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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, January 2nd, 2016 - 7:00AM
Expires
Sun, January 3rd, 2016 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

Up to 2 feet of new snow and strong wind overnight has rapidly loaded the mountains surrounding Turnagain Pass and created a HIGH avalanche danger in the Alpine zones (above 2,500′). A CONSIDERABLE avalanche danger exists at Treeline (1,000′-2,500′). Natural wind slab avalanches, 2-6′ thick likely occurred overnight and are possible through the day today. Human triggered avalanches are likely on steep wind-loaded slopes. Sticking to mellow terrain (off and out from under slopes 35 degrees and steeper) and avoiding avalanche terrain is recommended in Alpine zones. In the Treeline zones, careful snowpack assessment and conservative route-finding will be necessary. A MODERATE danger exists in runout zones below 1,000′ where debris from a slide releasing above may reach.

Don’t forget to check the weekly summary for Summit Lake (on the Kenai). This summary is produced on Saturday mornings at 7am.

OUTSIDE OF CNFAIC ADVISORY ZONE:

*Photo below is of a large wind slab avalanche on Flattop Mountain, just above the Glen Alps parking lot in Anchorage (credit: Bob Lohr). This is thought to have slid on Dec 31st, with debris crossing the summer trail. Strong winds and snowfall are creating dangerous avalanche conditions throughout the region. More details on this slide can be found HERE  as well as a large human triggered wind slab in the Eagle River area, HERE.  Keep in mind summer trails can pass through avalanche paths such as the Flattop trail, as well as year around trails such as  the Powerline trail.

If you are thinking of heading North today to Hatcher Pass – be sure to check out their Saturday morning advisory at hatcherpassavalanchecenter.org.  

Special Announcements

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Sat, January 2nd, 2016
Alpine
Above 2,500'
4 - High
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
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. 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
  • 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

Another day, another storm. Since the last dry spell, just before Christmas and during the Solstice, we have seen roughly 8 feet of snow fall above treeline (a rain/snow combo below this). During this time we have also seen alders continue to be laid over and covered up, terrain features smoothed out and, as of today, a 13 minute gain of daylight. Just overnight, another intense pulse of moisture added 20+” of dense snow at treeline on Turnagain Pass with a rain/snow line around 800′. Precipitation and wind has abated this morning, yet the avalanche conditions remain dangerous.

If you are thinking of heading to Turnagain Pass today, keep in mind the mountains have just received a ‘rapid loading’ event. Not only is 20+” of new snow loading slopes (2.5″ of water equivalent), but very strong Easterly winds have loaded leeward slopes and cross-loaded gullies. Wind slabs in the new snow could be as thick as 6′ in places. 

Many different avalanches are possible today:

1) Wind slab: Triggering a wind slab 2-6’ thick is likely on steep wind loaded slopes. Winds are forecast to remain from the East in the 20-30mph range. If this is the case, fresh and sensitive wind slabs will remain through the day. 

2) Cornices: With such strong winds and warm snow at the ridgetops, we can expect cornices to be tender! Not only can these trigger an avalanche below, but may also trigger a slide breaking deeper in the snowpack – creating a very large avalanche. 

3) Wet loose avalanches: More rain on snow will saturate the upper layers of the snowpack below 1500’. This is more of a concern in areas with terrain traps and in steep channeled terrain where an avalanche from above will be impossible to escape. 

Yesterday’s field day, just prior to the heavy snowfall, showed the December storm snow bonding well at the mid-elevations. Keep in mind as you watch the video that another 20+” of snow has fallen.

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

As mentioned in the video above, sitting under 5-7′ of settled storm snow sits an old layer of faceted snow above the Thanksgiving Rain Crust (TRC). This layer extends to around 3,000′ and continues to be on our radar. Over the past several days, we have had reports of avalanches breaking into older weak snow, this is primarily in the periphery of our advisory area such as Summit Lake and Girdwood Valley. As the snowpack gets yet another load, we will be monitoring whether slides are breaking in older/weaker snow.

Weather
Sat, January 2nd, 2016

Obscured skies and strong Easterly winds were over the area yesterday. Light snow fell above 1,500′ while rain fell below 1,400′. Beginning yesterday evening, snowfall increased and the rain/snow line dropped to ~800′ at Turnagain Pass. Overnight, we have seen over 2″ of water equivalent from Girdwood to Turnagain Pass.  

As of this morning, the main ‘fire hose’ of sub-tropical moisture has moved off to the East, yet strong Easterly winds and light snowfall will remain today. The rain/snow line should remain just below 1,000′ and we could see another 2-5″ of snow at the mid-elevations. Winds are forecast to average in the 20-30mph range from a generally East direction. Temperatures will remain in the mid 20’s F on the ridgetops and the low 30’sF at 1,000′.

This warm and stormy pattern looks to be over us through Monday with a possible break between storm Tues/Wed.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 31   20   2.5   89  
Summit Lake (1400′) 33   8   0.8   26  
Alyeska Mid (1700′) 32   9 (very wet snow) 2.2   64  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 24   ENE   41   99  
Seattle Ridge (2400′) 27   N/A   N/A     N/A    
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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.