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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sun, March 11th, 2018 - 7:00AM
Expires
Mon, March 12th, 2018 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line

A  CONSIDERABLE  avalanche danger remains at all elevations and on all aspects. Large slab avalanches 2-4′ thick are likely to be triggered on slopes steeper than 30 degrees.  These may be triggered from a distance or from the bottom of a slope.  Cautious route finding and conservative decision-making will be essential for safe backcountry travel.

Summit Lake area – see Saturday’s  snowpack and avalanche summary.

*A  Special Avalanche Bulletin  has been issued through the National Weather Service.  This will remain in effect through 6pm tonight.  

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Sun, March 11th, 2018
Alpine
Above 2,500'
Considerable (3)
Avalanche risk
Treeline
1,000'-2,500'
Considerable (3)
Avalanche risk
Below Treeline
Below 1,000'
Considerable (3)
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
Considerable (3)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Avalanche Problem 1
  • 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

Triggering a large and unmanageable slab avalanche 2-4′ thick remains our primary concern. Yesterday, a snowmachiner remotely triggered two slabs on Seattle Ridge from a meadow below the slope; it’s important to remember that these avalanches can be triggered from the flats. These slabs were on the North end of the ridge above the powerline trail. This longtime Turnagain rider noted that avalanches in this area are not often seen. Additionally, similar slab avalanches were triggered in Girdwood Valley in the Notch Mtn area. 

The avalanche activity yesterday was all in the mid to lower elevation bands. Various layers of old faceted snow sit under the 2-4′ of storm snow from Friday (photo of this below). The faceted layer has been overloaded by the new snow and is failing, creating the current ‘persistent slab’ avalanche problem. At the higher elevations, above 2,500′, the facets are not as pronounced, yet there are old layers of buried surface hoar that are a concern. How likely a slab is to be triggered in these weak layers at the higher elevations is uncertain at this time. Of note is very little to no traffic was seen in the upper elevation bands yesterday. 

If you are headed out into the backcountry today things to keep in mind are:

  • The mountains are still adjusting to the several feet of new snow that fell two days ago
  • Watch and listen for whumpfing (collapsing) in the snowpack. This was noted by many folks yesterday, including the rider/skiers that triggered slabs. This means the snowpack is highly unstable and will almost certainly avalanche if the slope is steep enough.
  • Stick to slopes less than 30 degrees and ease into steeper terrain slowly – after careful snowpack assessment. Evaluate the consequences if the slope releases; where will the debris go?
  • If the visibility turns poor, be extra cautious to avoid being in a runout zone 

Slab avalanches on East faceing Seattle Ridge (looker’s right of the up-track). Two of these could be the avalanches that were remote triggered from meadow below yesterday.

Loose snow sluffs on the West face of Magnum

 
Annotated photo of the storm slab bonding to the old snow surface, but the old snow surface is weak and failing. Hence, the new snow has ‘overloaded’ the older weak snow.

Three feet of new snow and wind effect on Tincan Ridge (left) and Seattle Ridge’s Repeat Offender and uptrack zone (right). It’s good to see the new snow, but slopes need time to adjust. 

  


Turnagain Pass flats – plenty of safe places to play that are out of avalanche terrain. (Photo: Allen Garrett) 

Additional Concern
  • Cornice
    Cornice
Cornice
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
More info at Avalanche.org

Cornices have grown and are suspect for breaking while traveling along ridgelines. Give these an extra wide berth and minimize any time below them. Cornice falls can trigger avalanches on slopes below.

Weather
Sun, March 11th, 2018

Yesterday cloudy skies with light snow showers gave way to breaking skies in the afternoon. Total 24-hour accumulation was 2-5″ of low density snow in favored areas and no snow in others. Ridgetop winds were light to moderate from the East (5-20mph). Temperatures were in the mid 30’sF at sea level and in the teens along ridgetops.

For today, Sunday, a low pressure spinning East of us in the Gulf may push some moisture our way. Clear skies this morning are forecast to turn cloudy later today along with a chance that 1-3″ of light snow will fall. Ridgetop winds look to shift to the North and West and stay light (5-10mph) as we are on the back side of the low pressure. Temperatures will remain cool for March, in the mid 30’s again at sea level and teens along ridgetops.  

Looking ahead to the work week, Monday another system passes through with a chance for snow followed by clearing and the chance for a nice day on Tuesday. Unsettled weather looks to continue later in the week.

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

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 28   4   0.4   93  
Summit Lake (1400′) 24   0   0   36  
Alyeska Mid (1700′) 27   3   0.2   82  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 17   NE   14   38  
Seattle Ridge (2400′) 23   *n/a   *n/a     *n/a    
Observations
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Date Region Location
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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.