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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, February 15th, 2017 - 7:00AM
Expires
Thu, February 16th, 2017 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE  for  all elevations around Turnagain Pass, Placer Valley and Girdwood due to dangerous avalanche conditions. Natural avalanches are possible and human triggered avalanches are likely.  Careful snowpack evaluation, cautious route-finding and conservative decision-making are essential today.

Summit Lake:  Expect the avalanche danger to remain elevated. Click  HERE  for the weakly Summit Lake Summary.  

Special Announcements

Recent precipitation, strong winds and above freezing temperatures have created dangerous avalanches conditions throughout Southcenral, AK including Seward/Lost Lake, Anchorage Front Range, and Hatcher Pass.  

Have you purchased your tickets yet for the  3rd Annual SNOWBALL??  If not, get them  HERE!!  Come out, get your groove on, win some door prizes, bid on some amazing silent auction items and support avalanche safety and education!  All proceeds to benefit  Alaska Avalanche School  and  Friends of the Chugach National Forest Avalanche Information Center.

 

 

Wed, February 15th, 2017
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
  • 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

Natural avalanches were again observed yesterday in the advisory area as the second wave of the storm brought another round of strong winds, heavy precipitation and rising temperatures. Rain fell up to approximately 2000′ and saturated the upper layers of the snowpack at lower elevations. In the past 2.5 days Turngagain Pass has received 3 inches of water (SWE) and over 3.5 was recorded in Girdwood. This water weight (either as snow or rain) combined with strong winds overloaded the existing snowpack. Many of the avalanches observed were releasing at the new snow/old snow interface but a few looked to have released farther into the snowpack on the old faceted layers. 

It is important to remember that today a variety of avalanche concerns are present and will likely be triggered if you venture into the mountains. Snow is expected to fall today will an additional 5-15″ forecasted as temperatures cool down. Storm slab avalanches at all elevations, wind slabs in leeward terrain, large cornices along ridgelines and wet snow at lower elevations should all be taken into consideration. Natural avalanches are still a possibility today and it’s crucial to understand that human triggered avalanches are likely. Keep slope angles less than 30 degrees and avoid being in avalanche runout zones. Pay close attention to signs of instability: recent avalanches, cracking and collapsing (whumpfing). 

Large avalanche on the west face of Pyramid observed yesterday from Tesoro.

Natural avalanches on Seattle Ridge observed from the motorized parking lot yesterday.

Rain runnels

 

 

Avalanche Problem 2
  • 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

Sustained high ENE-SE winds continued to move snow yesterday and obvious cross-loading and scouring was observed. Expect leeward areas to be loaded and sensitive to triggering. Winds are not forecasted to be very strong today but with additional snow in the forecast watch for changing conditions if they do increase and start to move the snow around.  Look for cracking and avoid pillowed or drifted areas. 

Warm, wet snow and high winds are the perfect cornice building conditions. Steer clear of ridgelines. Cornices may break farther back than expected and poor visibility may make it really hard to judge where they are. If they do fail they could trigger a slab avalanche below. 

 

 

Wind transporting snow and obvious cross-loading on Seattle Ridge yesterday. 

Additional Concern
  • 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

Weak snow (facets and depth hoar) in the lower layers of the snowpack continues to be a concern in our advisory area. Avalanches occurring in the upper layers have the potential to step down and release the entire snowpack in some places. If this does happen the volume will be large and could run long distances. As more and more weight is added to the snowpack this becomes more of a concern. A few of the naturals yesterday looked to have run into older faceted snow. The possibility of these large avalanches is another reason for conservative terrain choices today. 

Weather
Wed, February 15th, 2017

Yesterday was mostly cloudy with a window of clearing in the afternoon. Snow and rain fell throughout most of the day. Rain/snow line was approximately 2000′. Temperatures were in the high 30Fs in the valleys and mid to high 20Fs at upper elevations. ENE-SE winds were gusting up to 100 mph early in the morning.  They stayed in the 30s with gusts in the 40-50s for much of the day. Snow and rain showers continued overnight and winds mellowed out early this morning. Temperatures cooled slightly in the last few hours.

Today will be mostly cloudy and another pulse of moisture is expected to bring 5-15″ of snow. Winds are expected to shift to the SW and be 10-20 mph with gusts into the 30s. There is a general cooling trend forecasted throughout the day with cold air moving in from the south (yep sounds odd). From the NWS discussion this morning, “Arctic air has wrapped all the way around from the  Bering Sea within the larger scale surface low.” There is another pulse of moisture tonight and a generally unsettled pattern forecasted throughout the week.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′)  36 4   1    74
Summit Lake (1400′)  35    rain  .2  25
Alyeska Mid (1700′)  35 6    1  61

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  24 ENE    35 107  
Seattle Ridge (2400′)  26*   SE*    32*  51*

*Seattle Ridge is recording intermittenly, data from 6am-6am is incomplete.

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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.