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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, December 26th, 2017 - 7:00AM
Expires
Wed, December 27th, 2017 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

There is a  MODERATE  avalanche danger today in the alpine and treeline elevation bands where triggering a hard wind slab will be possible on steep, unsupported slopes.  Above 3,000′ triggering a much larger and more dangerous deep slab avalanche is also possible due to weak snow near the ground.  Heightened avalanche conditions exist on specific terrain features. Evaluate snow and terrain carefully.

There is no hazard rating below 1,000′ due to a lack of snow.  

**Click  HERE  for the Summit Lake Summary posted Saturday.

*Please remember your safe travel practices! This includes, exposing one person at a time in avalanche terrain, watching your partners, being rescue ready and having an escape route planned.

Tue, December 26th, 2017
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. 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
  • 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

Be suspect of unsupported slopes steeper than 30 degrees that have a fat, smooth, pillow-type shape to them. Strong Easterly winds over the weekend changed the surface conditions of our forecast zone, creating hard wind slabs on leeward features. The wind slabs may be farther down slope due to the high wind speeds or only found on one side of gully features due to cross loading. Observations Sunday found hard supportable wind slabs as low as 1600’ on Eddies. These slabs averaged around 5” thick and were failing in hand pits on old weak snow. Any wind slabs today will be hard and supportable (to a skiers weight), they could be stubborn to initiate and may lure a skier well onto a slope before it fails. Listen for that hollow, drum-like sound below your skis or use a pole to probe for that hard snow over weak set up.  These slabs could be as thick as 1-2’ deep in upper elevations and if triggered in the alpine have the potential to initiate a much larger and more dangerous deep slab avalanche. More on this below in the secondary concern. 

Cross loading on Seattle Ridge.

 

Avalanche Problem 2
  • Deep Persistent Slabs
    Deep Persistent Slabs
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.
More info at Avalanche.org

Deep slab avalanches have been a concern in the advisory now for two weeks. We are not trying sound like a broken record but the message is the same. This set-up continues to warrant elevated caution and respectThis is a high consequence avalanche problem that is impossible to outsmart. The snowpack recipe for deep slab avalanches has been found in the upper elevations of our forecast zone, above 3000’ on slopes that did not avalanche during the early December storm cycle. The snowpack ranges from 3-5+’ thick and is sitting on weak basal facets. Observations over the last few weeks indicate this poor structure is widespread across our region in the alpine elevations.  

When dealing with a deep slab avalanche problem, keep in mind:

  • Large snow covered slopes that do not have piles of old debris under them are all suspect 
  • Thinner areas of the snowpack (1-2’ thick) are likely trigger spots as well as scoured areas near rocks 
  • It is possible to trigger this avalanche from below and it could run further than expected. Choose terrain wisely!
  • Due to strong winds over the last month the snow depths are highly variable and there may be more trigger spots than we realize
  • Thicker areas (3-5+’ thick) will be difficult to trigger and several tracks may be on a slope before someone finds a trigger point

Obvious clues like ‘whumpfing’ and shooting cracks may not be present until it is too late. Evaluate snow and terrain very carefully and steer clear of large loaded slopes. 

Poor snowpack structure in the alpine, hard snow over weak snow near the ground. 

 Pastoral. Remotely triggered Deep Slab avalanche that occured on December 20th, 2017.

 

Weather
Tue, December 26th, 2017

Yesterday was mostly cloudy with a few light snow showers and drizzle below 500′. The winds were light and easterly and temperatures were in the low 30Fs to mid 20Fs. Overnight skies cleared and temperatures dropped a bit into the the low 20Fs and teens. Winds shifted to the west and remained light.

Today will be mostly sunny with some patches of valley fog and temperatures in the low 30Fs to mid 20Fs. Winds will be northwesterly 5-15 mph. Tonight temperatures will drop down into the teens and skies will be partly cloudy.  

Wednesday is forecasted to be partly sunny with temperatures in the 20Fs and calm winds. Thursday looks to be pretty similar with some more sunshine on tap. Stay tuned for possible snow over the weekend.  

*Center Ridge SNOTEL is reporting erroneous temperature data. See  Turnagain Pass DOT weather station  for accurate temperature at 1000′.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) *31   0   0   27  
Summit Lake (1400′)  23 0 0  10
Alyeska Mid (1700′)  29 0   0  29

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 21   ENE   5   22  
Seattle Ridge (2400′)  25 SE    7 18  
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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.