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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Fri, January 19th, 2018 - 7:00AM
Expires
Sat, January 20th, 2018 - 7:00AM
Forecaster
Heather Thamm
The Bottom Line

A  CONSIDERABLE  avalanche danger remains at elevations above 2,500′ in the Turnagain Pass area. Triggering an isolated storm slab or a very large Deep Slab avalanche (3-8+ feet) is trending towards possible today. However, it is only the 2nd day after a storm and a lot of uncertainty exists about how the snowpack is adjusting to its new load. Today is a day to follow the ‘travel advice’ for  CONSIDERABLE  danger:  Careful snowpack evaluation, cautious route-finding and conservative decision making.

Below 2500, treeline, the avalanche danger is MODERATE where a crust is helping to stabilize the snowpack, but an avalanche from above could run into this zone.  There is  LOW  avalanche danger below treeline where very little snow exists.

**Heightened avalanche conditions exist in the Summit Lake zone. Click HERE for recent observations.  

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Fri, January 19th, 2018
Alpine
Above 2,500'
Considerable (3)
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
Considerable (3)
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
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.

Likelihood of Avalanches
This graphic depicts how likely you are to trigger avalanches or encounter natural avalanches while traveling on avalanche prone slopes. Unlikely means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. The chance of triggering or observing avalanches increases as we move up the scale. Certain means that humans will be able to trigger avalanches on many slopes, and natural avalanches should be expected.

Size of Avalanches
This graphic depicts the potential size and destructive force of expected avalanches. 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, large enough to bury or destroy vehicles and break a few trees, and large enough to destroy railway cars, buildings, or a substantial amount of forest. Historic avalanches are massive events capable of destroying villages and gouging or altering the landscape.
More info at Avalanche.org

Several storms this week brought 3-6+’ of snow to the upper elevations and ended on Wednesday. This last storm caused a widespread avalanche cycle including: a very large D3 avalanche on the SW face of Sunburst, and dozens of large wet avalanches in the mid elevations. Above freezing temps were noted in the alpine during this cycle and rain may have reached ~3000’ at some point, but this has not been confirmed with field data. What we do know is this wet snow has frozen into a solid stable crust below 2500’, but in the alpine this moist snow layer gradually goes away and is not obvious due to new snow covering it up. Basically there is an invisible gray line between Treeline and the Alpine where we go from a stable snowpack to a Deep Persistent Slab problem, where several weak layers (buried surface hoar and facets) are sitting below 3-8+ feet of snow. This avalanche problem comes with a lot of uncertainty due to how deeply buried these layers are. It will be impossible to know where the thinner areas of the snowpack are – likely trigger spots. A snowmachine or a person may get away with riding on steeper slopes, but if a thinner area is found, the consequences could be unsurvivable if caught. This is no beast to mess around with. Keeping your terrain choices conservative in the Alpine will be key. This means choosing low consequence terrain and avoiding large steep slopes.

 Keep in mind:

  1. Obvious signs of instability like cracking or collapsing “whumpfing” may not be present until it’s too late
  2. Be aware of people above and below you, an avalanche from above could run into the treeline elevation band
  3. Several tracks could be on a slope before someone finds a trigger spot
  4. Remote triggering from above, below or adjacent to a steep slope is possible from a thinner area of the snowpack
  5. Snow pits and stability tests may not be representative due to how deeply buried these weak layers are

 

Yesterday we investigated the avalanche on Sunburst and found the New Year’s buried surface hoar to be the most likely culprit. This avalanche wraps around the West Ridge near the skin track all the way to the gully below the weather station (~3/4 mile) and filled Taylor Creek up with debris. 

 

 

Debris in Taylor Creek

 

 

Snow pit at 3300′ just above the crown on a W aspect of Sunburst. 

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
This graphic depicts how likely you are to trigger avalanches or encounter natural avalanches while traveling on avalanche prone slopes. Unlikely means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. The chance of triggering or observing avalanches increases as we move up the scale. Certain means that humans will be able to trigger avalanches on many slopes, and natural avalanches should be expected.

Size of Avalanches
This graphic depicts the potential size and destructive force of expected avalanches. 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, large enough to bury or destroy vehicles and break a few trees, and large enough to destroy railway cars, buildings, or a substantial amount of forest. Historic avalanches are massive events capable of destroying villages and gouging or altering the landscape.
More info at Avalanche.org

Triggering a storm slab in the top 1-3’ of snow is becoming less likely as we move away from the storm that ended Wednesday. Cold temps and benign weather is helping stabilize any mid-storm weaknesses within the new snow. Storm slabs may be lingering in the alpine, above 2500’, on steep to very steep terrain. This is an additional reason to avoid larger terrain and stay off of slopes steeper than 35 degrees. 

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 been growing over the last week and could be easy to trigger. Give these features a lot of space and remember they can break further back from a ridge than expected.  A cornice fall also has the potential to trigger a very large avalanche on the slope below.

Weather
Fri, January 19th, 2018

Yesterday skies were clear becoming partly cloudy in the afternoon. Temperatures were between 20-25F and winds were calm. No precipitation was recorded.  

Today skies will be overcast and valley fog and low lying clouds are also possible. Temperatures will be in the mid to low 20F’s and may drop into the teens overnight. Ridge top winds are expected to remain light and variable.  

Benign weather is expected tomorrow through the weekend with mostly overcast skies and a possibility of snow flurries.   Temperatures will be in the teens to low 20F’s and winds are expected to remain light through the weekend.  

**Seattle Ridge weather station stopped recording wind data 1/17/18 rime covering the anemometer.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 23 0   0   56  
Summit Lake (1400′) 17 0   0   15  
Alyeska Mid (1700′) 26   0   0    43

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 20   variable   4   16  
Seattle Ridge (2400′) 23    *n/a  *n/a    *n/a  
Observations
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Riding Areas
Updated Mon, December 02nd, 2019

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
Closed.
Placer River
Closed
Closed.
Skookum Drainage
Closed
Closed.
Turnagain Pass
Closed
Closed.
Twentymile
Closed
Closed.
Seward District
Carter Lake
Closed
Closed.
Lost Lake Trail
Closed
Closed.
Primrose Trail
Closed
Closed.
Resurrection Pass Trail
Closed
Closed. Will be open for the 2019/20 season pending adequate snow cover.
Snug Harbor
Closed
Closed.
South Fork Snow River Corridor
Closed
Closed.
Summit Lake
Closed
Closed.

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