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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, April 2nd, 2018 - 7:00AM
Expires
Tue, April 3rd, 2018 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

A generally  LOW  avalanche danger exists in the core zone of Turnagain Pass, while areas on the periphery continue to trend from a  MODERATE  danger to a  LOW  danger. Triggering an old hard slab avalanche 2-4′ thick is unlikely, but not out of the question. Areas of most concern are above 2,000′ and in shallow snowpack zones such as Girdwood Valley and South of Turnagain Pass toward Johnson Pass and Summit Lake. Pay attention to afternoon warming and give cornices a wide berth.  

If you are headed South of Turnagain Pass check out the  Summit snowpack and avalanche summary.

Mon, April 2nd, 2018
Alpine
Above 2,500'
1 - Low
Avalanche risk
Treeline
1,000'-2,500'
1 - Low
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
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.
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 sunny skies and high pressure that sit over our region will be present again today; clouds may move in tomorrow and a chance for snow exists on Wednesday. Despite the sunshine, chilly and breezy Northerly winds are likely to keep the sun from warming the snow surface too much today, limiting the chance for wet snow avalanches. What this means avalanche-wise is another day with no contributing factors to snowpack instability. We remain in a holding pattern of sorts as the chances for triggering an old hard slab avalanche are becoming more unlikely as time goes on.

The current snowpack has a poor structure with many weak layers that sit 2-4′ below the surface. These layers are composed of facets and buried surface hoar. The slab on top is hard and varies in thickness due to prior wind effect. The snow surface has been beat up by wind and sun over the past three weeks. Although the weak layers continue to adjust and the slab on top is losing its ability to propagate, we are still concerned someone could find just the wrong spot on just the wrong slope and trigger a dangerous avalanche. Places most suspect are thin snowpack zones such as the Girdwood Valley and the South end of Turnagain Pass towards Johnson Pass, Lynx Creek and Summit Lake. Additionally, North and Easterly slopes in general have a thinner pack making them more suspect. Trigger points in this situation are often in thin areas near rocks or in scoured areas along ridges. As always, practice safe travel habits, expose one person at a time and minimize exposure in avalanche terrain. 

Although winds are likely to limit surface warming by the sun, keep this on your radar in the afternoon. If the snow heats up enough to become wet and punchy, it’s time to move to another aspect. Photos below are from Portage Valley on Saturday. This close up view on the right is a solar aspect of Maynard Mtn on the North side of Portage Lake. This slope is one of the first to have natural wet avalanches on sunny warm days. 

 

 

Note the snowfall during March in the chart below. One large storm right before Spring Break that brought 2-3 feet of snow to the region. Since then, only a few inches has fallen and since March 20th, no snow has fallen. 


–

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

Watch out for cornices along ridgelines. It’s that time of year where cornices will begin to slowly warm during daytime heating, making them easier to break off. As always, give cornices plenty of space and limit exposure underneath them. 

Weather
Mon, April 2nd, 2018

Yesterday sunny skies prevailed again over the region. Ridgetop winds were light (5-10mph) with gusts in the 20’s mph from the Northwest. During the day, temperatures warmed to the low 40’sF at sea level and to 30F along ridgetops. Overnight, temperatures have dropped to the teens to the low 20’s at sea level and similar along ridgelines.

Today, Monday, we can expect clear skies again. Ridgetop winds may bump up slightly from the North to the 10-15mph. Temperatures should climb at sea level to near 40F again today and to near 30F along ridgelines.  

For Tuesday, clouds may move in ahead of a low-pressure that looks to move into our region from the East on Wednesday. This may also bring a chance for light snowfall on Wednesday. Stay tuned!

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 31   0   0   75  
Summit Lake (1400′) 30   0   0   30  
Alyeska Mid (1700′) 31   0   0   70

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 24     NW   7   23  
Seattle Ridge (2400′) 29    NW 9   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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