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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, April 10th, 2018 - 7:00AM
Expires
Wed, April 11th, 2018 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

Warm weather, a second night with no freeze and rain have increased the danger below 2500′ to CONSIDERABLE. Human triggered wet snow avalanches are likely and naturals are possible. In the Alpine triggering a slab avalanche 1-2′ thick remains possible on Northerly aspects where drier snow exists. Give cornices a wide berth.

Tue, April 10th, 2018
Alpine
Above 2,500'
2 - Moderate
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
  • 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

Rain on snow breaks up bonds and adds load to the snowpack. Yesterday saw rain showers throughout today and the precipitation intensity picked up this morning. Rain continuing today combined with another day of warm temperatures and last night being the second night in a row with no freeze below 3000′ has increased the likelihood of both wet loose avalanches and wet slab avalanches. The snowpack will continue to get more saturated throughout the day. If you do decide to recreate today pay attention to the depth you are sinking in and get off the slope if the snow is over your boot tops or your machine is trenching in. Expect loose snow moving to entrain more as moves downhill. Yesterday observers reported large roller balls/pinwheels and the size of human triggered loose avalanches increasing later in the day. Anywhere that had softer drier snow becoming wet was the most tender. 

Wet slab avalanches:  We have not seen any wet slab avalanche activity yet, but it’s not out of the question that a wet loose slide (or the weight of a skier or snowachiner) could trigger a wet slab today as water penetrates to lower layers. 

Wet loose avalanches on the North Corbiscuit chutes yesterday. 

 Wet loose avalanches on Pete’s South. Expect the size of wet loose avalanches to increase today. 

 

Avalanche Problem 2
  • 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

Above the rain line triggering a slab avalanche 1-2′ thick on North shaded aspects is still possible. Overall we have seen that the snow from last week has been bonding but haven’t forgotten that there is a weak layer under the old storm slab.  The slab/facet combo remains suspect and may be more tender in the Alpine today as heavy wet snow falls. Wind loading may also add stress, as the ridgetop winds remain strong.  

Facets found beneath last weeks storm snow in a pit on the North side of Tincan at 3,200′. Photo: Eric Roberts

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

It’s that time of year where the snowpack is warming up and cornices will begin to break. Give cornices a wide berth.

Weather
Tue, April 10th, 2018

Yesterday was mostly cloudy with rain showers throughout the day. Temperatures were in the 40Fs at sea level and the high 20Fs to mid 30Fs at upper elevations. Winds were easterly 20-30 mph gusting as high as 67 on Sunburst.  

Today is forecast to mostly cloudy with around a half an inch of rain expected. Temperatures will remain in the 30Fs and 40Fs. Winds will continue from the east 15-25 mph with gusts into the 40s. Temperatures will stay warm overnight and rain showers will continue.  

The next couple of days look to remain cloudy with a continued chance of rain as another low moves into the Gulf. There is a chance for some clearing this weekend but the long-term forecast discussion keeps temperatures warm and the overall pattern active. Springtime in AK!  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 37   0   0.4    71
Summit Lake (1400′) 39    0      0.4      30
Alyeska Mid (1700′)  36      0     0.34    71

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  28 ENE   25   65  
Seattle Ridge (2400′)  31  ESE     21   47  
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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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