ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Thu, January 4th, 2018 - 7:00AM
Expires
Fri, January 5th, 2018 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line
The avalanche danger is CONSIDERABLE on all aspects and elevations near 2,000′ and above. Human triggered slab avalanches 1-2′ thick are likely where surface crusts are either thin or do not exist. Additionally, remote triggered avalanches from the side, below and from the top of slopes is possible. At elevations above 3,000′ there is still the possibility of triggering a very large slab avalanche that breaks in weak snow near the ground. Cautious route-finding and conservative decision-making are essential. Below 2,000′ the danger is MODERATE in avalanche paths where a slide occurring above may send debris.
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Thu, January 4th, 2018
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
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.
After several days of snowy weather, low visibility and a natural avalanche cycle, mostly clear skies are expected today. This will make travel above treeline inviting. If you are headed out in the backcountry please understand there are dragons lurking in the snowpack. Two dragons to be exact: 1) a layer of buried surface hoar sits 1-2′ below the surface and is showing signs of being reactive, 2) weak snow near the ground exists at elevations above 3,000′. These issues are also being seen in the Summit Lake region.
The New Year’s storm that began Dec 31st and ended yesterday, Jan 3rd, added 1-2′ of snow in most areas, possibly more at the higher elevations. This snow fell on a layer of surface hoar and near surface facets, creating a persistent slab problem. Rain fell up to 2,300′ at one point during the storm, which has formed crusts in the top foot of the snow at elevations below 2,000′. Above ~2,000′ where moist to dry snow exists without crusts are where triggering an avalanche is most likely. Things to keep in mind:
1- Human triggered avalanches 1-2′ thick are most likely above 2,000′ 2- Remote triggering from the top, side or bottom of a slope is also likely 3- Avalanche size is expected to be large enough to bury and kill a person 4- The larger the slope the larger the avalanche
Performing quick pits to assess the bonding of the New Year’s snow to the weak snow below is a great idea, but not something to completely hang your hat on. The buried surface hoar may have been blown down in your pit, yet remains lurking on the slope you plan to ride. Today and tomorrow are days to be extra cautious as there are too many signs pointing to an unstable snowpack. Along with the buried surface hoar problem, wind slabs and cornice falls are possible along ridgelines. One of these avalanches can trigger a larger avalanche below.
Check out these videos. The first is the general snowpack set-up, the second is a close up/snow motion video of the collapse and propagation of the buried surface hoar found yesterday on Sunburst:
Runnels from the rain exists around 1,600′. At these lower elevations the wet snow is freezing into a crust and triggering an avalanche is unlikely.
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.
In the Alpine, above 3,000’, weak sugary snow (basal facets) sits near the ground. The storms over the past few days have added additional load to slopes that already have a hard slab 3-5+ feet thick.At these elevations, human triggered, large and dangerous deep slab avalanches ARE still possible. This is a high consequence avalanche problem that is impossible to outsmart and can take a long time to heal. Keep this in mind as improving visibility in the next few days may allow for travel to the Alpine. It is really important to remember that triggering an avalanche in the upper layers of the snowpack may then initiate a deep slab avalanche. Cautious route-finding is essential. This includes thinking about the remote trigger potential from below.
Weather
Thu, January 4th, 2018
Obscured skies and light snow showers filled the area yesterday. Snowfall yesterday at the mid elevations accumulated to 2-3″ at most locations with .2-.3″ of water equivalent. An additional couple inches is likely to have fallen in the upper elevations. Winds turned Westerly yesterday and were light in the 5-10mph range during the past 24-hours. Temperatures have dropped overnight with clearing skies and are sitting in the mid-20’s at 1,000′ and near 20F on ridgetops.
Today, mostly clear skies are forecast with valley fog to be expected. Ridgetop winds will be Westerly in the 5-10mph range. Temperatures will continue to decrease to the teens along ridgelines and the mid 20’sF at 1,000′ as cold air streams in with the winds.
Another clear sky day is on tap for tomorrow before a round of snow (and it looks like snow to sea level) moves in on Saturday – 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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