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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, December 24th, 2016 - 7:00AM
Expires
Sun, December 25th, 2016 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is MODERATE this morning in the Alpine due to the possibility of fresh wind slabs on leeward aspects as winds have picked up and there is snow available for transport. At Treeline and below the danger is  LOW. The danger level may rise to CONSIDERABLE at all elevations  as a warm, wet and windy storm impacts the advisory area today. Watch for rapidly changing conditions, have a conservative mindset and adjust travel choices appropriately. Natural avalanches may be possible as the day progresses and human triggered likely.  

Check out the Summit Lake Summary HERE.  

Special Announcements

***Carter Lake and Snug Harbor areas are now open to motorized use as of Tuesday, December 20th. Please respect other closures across the Forest. The Forest Service is monitoring conditions daily and will open more areas just as soon as there is enough snow to prevent resource damage to underlying vegetation. Thank you for your patience!

Sat, December 24th, 2016
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
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
  • Wind Slabs
    Wind Slabs
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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

Soft settled snow will easily move as winds pick up in advance of the approaching storm. Wind slabs may be tender and easily triggered in the Alpine. As the snow starts today this will only become more likely. Wind speeds are forecasted to be easterly 25-45 mph gusting to 60 mph. Look for cracking, drifting and blowing snow. Cornices may also grow and become more tender with increasing wind, new snow and rising temperatures. 

 

Avalanche Problem 2
  • 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

As the snow starts to fall today it will land on very weak surface snow and is not expected to bond well. It is important to remember there is widespread surface hoar as well as small facets on almost surfaces from road level high into the alpine. Expect new snow to act as a slab fairly quickly due to this interface. As temperatures warm throughout the storm and the wind blows this may become even more pronounced. Rain or rain/snow mix falling on the old snow may produce avalanches as well at lower elevations. Look for red flags i.e. recent avalanches, cracking and collapsing (whumpfing). Pay attention to changing conditions!

 

Rimed surface hoar over soft settled (faceting) snow on Sunburst ridge. Photo: Kevan Dee

 

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

We have been tracking a couple of weak layers this season. Buried surface hoar from November 16th and December 15th have both produced avalanches. These layers are still buried in the snowpack in some terrain. In addition there is faceted surface snow and new surface hoar waiting to get buried. New snow load may cause avalanches that step down into old weak layers. This may not happen initially as the storm starts but should be a factor that is remembered as it progresses. We have a weak foundation/snowpack structure. Patience will be key in the next couple of days. We could see a large avalanche cycle if this storm really does it’s thing. 

Weather
Sat, December 24th, 2016

Yesterday was clear with some valley fog. Temperatures were inverted with single digits in the valleys and high teens at ridge tops. Winds were light. Last night the temperatures rose into the 20s and winds picked up this morning gusting into the 30s.  

Today there is a Winter Weather Advisory for the forecast area due to a warm wet storm that is forecasted to move into the region late this morning. Easterly winds will increase with gusting into the 60s and 3-7 inches of snow is expected today. Snow may be heavy at times and there is uncertainty with how warm it will be. Rain may fall below 1000′. The storm is forecasted to continue tonight into tomorrow with an additional 6-10 inches of snow overnight and 4-6″ tomorrow. The brunt of the storm is forecasted to be this evening.  

...WINTER WEATHER ADVISORY REMAINS IN EFFECT FROM NOON TODAY TO
9 AM AKST SUNDAY FOR TURNAGAIN PASS AND PORTAGE VALLEY...

* LOCATION...TURNAGAIN PASS AND PORTAGE VALLEY.

* SNOW...12 TO 19 INCHES FOR TURNAGAIN PASS. 8 TO 15 INCHES FOR
  PORTAGE VALLEY.

* VISIBILITY...ONE HALF MILE OR LESS AT TIMES.

* WIND...EAST WINDS 25 TO 45 MPH WITH GUSTS TO 60 MPH.

 Snow showers will continue into early next week with a clearing and cooling trend towards the weekend.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 15    0  0 24  
Summit Lake (1400′)  7  1  .1  8
Alyeska Mid (1700′)  16  0  0  16

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) Under repair      Under repair   Under repair     Under repair    
Seattle Ridge (2400′) 15    SE 21   27  
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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.