ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Mon, December 19th, 2016 - 7:00AM
Expires
Tue, December 20th, 2016 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line
There is a CONSIDERABLE avalanche danger in the mountians surrounding Turnagain Pass, Girdwood Valley and Summit Lake on the Kenai. New snow and strong wind yesterday have loaded a very weak layer of faceted snow and human triggered avalanches are likely on all slopes above 1,000′. Slab avalanches ranging from 10-20″ could be triggered remotely from ridgelines or valley bottoms. Today’s message is simple: it is a day to play it safe and keep slope angles below 30 degrees with nothing above you.
Conservative terrain management and expert level snowpack assessment will be necessary for any travel in avalanche terrain.
***Portage Valley ice climbers and hikers: A MODERATE danger exists below 1.000′ where there is potential for avalanche debris, from a slide occurring above, to run through gullies.
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Mon, December 19th, 2016
Alpine Above 2,500'
Considerable (3)
Treeline 1,000'-2,500'
Considerable (3)
Below Treeline Below 1,000'
Moderate (2)
Alpine Above 2,500'
Considerable (3)
Treeline 1,000'-2,500'
Considerable (3)
Below Treeline Below 1,000'
Moderate (2)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Avalanche Problem 1
Persistent Slabs
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.
An exciting day was had in the backcountry yesterday. Both for weather and avalanches. It was a ‘rapid loading’ event as heavy snowfall from 6am till 5pm loaded slopes with medium density snow. Human triggered avalanches were numerous, yet we have not heard of anyone being caught up in these. From what we know, these slides were in “small terrain” which equates to smaller and more manageable avalanches. See all the reports sent in last night HERE – a big thanks to these folks for writing in!!
The snowpack is showing its hand and it’s not a good one. Weak faceted snow sits under a slab anywhere from 8-20+” thick. The slab is relatively soft and composed of yesterday’s new as well as the 4-8″ that fell during the end of last week. Strong winds have redistributed the snow above treeline and slabs will vary in depth and stiffness. Things to keep in mind today:
You are likely to trigger an avalanche on steeper slopes
You can trigger these from below or from the ridgeline
Facets are notorious for wide propagating slabs and remote triggering
Watch for obvious signs of instability (recent avalanches, cracking and collapsing)
Quick hand pits and poking the snow with your pole (away from a used up-track) are good ways to monitor the slab and weak snow underneath
If skies clear enough for travel outside of the trees, another thing to keep in mind:
Small terrain = small avalanches = relatively manageable
Larger terrain = larger avalanches = unmanageable
Photo: Crown of easily triggered 8″ slab avalanche that sympathetically triggered three more small slabs on the other side of the gully. If you look closely you can see one in the background. More on that in video below. This is the definition of a ‘touchy’ snowpack.
Photo: Soft slab avalanche into a terrain trap in the Tincan Trees (left side of photo). Small terrain – small avalanche. Thank you to Kakiko Ramos-Leon for the photo.
Photo: Remote triggered shallow slab avalanche on a rollover in the Eddies area, Andy Moderow.
Hopefully by this point you are convinced that once the visibility improves, very conservative travel is warranted in the backcountry. If you get out, please send us your photos and information – we will be monitoring the pack daily but your reports are invaluable!
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.
Expect cornices to be sensitive and easy to break off. They also are likely to trigger a slab avalanche below. If you choose to walk a ridgeline today, give these a wide berth and be aware of people below you.
Weather
Mon, December 19th, 2016
Yesterday was a bone fide storm day on Turnagain Pass. Heavy snowfall began in the morning and tapered off last night. See the snowfall and water numbers below. Ridgetop winds were strong, averaging in the 20’s mph with gusts into the 60’s. Temperatures were relatively warm; low 20’sF on ridgelines and 30F at 1,000′ in the parking lots.
Today we can expect partly clearing skies – to some degree – and a chance for localized instability showers. Another 1-2″ of snow could fall where these ‘showers’ take place. Temperatures will be slightly warmer, near 32F at 1,000′ and increase to the upper 20’s along the ridgelines. Ridgetop winds will be light, 5-10mph from a Westerly direction.
For this week, most of the weather is getting pushed South and we should see a mix between clearing skies and localized snow showers.
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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