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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, February 5th, 2019 - 7:00AM
Expires
Wed, February 6th, 2019 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger remains a ‘scary’  MODERATE  on slopes above 2,000′. A weak layer of buried surface hoar is lurking  2-3′ below  the surface and  it remains possible to trigger a large and dangerous hard slab avalanche; similar to those triggered last Friday. In addition look for recently wind-loaded slopes, avoid cornices and glide cracks and watch your sluff.    

SUMMIT LAKE / JOHNSON PASS:    A variety of weak layers exist in the snowpack and human triggered slab avalanches 1-3′ thick remain possible on slopes with recent wind loading.  

LOST LAKE:   New snow and wind on Sunday in the Lost Lake and Seward area is expected to have increased the avalanche danger. Look for signs of instability.  

Special Announcements
  • New snow and wind has created dangerous avalanche conditions regionally.  There was a partial burial at Hatcher Pass yesterday. For preliminary information click HERE and check out the Hatcher Pass Avalanche Center Facebook page for current conditions.  
  • GET  your tickets now! SNOWBALL is this Thursday – February 7th:  This annual fundraiser supports CNFAIC and Alaska Avalanche School with a silent auction and a raffle. Come celebrate the winter season with the funk band  Superfrequency  at the 49th State Theater in Anchorage!!!
Tue, February 5th, 2019
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
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
  • Persistent Slabs
    Persistent Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.
Likelihood of Avalanches
Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.
Size of Avalanches
Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word Size (D scale) Simple Descriptor
Small 1 Unlikely to bury a person
Large 2 Can bury a person
Very Large 3 Can destroy a house
Historic 4 & 5 Can destroy part or all of a village
More info at Avalanche.org

The problem with the MLK buried surface hoar sitting 2-3′ below the surface is the potential consequences and the uncertainty.  It was the culprit in all of the scary avalanches on the Seattle Ridge side. The most recently triggered avalanches were four days ago and snowpack tests on Saturday showed that the layer still has the potential to be triggered on that side of the road. On the non-motorized side there haven’t been avalanches triggered.  The buried surface hoar has been found in snowpits but recently has not shown the same potential for triggering. We don’t have a lot of data on this side. The fact that the MLK buried surface hoar is so widespread above 2000′ in the advisory area is concerning. Is there still a chance of triggering a large unsurvivable avalanche on both sides of the road? Or up Lynx or in Placer?   At this point we are still saying YES. This the type of weak layer that can’t be underestimated. Additional snow fall and wind loading also could add stress to a layer like this.

 What to keep in mind:

   1-  Around 2-3 feet under your feet or snowmachine sits a weak layer
   2-  The weak layer may or may not be reactive, this is the tricky part
   3-  If an avalanche is triggered in this deeper weak layer, it can be very large and propagate across the entire slope. 
   4-  No obvious signs of instability are likely to be seen before a slope releases and it may be the 1st track on the slope or the 20th…
   5-  Remotely triggering a slab from the ridge, sides or below is possible
   6-  Use safe travel protocol and assess the consequences if the slope slides. The larger the terrain = the larger the potential avalanche!

To complicate matters the Girdwood Valley has another weak layer of facets associated with a crust. This is now roughly 2-3′ below the surface. All this said, it’s important to keep in mind that the snowpack harbors buried weak layers. 

Buried surface hoar at 3200′ on Sunburst, 2-4-19.

 Large avalanches on the backside of Seattle Ridge, 2-2-19.

Avalanche Problem 2
  • Wind Slabs
    Wind Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.
Likelihood of Avalanches
Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.
Size of Avalanches
Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word Size (D scale) Simple Descriptor
Small 1 Unlikely to bury a person
Large 2 Can bury a person
Very Large 3 Can destroy a house
Historic 4 & 5 Can destroy part or all of a village
More info at Avalanche.org

Both the winds last night and on Sunday were enough to move the soft new snow around. Look for signs of wind loading in the Alpine. Fresh wind slabs maybe tender. Watch for cracking in the surface snow and stiffer snow over softer snow. Although wind slabs are likely to be shallow, they could be more dangerous if they were to step-down and trigger a large slab that breaks in the buried surface hoar discussed above. 

Loose snow sluffs:  Sluffs on steep slopes within the new snow should be expected.

Cornices:  We had a report of a large cornice fall in upper Seattle Creek drainage over the weekend. Give cornices an extra wide berth as they continue to grow. 

Cross-loading and wind texture on Seattle Ridge, 2-4-19. Photo: Nikki Champion

Additional Concern
  • Glide Avalanches
    Glide Avalanches
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
More info at Avalanche.org

Glide cracks are opening again. We know of one glide avalanche that has released recently in the Summit zone just north of Manitoba. Look out for glide cracks and limit exposure under them! 

Weather
Tue, February 5th, 2019

Yesterday:  Mostly clear skies with valley fog between 1000-2000′ decreasing throughout the day. Easterly winds were light during the day but increased to 10-15 mph overnight with gusts into the 20s. Clouds moved in late in the day.  

Today:  Mostly cloudy skies and light snow showers starting early this morning are expected to continue on and off throughout the day as a front pushes in from the southwest, 1-3″ of snow forecast.   Easterly winds 10-15 mph with gusts into the 20s. Temperatures should remain in the 20Fs at upper elevations, and continue to hover near 30F at sea level.

Tomorrow:  Mostly clear skies and calm winds with temperatures in the 20Fs. The next chance for snow looks to be over the weekend. Stay tuned!

*The Seattle Ridge anemometer (wind sensor) was destroyed. We have a replacement on the way and it should be operational by mid February.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′)  28 1    0.1 59  
Summit Lake (1400′)  25  1    0.1  28    
Alyeska Mid (1700′)  25  2   0.2   53  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  21  E  8 25  
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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