ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Mon, January 1st, 2018 - 7:00AM
Expires
Tue, January 2nd, 2018 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line
The avalanche danger is HIGH today at all elevations above 1,000′. Another round of strong wind and heavy snowfall will continue to overload a weak snowpack. Natural avalanches are likely today and tonight. Human triggered avalanches are very likely. As the new snow piles up on a weak layer of buried surface hoar and facets, avalanches are becoming larger and more dangerous. Travel in avalanche terrain is NOT recommended.
The avalanche danger is CONSIDERABLEbelow 1,000′ where debris from avalanches above may run.
Special Announcements
*All of us at the CNFAIC would like to wish everyone a Happy, and safe, New Year!! Thank you for your support!*
Dangerous avalanche conditions are expected in the Southern Kenai Mountains (such as, Seward and Lost Lake) due to heavy snowfall.
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
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.
Yesterday’s NYE storm (New Year’s Eve) greeted many snow enthusiasts with joy. Storm totals were 12-20″ at the mid and upper elevations from Girdwood Valley to Turnagain Pass. On the South side of Turnagain Pass, such as Sunburst and Johnson Pass, only 3-4″ of snow fell… Snowfall amounts were very different from one ridge to the other with this storm. The part of the puzzle that is not joyful however, is a layer of surface hoar underneath the new snow. This has created poor bonding between the new and old snow; proved yesterday by the plethora of human triggered avalanches. To make matters a bit worse, under the surface hoar are near surface facets (another weak layer) and then a hard layer of crust or wind packed snow that is acting as a bed surface. All these ingredients point to likely human triggered avalanches.
There is another storm headed in today. An additional 5-10″ of snow is expected with another 6-12″ tonight above 1,000′; rain/snow line should be around 500-700’. This will increase the load on the weak layers as well as the slab thickness to over a foot and up to 3′ in places. The storm is coming in warmer and windier, which will also contribute to the increasing avalanche danger. Even small(ish) slopes could become hazardous if the slab is 3 feet thick instead of 1 foot. Keep this in mind if you are headed to the backcounty this week. Also, keep in mind remote triggering is possible from the side or below a slope. Steering well clear of runout zones will be key.
Yesterday’s avalanche activity: Human triggered avalanches were widespread in the Tincan Trees yesterday. All avalanches were soft slabs or wind slabs around 1-2′ thick and failing on the buried surface hoar mentioned above. Photo below and comment from a group that triggered a larger avalanche in the Tincan Trees (16″ deep, 150 wide and running 200′ ). Triggered remotely by 3rd skier on adjacent slope – no one caught.
“We were pretty surprised at how much energy and how far it ran through trees.” (Photo Ray Koleser)
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Photos of skier triggered soft slabs in the Tincan Trees yesterday. Smaller terrain, smaller slab, smaller avalanches. (Photos by Trip Kinney)
Shooting cracks
A look at the new snow sitting on weak layers with a harder wind packed surface below. All known avalanche activity seen yesterday on Tincan was due to the buried surface hoar.
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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’, rapidly loading slopes may awaken a large and dangerous deep slab avalanche. At these elevations, a hard slab, 3-5+ feet thick, is sitting on top of weak sugary snow (basal facets) near the ground. As new snow increases the load over this snowpack structure during the current storms, there will be the potential for large natural avalanches. Between storms, human triggered deep slab avalanches will be 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 breaks between storms may allow for travel to the Alpine.
Weather
Mon, January 1st, 2018
Yesterday was a classic storm day on Turnagain Pass. Very low visibility, gusts winds and snowfall. Roughly 12 – 20″ of new snow has fallen in the upper elevations of Girdwood Valley and the Turnagain Pass area. 24-hour totals are below in the table. Lesser snow amounts were seen on the South end of Turnagain Pass and in the Summit Lake area. Ridgetop winds were Easterly averaging 25-45mph with gusts to 80mph. Temperatures were in the mid 20’sF at treeline and around 30F at 1,000′.
Today, New Year’s Day, into Tuesday we have another storm moving in. This one will be warmer, windier and possibly wetter. The rain/snow line looks to rise to around 500-1,000′ before lowering with cooler temperatures tomorrow. Snowfall amounts look to be 5-10″ (.8″ water) of heavier snow today with another 6-12″ (.9″ water) tonight. Ridgetop winds will be Easterly in the 30-50mph range with stronger gusts. Temperatures continue to climb to the mid 30’sF at 1,000′ and upper 20’sF on the ridgetops.
On Tuesday the storm begins to move out and cooler temperatures move in. A break in storms is looking to be Wednesday and Thursday before another low-pressure heads our way.
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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