ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Mon, January 8th, 2018 - 7:00AM
Expires
Tue, January 9th, 2018 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line
The avalanche danger is MODERATE on all slopes above 2,000′ in elevation. Slab avalanches breaking 1-3′ thick in weak layers under the New Year’s snow will be possible to trigger. Slabs may release after several tracks are on the slope and remotely triggering an avalanche from below or adjacent to a slope is possible. Additionally, triggering a larger slab breaking near the ground remains possible at elevations above 3,000′. The danger is LOW near 2,000′ and below.
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Mon, January 8th, 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
Persistent Slabs
Almost 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.
It has been four days since any known avalanche activity has been seen. The New Year’s storm snow has now settled to around 12 – 20″ at Turnagain Pass and to 20-30″ in the Girdwood Valley. The buried surface hoar that is the culprit for last week’s avalanche activity sits right under the New Year’s snow. This layer is slowly gaining strength and the slab is becoming more stubborn to trigger. However, we are not out of the woods. Snow pit results are still showing this layer to be reactive and to propagate – meaning human triggered slabs are possible if one hits the right spot on the slope. Additionally, there are slopes that avalanched in early December which have a thinner snowpack and harbor another set of facets under the pack. Essentially, we have a thin snowpack with various weak layers.
These problems are relegated to above ~2,000′; due to rain falling up to 2,000-2,300′ on Jan 2nd, which has turned the slab into crusts at these elevations and below. (Remember from your level 1 avalanche course, to have a slab avalanche you must have a slab, not just a weak layer.) Hence, many folks were out enjoying terrain around 2,000′ yesterday without incident, yet most terrain above this, and in the 3,000’+ range, remains untested. Here is a video from 3,000′ yesterday:
*For those riders and skiers headed out for today’s sun, keep in mind that travel in the upper elevations is where triggering a slab is most likely. This is above 2,500′ where NO crusts exists in the top foot of the snowpack and the snow quality is best. Remotely triggering a slab is possible, several tracks may be on the slope before a slab releases and no signs saying ‘the slope is unstable’ are likely to be present. Larger slopes are more suspect as well and those with rocky features. Safe travel habits are always key, but especially when dealing with persistent and deep persistent slab avalanche problems: exposing one person at a time, having an escape route planned and watching your buddies closely.
The SLAB and WEAK LAYER we are talking about – the “thin grey line”…
In the photo below, the SLAB fails and cracks but doesn’t quite want to fall into the pit on this low angle slope when a snowmachiner side hills just over the pit wall.
Many folks out enjoying terrain in the near 2,000′ elevation zone and below where triggering an avalanche is unlikely. (Photo from Main Bowl/1st Bowl in Seattle Creek drainage)
Old avalanche debris from Jan 2nd – this is at 2,000′ and these lower slopes that were rained on when the avalanche occurred are now frozen and stable. (photo Ray Koleser)
Avalanche Problem 2
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.
At high elevations above 3,000’, human triggered large and dangerous deep slab avalanches are still possible. Weak sugary snow (basal facets) near the ground is creating a low probability/high consequence avalanche problem that is impossible to outsmart and will take a long time to heal. A big trigger like a snowmachine or a slab avalanche in the upper layers of the snowpack may be enough force to initiate a deep slab avalanche. Likely trigger spots will be in thinner areas of the snowpack that are connected to large, loaded slopes. Cautious route-finding is essential. This includes thinking about the remote trigger potential from below.
Broken skies and valley fog were over the region yesterday. Winds were light and variable and temperatures were in the mid 20’sF. Overnight, valley bottom temperatures have dropped to the teens as an inversion has developed.
Today, we can expect mostly sunny skies and light and variable winds. Temperatures should climb to the mid 20’sF at 1,000′ by the afternoon and remain in the mid 20’sF along ridgelines.
Snow flurries are on tap for tomorrow with little accumulation expected and temperatures should remain cool enough for any flurries to fall to sea level. This weekend looks to be a chance for another shot of snow – stay tuned!
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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