ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Fri, March 30th, 2018 - 7:00AM
Expires
Sat, March 31st, 2018 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line
The avalanche danger continues to be MODERATE above 1500′ in the periphery of the forecast zone, such as Girdwood Valley and South of Turnagain Pass. Triggering a hard slab avalanche 2-4 feet thick remains a possibility and may be triggered remotely. A generally LOW danger exists in the core zone of Turnagain Pass where triggering this type of avalanche is trending toward unlikely. Pay attention to afternoon warming and give cornices a wide berth.
Similar avalanche concerns exist in the Summit Lake area and other zones on the Kenai.
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Fri, March 30th, 2018
Alpine Above 2,500'
Moderate (2)
Treeline 1,000'-2,500'
Moderate (2)
Below Treeline Below 1,000'
Low (1)
Alpine Above 2,500'
Moderate (2)
Treeline 1,000'-2,500'
Moderate (2)
Below Treeline Below 1,000'
Low (1)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Avalanche Problem 1
Deep Persistent Slabs
Almost 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.
As the days go by with little in the way of weather to contribute to avalanche danger, the chances for triggering an avalanche are decreasing. It has been over two weeks since the last snowfall, a week since the last human triggered avalanche and the snowpack is adjusting to the weak layers that sit in the middle and base of the snowpack. Terrain in the core Turnagain Pass zone is trending towards a LOW danger, meaning triggering a slab avalanche is unlikely – but not out of the question. Areas on the periphery, such as Girdwood Valley and the South end of the Pass towards Johnson and Lynx, we are more concerned with as they have a thinner snowpack and poorer structure.
Yesterday a large collapse was triggered under Goat Mountain in the Girdwood Valley (see Heather’s video below). This is a sign the snowpack can still produce a dangerous slab avalanche. The slabs in questions are hard slabs 2-4′ thick. The weak layers in question were formed in January and are various layers of facets and buried surface hoar. Though these layers are becoming harder and harder to trigger, there is still a chance someone could hit the wrong spot. This is a low probability, high consequence situation and makes travel in avalanche terrain tricky. No obvious signs of instability may be present before a slope releases and it may be the 10th skier or snowmachiner onto a slope that finds the trigger point. These trigger points are often in thinner areas of the snowpack near rocks or in scoured areas along ridges.
Sunshine: If the sun shines in full force today be aware of warming later in the day on Southerly slopes. Especially steep southerly slopes with rocks that will help heat up the snow around them, aiding in possible wet avalanche activity. As we gear up for a weekend of sunny skies, keep the afternoon warming on your radar!
Poor snowpack structure and poor pit results found in the Girdwood Valley yesterday. This video was taken in Gulch Creek under Goat Mountain near 3,000′.
A loot at Turnagain Pass from the top of the uptrack on Tuesday. It has been over two weeks since snowfall and surfaces are hard and crusty from either the sun, wind or both.
–
Additional Concern
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.
Cornices are large in many places. As we head into warmer, sunnier weather remember this can help de-stabilize them. As always, give cornices plenty of space and limit exposure underneath them.
Weather
Fri, March 30th, 2018
Partly cloudy skies were over the region yesterday. A few snow flurries were seen in the morning but no snow accumulated at Turnagain Pass. A few inches were found in the upper Girdwood Valley. Ridgetop winds have been light from the North and West during the past 24 hours (5-10mph). Temperatures were in the teens yesterday along ridgetops and in the mid 30’sF in valley bottoms. Overnight, valley bottom temperatures have cooled slightly to 25-30F.
Today, Friday, partly cloudy skies this morning should give way to mostly sunny skies this afternoon as high pressure has built in over our area. Ridgetop winds look to remain light from the North in the 5-10mph range. Temperatures are expected to stay in the teens along ridgetops and warm to the mid 30’sF again at 1,000′.
For the weekend, we can expect high pressure to remain, which will bring mostly sunny skies and Northerly winds. Cool temperatures look to stay in place for Saturday, but warmer a air mass may push in Sunday or Monday. This could bring more spring-like conditions.
Status of riding areas across the Chugach NF is managed by the Glacier and Seward Ranger Districts, not avalanche center staff.
Riding area information is posted as a public service to our users and updated based on snow depth and snow density to prevent resource damage at trailhead locations.
Riding area questions contact: mailroom_r10_chugach@fs.fed.us
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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