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Sat, December 30th, 2017 - 7:00AM
Sun, December 31st, 2017 - 7:00AM
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger could rise to CONSIDERABLE late in the day due to increasing wind and new snow landing on weak surface snow. Pay attention to changing conditions. There is a Winter Storm Watch in effect.

Before the snow and wind start:

There is a  MODERATE  avalanche danger in the alpine, above 3,000′ where triggering a large and dangerous deep slab avalanche is possible due to weak snow near the ground. Triggering an isolated hard wind slab is still possible on steep leeward features or on unsupported terrain in the alpine.  Heightened avalanche conditions exist on specific terrain features. Evaluate snow and terrain carefully.

The avalanche danger is  LOW  at Treeline.  LOW danger does not mean NO danger. Pockets of unstable snow are not out of the question if you find yourself in very steep terrain, below 2500′. There is no hazard rating below 1,000′ due to a lack of snow.      

**Click  HERE  for the Saturday Summit Lake Summary.

Special Announcements

Check out the Near Miss Final Report for the December 20th human triggered Pastoral avalanche HERE. Thanks again to the party involved for sharing their story.

If you are headed to Hatcher Pass get the Saturday forecast here at hpavalanche.org.  

Sat, December 30th, 2017
Above 2,500'
3 - Considerable
Avalanche risk
3 - Considerable
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
  • Storm Slabs
    Storm Slabs
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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

Today is a transition day. We have had mostly cold clear weather with no precipitation for the last few days. This afternoon the winds are forecasted to pick up and snow should start falling. The new snow will be falling on very weak surface snow. There is widespread surface hoar, with near-surface facets below or hard wind crust or firm melt-freeze crust.  None of these old snow surfaces bode well for snow bonding to them. As the weather comes in today it will be important to pay attention to changing conditions. Slabs may form quickly, especially in leeward terrain. Quick hand pits can help check if the new snow is sticking to the old snow surfaces or not. The weak surface snow can also act as a few inches of fast moving loose surface snow “sluff” that could catch you by surprise if you’re not expecting it in steep terrain.

In addition, as you are out recreating today keep in mind triggering an old wind slab is still possible on very steep terrain in the alpine zone. Places you might find a hard wind slab will be in steep couloirs, large unsupported terrain features, or in thin rocky areas. Triggering a wind slab in the Treeline zone is becoming less likely, but is not out of the question in high consequence terrain. Triggering a wind slab could take you for an undesirable ride, and has the potential for initiating a much larger and more dangerous avalanche above 3000′. Be suspect of any slopes that may harbor a deep slab problem in the upper elevations.

Surface hoar over near surface faceted snow over a melt freeze crust 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

It has been over a week since a large avalanche was triggered on Pastoral by two skiers traveling below the NW face. If you have been reading the forecast regularly we are not trying sound like a broken record but the message is the same. This snow pack set-up continues to warrant elevated caution and respect. It is a high consequence avalanche problem that is impossible to outsmart and can take a long time to heal. The ingredients for a deep slab avalanche have been found in the upper elevations of our forecast zone, above 3000’ on slopes that did not avalanche in the early December storm cycle. This is a hard slab, 3-5+ feet thick, sitting on top of weak sugary snow (basal facets) near the ground. Observations over the last few weeks indicate this poor structure is widespread across our region in the alpine elevations. As the load increases over this snowpack structure during the upcoming storms there will be the potential for large natural avalanches.

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

  • Large snow covered slopes that do not have large piles of old debris under them are all suspect 
  • Thinner areas of the snowpack (1-2 feet 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 
  • Snow depths are highly variable and there may be more trigger spots than we realize
  • Several tracks on a slope do not mean the conditions are safe. It could be the 2nd, 3rd or 10th person on a slope before someone finds a trigger spot.

Weak faceted snow near the ground continues to be reactive and signal propagation potential in test pits in the alpine. Check out a video HERE and observation from Magnum on 12/27/17 HERE

Sat, December 30th, 2017

Yesterday was overcast with temperatures in the teens and low 20Fs. Winds were easterly 5-15 with gusts into the 20s. Overnight the temperatures rose a little.  

Today will be mostly cloudy with a chance of snow showers in the morning and snow likely in the afternoon. 1-5″ during the day. Winds will be easterly building this afternoon into the evening, 10-20 mph with gusts into the 30s and 40s. Temperatures will be in the 20Fs.  

Tonight the snow is forecasted to be heavy at times, 6-12″. Easterly winds could into gust into the 60s. Temperatures stay in the 20Fs. There is a Winter Storm Watch in effect.  

Tomorrow will be cloudy with snow likely and heavy at times, 7-10″. Winds will be 10-20 mph with gusts into the 30s. Temperatures will slowly rise with a potential for some “mixed” precipitation at lower elevations. The storm will continue into the New Year.  The pattern is very active this week with another system on track for Tuesday. Stay tuned!

The National Weather Service is forecasting 2’+ for Turnagain Pass by Monday.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 20   0   0   30  
Summit Lake (1400′) 17   0 0   12  
Alyeska Mid (1700′) 21   0    0  26

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  15 E   10 21  
Seattle Ridge (2400′)  17 E 15   28  
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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.