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Issued
Sun, December 27th, 2015 - 7:00AM
Expires
Mon, December 28th, 2015 - 7:00AM
Forecaster
Heather Thamm
Avalanche risk The Bottom Line

CONSIDERABLE avalanche danger exists in the Alpine where a human could easily trigger a slab 2-3′ thick in steep terrain. Below 2500′  MODERATE avalanche danger exists where triggering 20 € of low density snow in a terrain trap could have high consequences. Remember this new snow is sitting on a very weak pre-existing surface and not enough time has passed for the snowpack to adjust to its new load. Large numbers of people will be difficult to manage and warrants extra patience today. Change plans if crowds get too large and multiple people in avalanche terrain becomes a safety concern.

*Another large storm is moving into the area late this evening. If Easterly winds exceed the forecasted amount of 20-40mph today, expect avalanche danger to increase rapidly and adapt to changing conditions.

Sun, December 27th, 2015
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
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

Large holiday crowds combined with a recent rapid loading event of 20” of new snow will make safe travel in the backcountry challenging. Today another 3-5” of snow and 20-40 mph ridgetop winds will be adding additional stress to the snowpack. Should crowds start pushing into steeper and more dangerous terrain human triggered avalanches in the alpine are likely. 

Wind Slabs: Today’s Easterly winds will be forming new wind slabs on already wind loaded features. These slabs are sitting on a widespread weak layer and could easily produce slabs up to 3’ thick. Be on the look out for blowing snow, shooting cracks, and hollow sounding snow. Avoid steep wind loaded features where the snow is denser on the weaker snow below.

Storm Slabs: In places protected from the wind new snow could be tender and break easily on steep features. As this new snow settles and the slab become more cohesive the weak snow below is something to investigate. Hand pits will allow you to see how well the new snow is bonding to the weak snow below. Experiment on small, steep terrain features with low consequences, and avoid terrain traps.

Loose Snow Avalanches: This problem is a concern in areas with terrain traps, like at the bottom of a narrow gully or abrupt landing, where triggering loose snow could easily bury a person.

Today’s avalanche concerns combined with large holiday crowds are something to be very aware of.  Remember not everyone reads this forecast and understands how to travel in avalanche terrain when hazards are elevated. Here are a few tips to keep in mind:

  • Anticipate others above and below you
  • Wait your turn to avoid putting multiple people on/under large slopes
  • Coordinate with other groups about how to best navigate the terrain
  • Change your plans if crowds get clustered 

Large groups of people on Tincan yesterday made it difficult to know if other groups were above or below you.

 

 20″ of low density snow sitting on a weak layer of snow below. Today winds and new snow will be adding stress to this weak layer. Be aware of changing conditions. 

Avalanche Problem 2
  • Cornice
    Cornice
Cornice
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
More info at Avalanche.org

Recent strong winds combined with 20” of new snow have been adding stress to this unpredictable trigger.  Avoid being under cornices and anticipate that these loaded guns will break back into a ridgeline further than expected.  Give cornice features a lot of space; should one break it could easily trigger a large slab and send you into dangerous terrain.

Weather
Sun, December 27th, 2015

Yesterday scattered showers brought another 5 inches of new snow to Turnagain Pass. Winds were light 10-20mph from the East. Rain/snow line was around 500′.

This morning ridgetop Easterly winds are already averaging 28mph at Sunburst weather station. Todays winds are expected to be in the 20-40 mph range with another 3-5 € of new snow. Rain/snow line could be as high as 800′, but temperatures are expected to increase through out the day.

Another large storm is moving into the area this evening bringing strong Easterly winds and more snow. Another 12 € is expected overnight and rain/snow line may rise to 1500′.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 29F   4   0.4   64  
Summit Lake (1400′) 28F    0 0   17  
Alyeska Mid (1700′) 30F   5   0.46   46  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 22F   ENE    18 45  
Seattle Ridge (2400′) 25F   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.