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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, December 28th, 2015 - 7:00AM
Expires
Tue, December 29th, 2015 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

Heavy snowfall and strong winds are creating a HIGH avalanche danger in the Alpine areas of the backcountry today. A HIGH avalanche danger also exists at all elevations in Portage Valley and Girdwood Valley. Travel is not recommended in avalanche terrain above treeline where natural avalanches are likely occurring due to rapid loading. At the Treeline elevation band on Turnagain Pass, where lesser snow amounts have been seen, there is a CONSIDERABLE avalanche danger. In this area natural avalanches are possible and human triggered avalanches are likely. The type of avalanches expected today are 1-3′ thick slabs that release on slopes 35 degrees and steeper. Additionally, these slabs could ‘step down’ into older weak snow; allowing for much larger slides to occur, which have the potential to connect across terrain features and run into the flats.

A MODERATE danger exists below 1,000′ where debris from an avalanche releasing above may run.

** If you are headed out today, keeping to mellow terrain (slopes under 35 degrees) and staying well away from runout zones is recommended. The visibility will likely be very limited so a good prior knowledge of the terrain you are headed for, and knowing there is nothing steeper above you, will be key. This advice will likely extend through the next couple days as another storm is right on the heels of this one.

Special Announcements

The CNFAIC would like to give a big shout out to the AKDOT plow drivers for doing an exceptional job clearing the parking lots on Turnagain Pass!  When finding a place to park during storms or ongoing plowing operations, please consider their operations as well as your safety and that of others driving the road.  

Mon, December 28th, 2015
Alpine
Above 2,500'
4 - High
Avalanche risk
Treeline
1,000'-2,500'
4 - High
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
  • 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

After a break in storm systems yesterday, we have another strong Pacific storm over us currently. This system is a bit warmer and bringing varying snowfall amounts around the region. The rain/snow line is hovering around 800′ (good news for keeping the snow at Turnagain Pass, which is at 1,000′). It is also a “Girdwood Special” as far as snowfall totals go: Girdwood Valley has picked up 16″ so far, Turnagain Pass 8″ with Summit Lake only seeing 2-3″. Snowfall will continue through the day with an additional 8-14″ expected.

What this all means for avalanche conditions is another HIGH hazard day in the backcountry. If you are considering heading out keep in mind:

1) Natural wind slab avalanches are likely on slopes being loaded by the winds. These will be 1-3′ thick and have the potential to break into older snow. 

2) If slides break into older weak layers, we could see very large avalanches, up to 6′ thick (more on this below).

3) In sheltered areas, out of the wind, slabs up to 2′ thick are still possible due to a layer of surface hoar and near surface facets that sat 10-15″ below the surface yesterday. Very careful snowpack assessment is needed if you are in a sheltered zone – in the Tincan Trees for example.

4) Cornices: With such strong winds and warm snow at the high elevations, we can expect cornices to be forming and breaking off. These ‘backcountry bombs’ are likely triggering avalanches below. Even if the winds subside today, these can still break naturally

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

There are two older weak layers in the pack we are watching. These can be seen in the photo below. Both layers are composed of a mix of surface hoar and near surface facets. These are persistent grains types that can produce large avalanches during, and after, a ‘rapid loading’ event such as today. If today isn’t enough load, then possibly the next storm cycle coming tomorrow through Wednesday. An avalanche breaking this deep in the pack can be very large, connect across terrain features and run to valley bottoms. We are unsure if this could happen today, but it is definitely on our minds.

Weather
Mon, December 28th, 2015

Overcast skies, light snowfall and moderate to strong winds covered the region yesterday. Only around 1-2″ of snow accumulated with rain falling below ~600′. Ridgetop winds averaged in the 20’s mph from the East with gusts in the 40’s mph.

Overnight, a strong low pressure system moved into the Gulf and is bringing warm air and moisture with it. See the Satellite image below. Sunburst is averaging Northeast winds in the 60’s this morning with a peak gust of 110mph! The rain snow line has crept up to ~800 feet and models are showing it rising to 1200 feet today. Precipitation amounts vary significantly across the region, see the 24-hour charts below.  The storm will begin to move out today but strong Easterly ridgetop winds and lingering snowfall (rain below 1200′) will remain. Ridgetop winds are slated to average in the 30-40’s mph with another 8-14″ of snow above treeline and around 1″ of rain at sea level.

Another warm, wet and windy storm is on tap for Tuesday and Wednesday. We are watching the rain/snow line closely and so far it looks to fluctuate between 500 and 1200′ for this one.

Image below is from the NWS: 5am infrared satellite picture of the strong Pacific storm over us today.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 31     8   0.7   68  
Summit Lake (1400′) 32    2 0.3   19  
Alyeska Mid (1700′) 31    16 1.35    57  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23    NE 38   110  
Seattle Ridge (2400′) 25   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.
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