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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, March 14th, 2018 - 7:00AM
Expires
Thu, March 15th, 2018 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE.  Large slab avalanches 2-4+ feet thick are likely to be triggered on slopes steeper than 30 degrees.  These may be triggered from a distance or from the bottom of a slope.  Cautious route finding and conservative decision-making are essential. If the sun comes out today natural avalanches are possible, especially on southerly and easterly slopes. Pay attention to changing conditions.

A skier remotely triggered an avalanche on the skin track in Summit Lake Monday – see Saturday’s  Summit snowpack and avalanche summary  and recent observations HERE.  

 

Special Announcements

There will be intermittent traffic delays Wednesday March 14, 2018 on the Seward Highway  for avalanche hazard reduction work. Near mileposts 100 to 95, Bird Flats.  Motorists should expect delays of up to 45 minutes between 10:00 AM and 2:00 PM.  Updates will be posted on the 511 system.    http://511.alaska.gov/

Wed, March 14th, 2018
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
  • 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

What happens when you mix rapid warming and buried weak layers??? March Madness? Welcome to spring…Yesterday was an interesting day for avalanches. The sun came out, temperatures rose (some weather stations recorded temperatures into the 40Fs), high clouds came in and the heat was trapped. This “greenhousing” is believed to have been the cause of a natural avalanche cycle that started mid day and peaked around 5 pm. The highlight of this was a large natural slab avalanche that was observed on an NE aspect of Skookum. This appears to have run on the mid January facet/crust combination that has been a layer of concern in mid elevation terrain. This layer seems to be the culprit in the majority of the avalanche activity since last week’s storm. What does this mean for today? We know that there is a slab approximately 2-4 feet thick over persistent weak layers that have been well documented and that are widespread across our region. Large avalanches can be triggered by the weight of a skier or snowmachiner, especially in a thinner spot near rocks or ridges. There were avalanches triggered on Monday. If the sun comes out today there is the potential that a similar warm up may happen and naturals will be possible. It is important to recognize that the snowpack can be even more unstable as warm air and solar radiation affect cold snow and buried weak layers. Extra caution is advised.

If you are headed out into the backcountry today things to keep in mind are:

  • Several feet of new snow fell last week on buried weak layers. There is poor snowpack structure. If triggered avalanches could be large and very dangerous.
  • If the skies clear pay attention to changing conditions. Solar radiation and warm air temperatures can quickly make the snowpack more unstable. Natural avalanches will be possible. 
  • Avalanches can be triggered from the flats or remotely from an adjacent slope – be extra cautious to avoid being in a runout zone 
  • Stick to slopes less than 30 degrees and ease into steeper terrain slowly. Evaluate the consequences if the slope releases; where will the debris go?
  • Realize you may not see signs of instability before the slope releases. 

Natural slab avalanche in Skookum that released around 5 pm.

Natural slab on Seattle Ridge that releases around 1 pm.

 Crown of one of the skier triggered Sunburst avalanches that occcurred Monday. 

 Mid elevation layer of concern: buried facets over a melt freeze crust. 

Avalanche Problem 2
  • Wet Loose
    Wet Loose
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Wet Loose
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
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

Solar radiation and warm temperatures yesterday caused widespread wet loose avalanches. Some of these also triggered slabs on Southerly and Easterly slopes. If the sun comes out today pay attention to changing conditions. Naturals may run and could surprise you in the wrong spot. Wet loose avalanches may also be triggered from your skis or snowmachine in steep terrain on the solar aspects. 

Natural wet loose avalanches and a slab avalanche on Seattle Ridge yesterday

Additional Concern
  • 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

Cornices: Cornices have grown and are suspect for breaking while traveling along ridgelines. Give these an extra wide berth and minimize any time below them. Cornice falls can trigger avalanches on slopes below.

Wind slabs: Shallow wind slabs formed on some leeward features Monday due to moderate Easterly winds in the Alpine. Triggering a wind slab will likely be shallow, but could step down to a deeper layer of the snowpack and create a much larger and more dangerous avalanche. 

Weather
Wed, March 14th, 2018

Yesterday skies cleared earlier than forecasted and the sun came out. High clouds built in the afternoon.Temperatures rose into the low 30Fs to mid 40Fs. Winds were Easterly 5-15 mph gusting into the 30s. Skies became cloudy overnight and temperatures dropped into the 20Fs and low 30Fs. Winds shifted to the west and were light.

Today will be mostly cloudy in the morning but could become sunny in the afternoon. There is a chance of snow showers in the morning. Winds are forecast to be light and Northwesterly. Temperatures will be in the 20Fs to high 30Fs. Tonight will be mostly cloudy with temperatures in the 20Fs to low 30Fs and calm winds.

Tomorrow the next system approaches form the Southwest bringing snow showers in the afternoon and increasing winds. Snow is in the forecast into Friday. Stay tuned for snow amounts. The NWS described a persistent unsettled pattern continuing into next week.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′)  35 0   0   87  
Summit Lake (1400′)  30  0     0    35    
Alyeska Mid (1700′)   34    1      .11    79    

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  25 E-W   9   34  
Seattle Ridge (2400′)   29    E 10    32    
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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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