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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sun, January 14th, 2018 - 7:00AM
Expires
Mon, January 15th, 2018 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line

The  avalanche danger is  CONSIDERABLE  on all slopes above 1,000′. Triggering a 2-3+’ large slab avalanche, composed of the new snow, is likely above 1,500′. Wet slab avalanches near and below 1,500′ are possible.  Naturally occurring avalanches are also possible.  Additionally, avalanches could break in deeper layers of the snowpack, causing a much larger slide. Below 1,000′ the avalanche danger is  MODERATE  for wet loose avalanches.  

*Today is a day to let the mountains adjust to the several feet of recent new snow.  Cautious route-finding and conservative decision-making will be essential.  

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Sun, January 14th, 2018
Alpine
Above 2,500'
Considerable (3)
Avalanche risk
Treeline
1,000'-2,500'
Considerable (3)
Avalanche risk
Below Treeline
Below 1,000'
Moderate (2)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
Considerable (3)
Treeline
1,000'-2,500'
Avalanche risk
Considerable (3)
Below Treeline
Below 1,000'
Avalanche risk
Moderate (2)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Avalanche Problem 1
  • Storm Slabs
    Storm Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Storm Slabs
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.

Signal Word Size (D scale) Simple Descriptor
Small 1 Unlikely to bury a person
Large 2 Can bury a person
Very Large 3 Can destroy a house
Historic 4 & 5 Can destroy part or all of a village
More info at Avalanche.org

A break in the weather has finally moved in after three days of rain, snow and wind. The rain/snow line rose to 1,500′ in places yesterday, and possibly higher, yet several feet of new snow is now in the mountains above 1,500′. During the past 24 hours between 1-2′ of new snow fell in the region. Favored locations were Portage Valley, Girdwood and the North end of Turnagain Pass. 

STORM TOTALS near 2,000′ above the rain – Thursday morning through 6am Sunday morning:

Turnagain Pass     20-30″  (2.6″ of snow water equivalent)
Girdwood Valley    25-35″  (3.2″ of snow water equivalent)
Summit Lake         10-15″  (1.0″ of snow water equivalent)

Avalanche activity was prevalent in the Tincan Trees yesterday due to heavy snowfall creating a ‘rapid loading’ event. All avalanches were failing under the total storm snow, at the new/old snow surface. The old snow surface is composed of small buried surface hoar (3-7mm) and near surface facets. These are persistent weak layers that don’t bond quickly. Hence, we can expect the storm snow to fail in these layers again today. With the storm past, natural avalanche activity will be decreasing but human triggered avalanches will remain likely. If you are headed out – keep these points in mind:

1-  Slabs triggered will be deep (2-3+ feet) – these are dangerous and unmanageable avalanches
2-  Avalanches could run further than expected
3-  Remote triggering an avalanche from below, or near a slope is possible
4-  Due to the depth of the storm snow, no signs of instability may be present before someone triggers a slab

*Giving the snowpack time to heal from these storms is key. Sticking to low angle terrain with nothing steeper above is recommended. Remember, it’s the first 2 days after a storm where most avalanche fatalities occur. Although there is nice powder at the upper elevations that can lure us, now is not the time to be sampling it.
 

Stom slab avalanches in the Tincan Trees yesterday. The slab on the left was believed to have been natural while the slab on the right was remotely triggered by a group ascending. Recent avalanches, Bulls Eye clues the snowpack is unstable.

 

Another storm slab avalanche triggered by a skier in the Tincan Trees, note the size of the slab (~2,5′ thick). Big thanks to the folks who sent this photo in to us.


 

Avalanche Problem 2
  • Wet Slab
    Wet Slab
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Wet Slab
Wet Slab avalanches are the release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very unpredictable and destructive.

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.

Signal Word Size (D scale) Simple Descriptor
Small 1 Unlikely to bury a person
Large 2 Can bury a person
Very Large 3 Can destroy a house
Historic 4 & 5 Can destroy part or all of a village
More info at Avalanche.org

Rain fell on snow yesterday up to 1,500′ and today, light rain is expected to fall up to 2,200′. With these warm temperatures persisting, wet slab and wet loose avalanches remain possible. There have been some breaks in cloud cover overnight, which has likely helped to start freezing the snow surface at these lower elevations. However, clouds and light rain should move back in today softening any crusts that may have formed. Keep in mind that steep slopes with a wet and saturated snowpack are likely to slide. Even a small slope could have high consequences if heavy wet debris is able to pile up on a person. 

Additional Concern
  • Deep Persistent Slabs
    Deep Persistent Slabs
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.
More info at Avalanche.org

Weak layers within the snowpack have the potential to re-activate with the added load of this week’s new snow (2-3+” of water weight). Additionally, avalanches in the storm snow, discussed above, could step down to these deeper layers and produce a very large avalanche depending on the size of the slope. In short, a layer of buried surface hoar from the New Year’s holiday sits roughly 3-6′ deep at this point and is a concern at elevations above 2,000′. Basal facets, near the ground, remain a concern at elevations above 3,000′. 

Weather
Sun, January 14th, 2018

Yesterday’s storm peaked around noon and brought 1-2″ of rain to 1,500′ in most locations with moist snow above this. The greater amounts of precipitation seen were in the Portage Valley area. Snowfall in the upper elevations varied from 1-2 feet; Girdwood Valley and the North end of Turnagain Pass saw near 2′ of snow while the South end of the Pass and Summit Lake saw 12-15″ of new snow. Ridgetop winds were strong from the East, averaging 30-60mph with gusts to 99mph. Temperatures were warm, around 36F at 1,000′ and the mid to upper 20’sF along ridgetops.  

Currently, the fire hose of moisture has shifted to our East  and precipitation has decreased significantly. Today we can expect light precipitation with 1-2″ of snow above 2,200′ and ~.1″ of rain below this. Skies could clear slightly at times before light precipitation moves back in for this evening along with cooler temperatures. We could see another 2-5″ inches of snow above 1,000′ and .3″ of rain below this. Ridgetop winds will remain Easterly and expected to be in the 15-25mph range.

For tomorrow, Martin Luther King Day, we can expect partly to mostly cloudy skies with light snow flurries. Temperatures should remain cool enough for snow to fall to 1,000′. The next system looks to move in on Tuesday and again is a warm one.  

*Seattle Ridge anemometer is covered in rime from this storm and not functioning

PRECIPITATION 24-hour data (6am – 6am)

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 32   11   1.3   66  
Summit Lake (1400′) 33   rain   0.9   15  
Alyeska Mid (1700′) 33   2   1.7   48  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 25   NE   39   99  
Seattle Ridge (2400′) 28   *n/a   *n/a     *n/a    
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Riding Areas
Updated Fri, May 01st, 2020

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

Area Status Weather & Riding Conditions
Glacier District
Johnson Pass
Closed
Placer River
Closed
Skookum Drainage
Closed
Turnagain Pass
Closed
Closed as of May 1. Thanks for a fun, safe season!
Twentymile
Closed
Seward District
Carter Lake
Closed
Lost Lake Trail
Closed
Primrose Trail
Closed
Resurrection Pass Trail
Closed
Snug Harbor
Closed
South Fork Snow River Corridor
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Summit Lake
Closed

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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.