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

The avalanche danger is  HIGH  today at all elevations.  Heavy rain, snow and strong winds have created very dangerous avalanche conditions. Natural avalanches are likely and human triggered avalanches are very likely.  Avalanches may be very large and run long distances.  Travel in the mountains is not recommended.  Avoid all runout zones.  

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Wed, January 17th, 2018
Alpine
Above 2,500'
High (4)
Avalanche risk
Treeline
1,000'-2,500'
High (4)
Avalanche risk
Below Treeline
Below 1,000'
High (4)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
High (4)
Treeline
1,000'-2,500'
Avalanche risk
High (4)
Below Treeline
Below 1,000'
Avalanche risk
High (4)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Avalanche Problem 1
  • 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

The region has received 2″ of rain in the past 24 hrs. The rain fell at least as high as 3000′. Another half an inch of rain is forecasted to fall today up to 2400′. As the water moves into the snowpack it break bonds, adds load and can lubricate weak layers and/or run along crusts, causing large wet slab avalanches. We know we have existing weak layers and crusts buried in the snowpack. Rain may also initiate large wet loose avalanches that could trigger the wet slabs. It is best to visualize the snowpack that has been affected by rain has now turned into a giant, scary slurpee that is out to get you. Avalanches were observed in many paths on Seattle Ridge yesterday and in channeled terrain along the Seward highway. Some of these avalanches were running to the valley bottom. This is very good reason to avoid travel in runout zones. Travel in the mountains is not recommended today. 

Debris in an avalanche path off of Seattle Ridge. 

Fresh debris overrunning old debris in one of the Petersen slide paths. 

                                                         Slurpee Monster 

 

Avalanche Problem 2
  • 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

At higher elevations the precipitation is falling as snow and there were strong winds in the last 24 hrs.  Large storm slabs may have developed in the Alpine depending on how well the new snow bonded to the layer below. Leeward slopes may be very loaded and cornices may be large and tender today. 

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

The additional load, either from rain or snow combined with high winds, will be stressing the persistent weak layers of snow we know exist in the snowpack. We have two layers of buried surface hoar, a variety of facet/crust combinations and weak faceted at the ground. Avalanches initiating in the upper snowpack may step down to these layers and cause large, deep, dangerous avalanches. 

Snowpack near the skier triggered avalanche that occured on Eddies Monday. Click HERE for full observation.

Eddies avalanche from below. 

 

Weather
Wed, January 17th, 2018

Yesterday skies were obscured and rain fell throughout the day. Rain/snowline was observed as high as 3000′ and may have gone higher. Around 2″ of rain fell across the region. Temperatures were in the 40Fs at sea level and 30Fs to high 20Fs at ridgetops. Winds were easterly 20-30 mph gusting as high as 70 mph on Sunburst.  

Today should see cloudy skies and continued rain and snow during the day tapering off in the evening. Rain/snowline is forecasted to be around 2400′. Temperatures will cool down this evening with lows in the 20Fs tonight. Winds will be easterly 25-35 mph with gusts into the 40s today and be light overnight.

Tomorrow looks to be clear and sunny with temperatures in the 20Fs and light westerly winds. Cooler temperatures and partly cloudy skies are forecasted for the weekend.  

 *Alyeska Mid stopped recording data at 6pm last night. Alyeska top of quad (2800′) weather station received 1.9″ of water in the last 24 hrs.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 33    1 2 57  
Summit Lake (1400′) 34     0   .4    15
Alyeska Mid (1700′)  *34  *0 *1.4   *45  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 27   ENE 27   70  
Seattle Ridge (2400′)  31  SE 25  63
Observations
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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
Closed
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.