and below treeline.   Human triggered wind slabs up to a foot in depth will also be possible in the treeline elevations.  

Avoid steep leeward slopes in elevations where new snow and wind is actively forming fresh wind slabs.   Avoid being in the runout of terrain that is above you.

Mon, December 29th, 2014
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
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

High ridge top winds, in the 40 mph range have been blowing out of the East since yesterday morning.  While the amount of snow available for transport has been minimal (in the 2-4” range) slabs have been forming and rapidly loading leeward slopes.  Due to the rapid nature of this loading, natural avalanches are possible today. When winds are at these speeds, slabs tend to form lower down on slopes and in a more scattered pattern.  Because of this it becomes possible to travel through starting zones without being on a wind slab only to find yourself on top of unstable snow part way down.   

Fortunately the weather today will make getting into starting zones very difficult, as poor visibility will make travel challenging in the alpine.  Despite this, should you find yourself in upper elevation terrain be on the lookout for signs of wind loaded snow.  Shooting cracks will be the first indicator that the snow below your feet is unstable.  Snow that has an upside down feel to it, is stiff and is on the leeward side of slopes or along cross loaded gullies should be avoided today. 

Expert level terrain assessment skills will be necessary for travel in the mountains today, as avalanches have the potential to release naturally and run through multiple elevation bands.

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

In the back of our mind today are persistent slabs.  Weak snow 2 feet below the surface is present in the snowpack.  From Turnagain Pass down to Summit Lake that layer is widespread buried surface hoar.  In the Girdwood Valley that layer is in the form of facets and exists in pockets.  Several skier triggered avalanches have occurred over the past 2 weeks on these layers and are worth keeping in mind when out in the mountains today.  

The buried surface hoar on Turnagain has quieted down for the time being.  Skiers in the Girdwood Valley triggered an avalanche on Saturday on a steep upper elevation slope with a thin snowpack.  Avalanches occurring in the new snow have the potential to step down to these deeper layers.  Because of this it will be possible for avalanches to increase in volume; avalanches occuring within the new snow could pull out deeper slabs and be large enough to injure and bury a person.

 

Weather
Mon, December 29th, 2014

Yesterday ushered in winds in the 40 mph range out of the East.   Temperatures have been creeping up into the low 30s at 1,000′.   Rain at sea level began overnight.   Snow has been falling above 1,500′ with the Center Ridge SNOTEL showing 3 € of accumulation in the past 24 hours.

Today expect a continuation of this pattern – warm, wet, and windy.   Temperatures will remain in the low 30s at 1,000′, ridge top winds will be out of the East at 40 mph, and   2-6 € of snow will fall in the higher elevations.   The rain snow line should hover around the 1,000-1,500′ elevation through the day.

The plume of moisture moving over us will continue to pull warm air from the South.   We should expect to see continued snow and rain, high winds and mild temperatures through Tuesday.

Alaska DOT has the Turnagain Pass Weather station back up and running.   This is a great resource for checking weather info and webcams at road level (1,000′).

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 31 3 .3 31
Summit Lake (1400′) 34 0 .1 6
Alyeska Mid (1700′) 31 2 .3 23.6

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23 E 35 87
Seattle Ridge (2400′) 24 SE 37 77
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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.