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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Mon, December 17th, 2018 - 7:00AM
Expires
Tue, December 18th, 2018 - 7:00AM
Forecaster
Aleph Johnston-Bloom
The Bottom Line

The avalanche danger is CONSIDERABLE above 1000′.  Human triggered storm slab avalanches are likely on slopes 35 degrees and steeper. Watch for signs of instability: recent avalanches, cracking and collapsing (whumpfing). Additional concerns: watch your sluff in steep terrain  and avoid traveling underneath  glide cracks.

GIRDWOOD VALLEY:  Storms have been favoring Girdwood Valley. Yesterday over a foot of snow with an inch of water weight fell adding to the snow from Friday night/Saturday morning. Expect slabs to be deeper and potentially more reactive.

SUMMIT LAKE:    There are more developed weak layers near the ground; increasing the chance a person could trigger a larger slab avalanche. Choose terrain carefully.  

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Mon, December 17th, 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

Expect storm slabs in terrain 35 degrees and steeper today, the bigger the slope the more potential hazard. Look for signs of instability. Slabs maybe quite soft and manageable but it will be important to ease into terrain and use small test slopes to get a sense of reactivity and depth.  Yesterday’s storm brought periods of heavy snow to the advisory area with observers reporting snowfall at an inch an hour in the afternoon. Storm totals were in the 6″ to 1′ foot range favoring Girdwood Valley. Expect deeper snow in the upper elevation terrain. Observers on Tincan and Notch Mountain reported cracking and reactive shallow storm slabs in the afternoon. The storm snow was a bit more cohesive yesterday than the snow that fell prior. Today this instability may settle fairly quickly but should be approached with caution.  Easterly winds gusting into the 30s yesterday may have also increased slab depth and slab cohesion, especially in the Alpine.

Loose snow avalanches: In areas where the snow does not act as a slab the sluffs may be quite large and pushy in steep terrain. These could knock you off your feet and carry you. Choose terrain carefully. 

 

Storm slab on Notch Mountain, 12-16-18. Photo: Andy Moderow

Slab depth in the mid-afternoon (snow continued through the night). Notch Mountain, 12-16-18. Photo: Andy Moderow

 

Avalanche Problem 2
  • Persistent Slabs
    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.

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

We are tracking buried layers of facets and crusts that sit 1-3′ under the snow surface. These layers are more prevalent in the mid-elevations (2000’ – 2700’). Snow pit data and a lack of avalanche activity has been pointing to an unlikely chance for an avalanche releasing deeper in the pack. However, observers yesterday on Sunburst were able to get propagation on weak snow near the ground at 2100′. Additional snow load especially in the Summit area could start to tip the balance. Evaluate the snowpack and terrain as you travel. The thinner the snowpack the more suspect it is. Observers keep finding spots that whumpf on facet/crust combinations reminding us not to forget about this potential avalanche hazard.

Additional Concern
  • Glide Avalanches
    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.
More info at Avalanche.org

Glide cracks are beginning to open up in many places, including Sunburst’s SW face under the weather stationSW face of Tincan Proper, Gold Pan area (behind Cornbiscuit/Magnum) and there was one glide avalanche in the Johnson Pass area. These cracks can release at any moment. They are not associated with human triggers and the best way to manage the hazard is to avoid being on or beneath slopes with cracks. 

Weather
Mon, December 17th, 2018

Yesterday:  Skies were obscured and it snowed throughout the day with periods of heavy snow in the afternoon and into the night. Temperatures were in the 20Fs. Winds were easterly 5-15 mph with gusts into the 30s.  

Today:  Mostly cloudy skies and snow showers are forecast for the day. Winds will be SE 5-15mph with gusts into the 20s. Temperatures will be in the 20Fs to low 30Fs depending on elevation.  

Tomorrow:  Skies will be mostly cloudy with a chance for snow, light winds and temperatures in the 20Fs. Tomorrow afternoon there is an increased chance of snow into Wednesday as a small low circulates in the Gulf. The pattern continues to be active into the weekend.    

*Seattle Ridge anemometer (wind sensor) is rimed over and not reporting.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 23   6   0.3   37  
Summit Lake (1400′) 20   6   0.4   17  
Alyeska Mid (1700′)  20     10   1.01   29  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 21   NE    9 35  
Seattle Ridge (2400′)  24      *no data *no data    *no data
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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
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Closed
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Closed
Skookum Drainage
Closed
Turnagain Pass
Closed
Closed as of May 1. Thanks for a fun, safe season!
Twentymile
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Closed
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Closed
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Snug Harbor
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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.