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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Wed, December 12th, 2018 - 7:00AM
Expires
Thu, December 13th, 2018 - 7:00AM
Forecaster
Heather Thamm
The Bottom Line

The avalanche danger is  MODERATE  in the Alpine and at Treeline where triggering a slab 1-3′ deep is possible. Keep an eye out for glide cracks and avoid traveling underneath this unpredictable avalanche hazard. Assess the snowpack as you travel, identify areas of concern and evaluate terrain consequences.

UPDATE: An unexpected sleeper storm is upon us!! We’ve seen 6-12″ in the Girdwood Valley and 3″ in Turnagain Pass between 6am and 10am. This is a very low density snow storm with little volume and little wind. Expect ‘sluffs’ in steep terrain and pay attention to new snow accumulation. In areas recieving a foot or more of snow the avalanche danger could increase to  CONSIDERABLE.

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Wed, December 12th, 2018
Alpine
Above 2,500'
Moderate (2)
Avalanche risk
Treeline
1,000'-2,500'
Moderate (2)
Avalanche risk
Below Treeline
Below 1,000'
Low (1)
Avalanche risk
Alpine
Above 2,500'		 Avalanche risk
Moderate (2)
Treeline
1,000'-2,500'		 Avalanche risk
Moderate (2)
Below Treeline
Below 1,000'		 Avalanche risk
Low (1)
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

Incremental loading over the last three days and more observations have nudged the mid-elevation band to MODERATE danger for triggering a slab 1-3’ thick releasing on weak snow. Yesterday on Magnum Mountain widespread ‘whumpfing’ caused a group to turn around at 2300’. A reactive layer of facets between two crusts showed easy propagation and they had low confidence in continuing into steeper terrain. This is a good reminder that it is still early season and limited snowpack observations exist across the forecast zone. Areas with a thinner snowpack are more suspect for weaker and unstable snow including the Southern-end of Turnagain Pass, Summit Lake and Girdwood Valley.

In the Alpine on the Northern end of Turnagain Pass stability tests have been showing a strengthening snowpack. However keep in mind North and East aspects have a tendency to be thinner and there is a zone in the mid-elevations where the snowpack quickly transitions to shallower depths. 

As always be on the lookout for red flag warnings like ‘whumpfing’, shooting cracks, new avalanche activity and any changes in weather. Today a 1-3″ inches of snow is expected and there is currently 4-8” of loose snow available for transport. Should ridgetop winds increase above the forecasted 5-15mph, keep an eye out for active wind loading. Any sign of increased winds could form wind slabs or add stress to a persistent slab.

One of three pits between 2000′ and 2300′ on Magnum’s NW shoulder that showed poor structure and propagation potential in stability tests

Avalanche Problem 2
  • Glide Avalanches
    Glide Avalanches
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
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.
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 several glide cracks were noted on the Southwest face of Sunburst below the weather station. A glide crack is the snowpack being pulled by gravity downhill along the ground. They can release at any moment without warning and are usually not associated with human triggers. The best way to manage this hazard is to avoid being on or beneath any slopes with cracks opening up. 

These are the first glide cracks of the season and we hope the last.  If you see any new glide cracks in other areas, snap a photo, and send us an observation HERE. This helps us provide better information in our forecasts.  

 

Weather
Wed, December 12th, 2018

Yesterday: Snow showers ended yesterday morning and skies were partly cloudy. Temperatures fell throughout the day from the upper-20Fs into the low-20F’s. Overnight ridgetop temps have fallen into the teens F. Winds were light and variable.

Today: Light snow flurries started early this morning. A few inches of snow (1-3″) is possible today across our region at all elevations. Temperatures will remain in the low 20F’s at 1000′ and low-teens F near ridgetops. Light Westerly winds 5-10 mph are expected to increase 10-15mph later this evening.    

Tomorrow: Temperature will remain in the low 20F’s to low teens. Partly sunny skies are in the forecast for tomorrow and light winds. Friday into Saturday looks like our next best chance for snow showers.

*Seattle Ridge weather station anemometer is rimed and not recording wind data.    

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 25   0   0    29
Summit Lake (1400′) 24   1   0.1   7  
Alyeska Mid (1700′) 25 2 0.16 13

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 17   variable   2   8  
Seattle Ridge (2400′) 23   *N/A   *N/A   *N/A  
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.

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