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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, December 19th, 2015 - 7:00AM
Expires
Sun, December 20th, 2015 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger will be rising to HIGH  today in response to heavy snowfall combined with strong winds in the Turnagain Pass and Girdwood Valley areas. Avalanches occurring naturally are expected at elevations above 1,000′ where new snow and winds are loading the steeper slopes. Human triggered avalanches in these areas are very likely. As the storm continues, traveling in avalanche terrain is not recommended. A MODERATE danger exists below 1,000′ where an avalanche occurring above may run into this zone.

*If you are thinking of heading into the backcountry today, terrain management will be the key in order to stick to mellow slopes and avoid slopes 35 degrees and steeper. This includes runout areas; for instance the bottom of the gullies along the front side of Seattle Ridge.  Once the snowfall stops and skies break up, very careful snowpack evaluation is necessary and a conservative approach is needed before deciding to enter slopes 35 degrees and steeper.  

Special Announcements

Don’t forget to come by the motorized lot at Turnagain Pass tomorrow for a  Free Avalanche Rescue Workshop!!. This event is hosted by the CNFAIC forecasters and is a great opportunity to practice beacon searches, learn strategic shoveling techniques. This workshop is open to everyone and anyone, novices and experts.

Sat, December 19th, 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

A moisture laden storm is currently moving through the Gulf bringing heavy snowfall to elevations above ~800′ and rain below in the Girdwood Valley and Turnagain Pass zones. Winds have been strong with the snowfall from the East – 30-40mph with gusts in the 70’s mph. Snow totals at the mid-elevations from overnight are:

Girdwood Valley:  8-10+”
Turnagain Pass:  9-11+”
Summit Lake:  2-3″

We are expecting an additional 10-15″ to fall today before the storm slows down tonight. A variety of storm snow avalanches are expected today. These include:

WIND SLABS:  With such strong winds associated with the snowfall, much of the avalanche activity is likely to be on wind loaded slopes. These ‘wind slabs’ are expected to be 1-3′ thick and standard storm-induced slabs that are the most unstable during wind loading and directly after, but can stability quickly.
STORM SLABS:  In areas sheltered from the wind we may see slabs release from denser storm snow falling over less-dense snow.
LOOSE SNOW:  Loose snow avalanches are likely on very steep and rocky terrain.

*In addition to the avalanches expected within the storm snow, there is a possibility that a slide could break into weaker layers that reside 2-3′ deep in the pack. If this is the case, then large avalanches could result. More on this below.

Photo: The approach of the storm. This photo was taken yesterday afternoon by Andy Moderow. A soft, but wind affected, pre-existing surface is now covered with 8-10″ of new snow from overnight with another 10″+ today. The good news is, there are no weak layers on yesterday’s surface and bonding with this new load should occur relatively quickly (in a day or so after the storm).

 

Additional Concern
  • 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

Around 2-3′ below the snow’s surface sits a layer of weak snow on top of either a hard crust (elevations below 3,000′) or a hard old wind surface (above 3,000′). The weak snow is comprised of facets that are bonding and in some cases surface hoar. In all our pit tests these weak layers are proving to be quite strong and unreactive. However, with a good shock to they system today – around 2″ of water weight rapidly added to the pack – we can’t rule out the potential that these old weak layers may become overloaded and a large avalanche release.

Weather
Sat, December 19th, 2015

We saw a brief break between storms yesterday as skies broke up and snowfall ceased for the middle part of the day. The Easterly winds, on the other hand, remained strong enough that several folks noted plumes on the peaks during the day.  

Overnight, a Pacific storm moved in and has, as of 6am this morning, added up to a foot of new snow at the higher elevations (above the snow stations). See tables below for snowfall and water numbers at the stations. The rain/snow line is hovering right around 800′ currently and expected to remain. Ridgetop winds picked up as well overnight into the 30-40 mph range with gusts in the 70’s. For today, we should see this very stormy weather continue though this evening before the storm starts thinking about moving out.

Tomorrow, we should see instability showers with moderate Easterly ridgetop winds. This means we could see either skies open up a bit or light snow showers. Stay tuned.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 29   9   0.7   55  
Summit Lake (1400′) 26   2   0.2   19  
Alyeska Mid (1700′) 31   9   0.66   39  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23   NE   20   77  
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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