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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, March 12th, 2019 - 7:00AM
Expires
Wed, March 13th, 2019 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger remains  CONSIDERABLE  in the Alpine today.  Human triggered slab avalanches 2-3′ thick are likely on steep slopes above 2000′. A MODERATE danger exists at Treeline and below where triggering an avalanche is still possible. Pay attention to changing conditions on solar aspects and with warming temperatures in the afternoon. Watch for roller balls and natural wet loose avalanche activity.   Extra caution is advised.  Give cornices a wide berth.

SUMMIT LAKE:    This area has a very shallow and weak snowpack. Strong winds combined with a few inches of snow during the weekend storm may have overload buried weak layers and caution is advised if heading to this area. Look for signs of instability.  

LOST LAKE:    Caution is advised in the Seward region. New snow and wind over the weekend have created dangerous avalanche conditions. The potential for large human triggered avalanches remains in this area as well.  

Tue, March 12th, 2019
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. 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

Today will be the first day after this weekend’s storm with a real chance of mostly clear skies and sunshine for a least part of the day. Yesterday the area received a couple inches of snow with rain below 1000′. Overnight skies cleared and temperatures dropped below freezing. On lower elevation terrain there will be a crust this morning on the surface. Due to the wet snow yesterday this may extend as high as approximately 2000′. Where this is crust supportable there is a lower likelihood of triggering an avalanche in the layers below. If you or your machine penetrate through crust or your traveling at an elevation that is crust free you need to consider the possibility of triggering an avalanche 2-3′ deep on a layer of buried facets/surface hoar. Overall the snowpack is fairly untested and good visibility may lure folks out on to bigger terrain. Slope angle and consequences of a large slide must still be taken into consideration. This type of set-up may not fail with the first skier or snowmachine on the slope. Snowpack tests still show the potential for triggering. As temperatures warm and the sun shines on steep slopes this may increase stress in the slab and make triggering more likely. Additionally look for loading patterns from the strong winds over the weekend. Wind loaded slopes could have deeper slabs and be more prone to triggering on steep unsupported slopes. Use safe travel protocols and ease into terrain. Pay close attention to changing conditions. Look for signs of instability. 

Slab over facets on Tincan, 2100′. 3-10-19. 

 Small skier triggered slab on Tincan, 3-11-19. Photo: Kyle Van Peursem

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

Direct sunshine today and warm temperatures in the afternoon may heat up the surface snow and initiate roller balls and progress to wet loose avalanches. Steep, rocky solar aspects will be the most suspect. There is the potential for these loose snow avalanches to trigger slab avalanches on the slopes below if the heating is sustained long enough. If you see roller ball activity starting avoid solar aspects and watch the terrain above you. 

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 grew larger during the storm this weekend. Be careful to avoid traveling to close to the edge. Remember cornices often break farther back than expected and could trigger a large avalanche on the slope below. 

Weather
Tue, March 12th, 2019

Yesterday:  Mostly cloudy skies with rain/snow showers and rain/snow line  fluctuating around 1000′. Temperatures were in the mid 20Fs to high 30Fs. Winds were mostly light and easterly shifting to the west in the evening. Overnight skies cleared and temperatures dropped ranging from just below freezing at sea level to the teens at upper elevations. Winds remained light.  

Today: Mostly clear and sunny in the morning with clouds building later in the day and a chance of snow showers in the evening. Temperatures will be in the 20Fs to mid 30Fs. Winds will be easterly 5-15 mph with gusts into the 20s.   Tonight will be mostly cloudy with temperatures in the 20Fs. Easterly winds will increase with gusts into the 30Fs. Snow showers overnight with 1-5″ forecast.  

Tomorrow: Mostly cloudy skies with snow showers throughout the day and snow likely in the evening. Temperatures will be in the 20Fs and 30Fs. Easterly winds blowing 10-20 mph with gusts into the 30s increasing overnight. The position of the low in the Gulf with this system is a still a little uncertain and could impact how much precipitation the area receives. The most intense part of this storm still looks to be Wednesday night.  

*Seattle Ridge wind sensor is not reporting.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 30    0 0.1   69  
Summit Lake (1400′)  28      0 0   27  
Alyeska Mid (1700′)  31      3 0.26    73    

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  21  NE  5 18  
Seattle Ridge (2400′)  27  *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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