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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Mon, February 18th, 2019 - 7:00AM
Expires
Tue, February 19th, 2019 - 7:00AM
Forecaster
Aleph Johnston-Bloom
The Bottom Line

The avalanche danger is CONSIDERABLE due to recent snowfall and wind loading. Human triggered slab avalanches 1-2′ thick are likely on slopes 35 degrees and steeper,  especially in wind loaded terrain. Additionally, give cornices a wide berth and avoid travel under glide cracks.  Careful snowpack evaluation and conservative decision-making are essential if headed into avalanche terrain. Look for signs of instability.  

SUMMIT LAKE / JOHNSON PASS:   This area has a very poor snowpack structure with multiple weak layers.  Watch for whumpfing, shooting cracks and recent avalanches. Triggering an avalanche in the new snow has the potential to initiate a more dangerous slab,  breaking deeper in the snowpack.  

SEWARD/ LOST LAKE:   Avalanche danger has risen in this region as well and storm snow avalanches are likely today.  

BYRON GLACIER TRAIL Hikers:    Remember this trail is in avalanche terrain and the popular snow cave is very dangerous and unstable.  

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Mon, February 18th, 2019
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

Triggering a 1-2′ thick slab avalanche on slopes 35 degrees and steeper is likely today. Wind loaded slopes are the most suspect. Yesterday the advisory area saw steady snowfall with an additional 4-10″ of snow (favoring Girdwood) and easterly wind loading throughout the day. Sunburst saw gusts into the 80s and Max’s into the 60s. Observers noted cracking and small test slopes being easily triggered later in the day. Visibility made Alpine observations difficult. The strong winds eased off in the evening and there was a break in the snowfall. Snow started again early this morning and is forecast to continue today throughout the advisory area. The old wind harden snow surface from before the storm and Saturday’s low-density snow will make it tough for the new snow to stick right away. ‘Upside-down’ snow was noted by observers yesterday. Today look for recent avalanches, shooting cracks and listen for whumpfs or hollow sounding snow. Choose terrain carefully. Cornices may be quite tender and should avoided. 

Storm slab triggered on Sunburst at 1300′,  2-17-19. Photo: Elliot Gaddy

Storm slabs triggered on Tincan later in the day, 2-17-19. Photo: Ray Koleser. 

 

 

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

New snow and wind loading have the potential to add stress to underlying weak layers. As we have been hammering home for weeks, in Turnagain Pass roughly 1-3′ below the snow surface sits a layer of buried surface hoar.  Periphery zones such as Summit Lake and Johnson Pass harbor a poor overall snowpack structure with a variety of weak layers. Although these persistent weak layers have not been reactive lately, additional load may start to tip the balance. It is good to keep in mind that triggering an avalanche today could to step down into old weak layers and initiate a larger more dangerous slide. This is more likely in Summit Lake and the central Kenai mountains. 

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 unpredictable, not associated with human triggers, and can release without warning at any time. The best way to manage this problem is to avoid traveling on slopes directly below glide cracks.

Weather
Mon, February 18th, 2019

Yesterday:  Skies were obscured with snow falling throughout the day. Winds were easterly 20-40 mph gusting as high as 88 mph on Sunburst. Temperatures were in the 20Fs to mid 30Fs. Overnight skies were mostly cloudy and winds were easterly 5-15 mph gusting into the 20s.    

Today: Mostly cloudy skies and snow showers, 3-8″ of snow forecast. Rain/snow line around 700′. Temperatures in the 20Fs to mid 30Fs. Winds will be southeasterly 5-15 mph with gusts into the 20s.  Light snow showers continue overnight with temperatures cooling into the low 20Fs.  

Tomorrow: Skies clearing in the early morning becoming mostly sunny. Winds shifting to the northwest and increasing to 20-30 mph with gusts into the 40s. Temperatures will be in the low to mid 20Fs. The next storm system is forecast to move into the area Wednesday afternoon.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′)  29  2 0.3   62  
Summit Lake (1400′)  30   2    0.2     28  
Alyeska Mid (1700′)  29   5   0.6    58    

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 20    NE 20   88  
Seattle Ridge (2400′) 25    E  13      38    
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.