ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sun, January 20th, 2019 - 7:00AM
Expires
Mon, January 21st, 2019 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line
A generally LOW avalanche danger exists in Turnagain Pass at all elevations. Pockets of MODERATE danger exist in the Alpine in zones that saw more snow last week, such as Girdwood, Portage and Placer Valley. In these areas, triggering a slab avalanche up to a foot thick is trending to unlikely, but may still be possible on steep slopes. In all areas, watch for small slabs in steep rocky terrain, sluffs in steep terrain and cornice falls. Glide cracks are still slowly moving and limit exposure under these.
SUMMIT LAKE / JOHNSON PASS: A poor snowpack structure exists in this area, which is very different than Turnagain Pass. Triggering a slab avalanche is trending toward unlikely, yet may not be out of the question.
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Sun, January 20th, 2019
Alpine Above 2,500'
Low (1)
Treeline 1,000'-2,500'
Low (1)
Below Treeline Below 1,000'
Low (1)
Alpine Above 2,500'
Low (1)
Treeline 1,000'-2,500'
Low (1)
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
Persistent Slabs
Almost Certain
Very Likely
Likely
Possible
Unlikely
Likelihood
Historic
Very Large
Large
Small
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.
Remember LOW danger doesn’t mean NO danger! In the periphery of the forecast zone, there are areas that saw up to a foot of snow last week; the majority of the mountains only saw 2-6″ of snow. This snow fell on weak near surface facets and buried surface hoar. There has been one known human triggered avalanche that failed in the facets below the new snow, which was last Thursday in the Placer Valley. As folks head out to enjoy the good weather above the fog and into the further reaches of the mountains, finding another slab that is reactive isn’t out of the question. These would most likely be around a foot thick, pockety and on the smaller side. Wind loaded steep slopes are the most suspect. Otherwise, the calm and cold weather pattern has moved us into the “normal caution” phase of avalanche concerns. These include:
An outlier slab avalanche:
Triggering a small slab avalanche would most likely be a lingering wind slab on an exposed ‘unsupported slope’ that sits above a cliff or steep rocky terrain.
Cornice falls:
Remember cornices often break farther back from ridges than expected. Give them a wide berth.
Sluffs on steep slopes:
Sluffs are slowly becoming larger as the cold weather eats away and weakens the surface layers, turning it to sugary facets.
*We can’t be lured into forgetting to look for signs of instability and using good travel techniques. Good habits keep us coming home at the end of the day.
South of Turnagain – Johnson Pass/Summit Lake zone: A very poor snowpack structure exists in these areas. Multiple mid-pack weak layers of facets and buried surface hoar have been found as well as a facet/crust combination in the bottom of the snowpack. No recent avalanche activity and calm weather has allowed the pack to slowly adjust. However, little is know of the high elevation at this point and it will be important to evaluate the terrain and snowpack as you travel.
Sluffs and even small slabs are still possible in steep committing terrain. This is the ‘normal caution’ regime.
Thank you to Brent Byrne for capturing the blue skies above the dense valley fog on Seattle Ridge. Also seen here is the crusty surface conditions with a new batch of surface hoar on top. A bad set up for the future and a new load of snow.
Additional Concern
Glide Avalanches
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
Glide cracks are still slowly opening. Although we have not heard/seen one of these release for over a week, limiting exposure under them remains wise. Known areas with cracks are Eddies, Tincan, Sunburst, Magnum, Cornbiscuit, Lipps, Seattle Ridge, Johnson Pass, Lynx Creek, Summit Lake, Petersen Creek, and Girdwood. This avalanche hazard is unpredictable and cracks can release without warning.
Glide cracks on the SW face of Magnum. The photo on the left with the skin track has the crack that seems to be the most active along this ridge.
Weather
Sun, January 20th, 2019
Yesterday: Clear skies sat above dense valley fog up to 2,000′. Winds have been light from the west along ridgetops (5-10mph). Temperatures have been continuing to cool yesterday and overnight, valley bottoms have dropped to the single digits and ridgelines are in the low teens.
Today: What looks to be one last clear sky day is on tap. Thick valley fog is expected to remain up to 1,500′ or 2,000′. Ridgetop winds will remain light (5-10mph) but switch to a more easterly direction. Temperatures should remain in the single digits in valley bottoms and in the teens F at the upper elevations.
Tomorrow: Clouds, warming temperatures and a chance for snow showers move in on Monday. The rain/snow line looks to stay near sea level through tomorrow and only a trace to a few inches of snow is expected. Another larger system pushes in Wednesday, bringing warmer weather and more precipitation, stay tuned!
*Seattle Ridge weather station was heavily rimed and the anemometer (wind sensor) was destroyed. We are currently working to replace it.
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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