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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sat, February 17th, 2018 - 7:00AM
Expires
Sun, February 18th, 2018 - 7:00AM
Forecaster
Heather Thamm
The Bottom Line

The avalanche danger is  MODERATE  above 1,000′ on all aspects where triggering an isolated wind slab or larger slab 2 feet deep remains possible. Be aware of radiation from the sun adding additional stress to solar aspects. Additionally, weak layers deeper in the snowpack may still be triggered, creating a larger avalanche.   Evaluate snow and terrain carefully.

The avalanche danger  below 1,000′ is LOW where triggering an avalanche is unlikely, but not out of the question. Avoid terrain traps where a pocket of unstable could have high consequences.  

The Summit Lake area has seen recent natural activity. Read the Summit Summary HERE.  

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Sat, February 17th, 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
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 persistent slab avalanche up to 2′ deep remains possible across the region, and a sunny Saturday may increase the odds. Several weak layers including widespread buried surface hoar (from Jan. 21) sits roughly 1-2′ below the surface. New snow combined with periods of strong ridgetop winds have created both soft and hard slabs on a variety of aspects. Be extra cautious on solar aspects and if you see moist surface snow or point releases near rocks, these are obvious clues the sun is adding stress to these aspects. Unfortunately obvious signs of instability may not be present today and all aspects are suspect due to poor structure. Assessing the terrain and the potential outcome of an avalanche breaking deeper in the pack is key. If you find yourself in a crowded area, consider changing your objective or waiting for people to clear a slope before continuing. 

 

Jan. 21st buried surface hoar  is ~2′ below the surface and continues to show propagation potential in stability tests. Snow pit at 2700′ on Tincan on a West aspect.  

 

A photo of some of the wind affected terrain near Squirrel Flats. Little snowpack information exists in Placer Valley/Skookum area. If you see or experience any avalanche activity please take a picture and send us an observation HERE

 

Avalanche Problem 2
  • 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
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 an isolated wind slab is possible on all aspects and will be more likely in steep terrain. Earlier in the week moderate to strong winds (from a variety of directions) loaded some slopes and scoured ridgelines. Pay attention to where the snow feels stiff, looks pillowed, sounds or feels hollow and watch for shooting cracks. Hard wind slabs tend to break when you are out onto the slope and often fracture above you.  Wind loaded convexities and cross-loaded gullies facing the sun are most suspect. 

Recent natural wind slab from earlier in the week in Zero Bowl, NW aspect of Seattle Ridge. 

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

Above 3,000′ in the Alpine, several old weak layers of facets and buried surface hoar sit near the ground and in the mid-pack. This structure is most pronounced in places with a thin overall snow cover, such as the South end of Turnagain Pass and the Summit Lake area.  As you plan your day, keep in mind that there are deeper persistent layers that could ‘wake up’ if you find a trigger spot in a shallow area. 

Weather
Sat, February 17th, 2018

Yesterday was mostly cloudy with light Westerly ridgetop winds and a brief period of moderate Westerly winds this morning. Temperatures in the upper elevations were in the high teens/low 20F’s and sea level temps bumped into the high 20F’s during the day and teens F’s overnight.  

Sunny clear skies and light Westerly winds are expected today.   Daily temperature swings should average in the upper 20F’s today and dip down into the teens (F) overnight. No precipitation is expected.  

Sunday will range from party to mostly cloudy with daily temperature swings in the low 20F’s to low 30F’s. Light winds will shift from the West to an Easterly direction. A similar pattern is expected through Monday followed by the possibility of warming trend starting Tuesday. However at this point low confidence remains in the long term forecast.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 22   0   0   64  
Summit Lake (1400′) 13   0   0   24  
Alyeska Mid (1700′) 20   0   0   57  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 19   W    7 25  
Seattle Ridge (2400′) 22   NW   5    17
Observations
Recent Observations for Turnagain Pass
Date Region Location
12/10/19 Turnagain Avalanche: Tincan and Sunburst from the air
12/10/19 Turnagain Observation: Tincan
12/08/19 Turnagain Avalanche: Tincan
12/06/19 Turnagain Avalanche: Sunburst
12/04/19 Turnagain Observation: Sunburst
12/03/19 Turnagain Observation: Hippy Bowl
12/01/19 Turnagain Observation: Tincan, All elevations
12/01/19 Turnagain Observation: Tincan
11/30/19 Turnagain Observation: Tincan Treeline Plateau/ Common Bowl/ Ridge
11/29/19 Turnagain Observation: Sunburst Ob #2
Riding Areas
Updated Wed, December 11th, 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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