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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Thu, April 12th, 2018 - 7:00AM
Expires
Fri, April 13th, 2018 - 7:00AM
Forecaster
Aleph Johnston-Bloom
The Bottom Line

Sunshine, warm temperatures, fresh snow and rain saturated snow will keep the danger at  CONSIDERABLE  today. Human triggered wet snow avalanches and storm slabs are likely and naturals are possible, especially this afternoon. In the Alpine triggering a persistent slab avalanche 1-2′ thick remains possible on Northerly aspects where drier snow exists. Give cornices a wide berth.

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Thu, April 12th, 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
  • Wet Loose
    Wet Loose
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Wet Loose
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

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

Yesterday as the sun poked out late in the afternoon a wet loose avalanche on Raggedtop mountain in Girdwood triggered a wet slab. Observers also noted new wet loose snow avalanches around the region. Rain/snow line was approximately 2700′. Conditions below that were described as “slop” in the saturated surface snow and above that new snow easily formed roller balls. A few skiers remarked on the “parade of pinwheels” initiating from their turns. There was clearing last night and a crust formed on the surface of the snowpack. This will slow snowpack warming down in the morning. However, the temperatures are forecast to rapidly rise and it is supposed to be clear and sunny with calm winds. Expect human triggered wet loose avalanches to become likely on solar aspects and watch for naturals later in the day. The wet loose avalanches may trigger wet slabs. If you do decide to recreate today pay attention to the depth you are sinking in and get off the slope if the snow is over your boot tops or your machine is trenching in. Watch for roller balls.  Expect loose snow moving to entrain more as moves downhill. Any fresh snow in the Alpine may be especially reactive on solar aspects. If the clouds move in early and trap the heat, fresh snow on shaded aspects may also become more reactive. Wet loose and wet slab avalanches today could be large and dangerous.

Wet slab avalanche on Raggedtop triggered by loose snow off of the rocks. 

Wet loose avalanches on Raggedtop initiated when the sun came out. 

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

Storm slabs: The precipitation over the past few days in the Alpine has fallen as snow. This rain/snowline has fluctuated so pay attention to changing conditions. Locations closer to the coast may have received up to a foot above 3000′. This storm slab may be especially reactive as temperatures rise today and on solar aspects as the sun hits. The winds have been strong as well and this may have created deeper slabs on leeward aspects. Look for wind drifts or pillowed snow, watch for cracking and remember conditions could change rapidly as the sun warms the snow. 

Cornices: This new snow and wind will have also added to already large cornices.  It’s that time of year where the snowpack is warming up and cornices will begin to break. Give cornices a wide berth.

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

In the Alpine on Northerly shaded aspects above the rain/snow line triggering a slab avalanche 1-2′ thick on weak faceted snow is still possible. The old slab/facet combo remains suspect and may be more tender with the additional snow and wind loading. 

 

Weather
Thu, April 12th, 2018

Yesterday was mostly cloudy with rain and snow showers on and off throughout the day. Skies became broken in the late afternoon and eventually cleared overnight. Temperatures were in the high 20Fs to mid 40Fs depending on elevation. The rain/snow line was around 2700′. Winds were easterly 15-25 mph with gusts into the 50s. Overnight temperatures dropped into the mid to low 30Fs and winds became light.  

Today is forecast to be mostly sunny and could be the warmest day of the season so far with highs getting close to 50F. Winds will be light and easterly. Tonight will be partly to mostly cloudy with temperatures dropping into the low 30Fs. Winds will become calm.  

Tomorrow is forecast to be partly to mostly cloudy with temperatures in the high 40Fs again. Temperatures may drop below freezing overnight Friday and skies are forecast to clear for the weekend. There is a chance of rain/snow early next week as another low tracks into the Northern Gulf.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 36    0  0.2  71

Summit Lake (1400′)

 38    

  0  0.1  26
Alyeska Mid (1700′)  36       0  0.4  68

 RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  27  ENE  19  59
Seattle Ridge (2400′)  32  ESE     20    52
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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

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Closed as of May 1. Thanks for a fun, safe season!
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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.