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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sat, January 12th, 2019 - 7:00AM
Expires
Sun, January 13th, 2019 - 7:00AM
Forecaster
Aleph Johnston-Bloom
The Bottom Line

Strong winds impacting the region today will cause the avalanche danger to rise to MODERATE at all elevation bands in exposed terrain as triggering a wind slab becomes possible. As these slabs build over the weak surface snow the danger may rise to CONSIDERABLE in the Alpine later in the day. Natural avalanches may become possible and human triggered avalanches may become likely. Pay attention to changing conditions. In addition, glide cracks may release into avalanches. Limiting/avoiding exposure under them is prudent and give cornices a wide berth.  

There is a Winter Weather Advisory issued for 9 pm this evening to 11 am Sunday for Portage Valley and Turnagain Pass.  

SUMMIT LAKE / JOHNSON PASS / LYNX DRAINAGE:  Keep in mind buried weak layers exist in the middle and base of the snowpack. More potential for triggering a large slab avalanche exists in this zone, especially with wind-loading. Choose terrain wisely.

 

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Sat, January 12th, 2019
Alpine
Above 2,500'
Considerable (3)
Avalanche risk
Treeline
1,000'-2,500'
Moderate (2)
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
Moderate (2)
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
  • Wind Slabs
    Wind Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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

After enjoying sunny skies and a mostly stable snowpack a series of storms will begin to affect the advisory area today. Temperatures are on the rise, winds are increasing and precipitation will start to fall later in the day. Today is the day to expect changing conditions and be on the lookout for signs of instability. Initially increasing winds may trigger small loose snow avalanches but as they continue and intensify, wind slabs will build in exposed terrain. There is soft snow available for transport and wind slabs may be quite tender as they develop over very weak surface snow formed during the cold weather. Well developed surface hoar was observed at lower elevations and along some ridge tops. Near-surface faceted snow was observed at all elevations. Below 1500′ there is a crust below the weak snow. This is not a good set-up for new snow and wind-loading. Look for blowing snow, cracking, collapsing and be especially heads-up if natural wind slabs start releasing. 

Rapidly warming temperatures and the additional blowing snow may also make cornices easy to trigger over the next few days. As always, give these a wide berth. 

 

 Weak surface and just below the surface snow on Center Ridge, 1-10-19. 

 Weak surface snow on the Gold Pan ridge, 1-11-19. Photo: Allen Dahl

Avalanche Problem 2
  • 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

South of Turnagain – Lynx Creek/Johnson Pass/Summit Lake zone:  As the weather pattern changes and the potential for wind loading increases it becomes even more critical to remember that a poor snowpack structure exists in these areas. The Christmas buried surface hoar has been found as well as concerning facet/crust combinations in the bottom of the snowpack. Avalanches may initiate near the ground and be quite dangerous. If you’re headed this way, evaluate terrain exposure and the snowpack as you travel. Be on the lookout for signs of instability. 

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

Identify glide cracks and avoid spending any time under these features. Glide cracks are opening and have avalanched within the last week. Glides are completely unpredictable and not human triggered. The rapid temperature rise and additional snow load over the next few days may or may not cause an increase in glide activity. 

Weather
Sat, January 12th, 2019

Yesterday:  Skies were clear and temperatures were cold with daytime highs in the single digits. Valley bottoms remained below zero. Winds were light and easterly. Overnight clouds bulit and temperatures began to increase.

Today:  Mostly cloudy skies with an increasing chance of precipitation throughout the day. Temperatures will continue to rise into the 20Fs. Easterly winds will build and blow 15-25 mph with gusts into the 40s. These are forecast to increase overnight. Snowfall will increase overnight and temperatures will rise with the potential for rain to fall at sea level.  

Tomorrow:  Cloudy skies and snow continues tomorrow with the brunt of the storm in the early morning. Winds will remain strong and temperatures will continue to rise with the warmest temperatures on tap for Monday. The series of fronts and lows are forecast to impact the region into the week.  

*Seattle Ridge weather station was heavily rimed and the anemometer (wind sensor) was destroyed. We are currently working to replace it.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 6   0    0  53
Summit Lake (1400′) -1     0    0   20    
Alyeska Mid (1700′) 8   0    0  41  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  6 SE    6 23  
Seattle Ridge (2400′)  8 *N/A   *N/A   *N/A  
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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.