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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Thu, February 1st, 2018 - 7:00AM
Expires
Fri, February 2nd, 2018 - 7:00AM
Forecaster
Aleph Johnston-Bloom
The Bottom Line

The avalanche danger is  MODERATE  above 2,500′ for triggering a slab avalanche 1-3′ thick.  Slopes with signs of wind loading will be the most suspect for triggering an avalanche. Additionally, watch for easily initiated  loose snow avalanches (sluffs) on steep slopes.

Below 2500′ the danger is LOW.  Remember low danger doesn’t mean no danger.  

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Thu, February 1st, 2018
Alpine
Above 2,500'
Moderate (2)
Avalanche risk
Treeline
1,000'-2,500'
Low (1)
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
Low (1)
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

Because last week’s storm loaded a widespread layer of surface hoar (a persistent weak layer) we are calling this a persistent slab issue and not just a wind slab. The main concern in the advisory area is finding a slope where the snow above the January 21st buried surface hoar is acting like a slab. We are not finding evidence of this below 2500′. Cold temperatures with the inversion have been quickly degrading the snow near and below treeline. Slopes near ridgelines in the Alpine, are where the winds over the weekend affected the snow and changed it from fluffy, low density powder to stiff and consolidated wind crust or slab. There were a couple reports of people intentionally triggering small slabs on wind-loaded slopes Tuesday that failed on the buried surface hoar, one skier triggered and one snowmachine triggered. These are a good reminder that despite the cold and clear weather now slowly faceting away the slabs; there is still the possibility of finding and triggering lingering slabs in leeward terrain today.  Remember that there is surface hoar is lurking underneath the recent snow and it’s important to assess areas affected by wind. Slabs can be deeper in loaded areas. Pay attention to slopes where the snow feels stiff, looks pillowed or sounds hollow and watch for shooting cracks. A small slab in the wrong terrain could have high consequences. The easterly winds have picked up this morning blowing into the teens and 20s. Look for changing conditions today if you see snow getting moved around.

Evidence of a natural wind slab avalanche in Ragged Bowl and wind-affected snow. This was likely triggered during the higher winds around the Girdwood area Sunday into Monday morning. Photo: Mike Ausman

Snow pit from Cornbiscuit yesterday. The January 21st buried surface hoar was easy to find but not reactive in this spot. 

January 21st buried surface hoar preserved under the slab.

Avalanche Problem 2
  • Dry Loose
    Dry Loose
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Dry Loose
Dry Loose avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.

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

As the cold weather continues to loosen and facet the surface snow, loose snow avalanches (sluffs) are becoming larger and faster by the day. Watch out for and manage your sluff in steep terrain features.

Natural loose snow avalanches and skier triggered sluffs on Magnum.

 

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

Triggering a deep slab is becoming unlikely, but is still not out of the question above 3000′. In the high elevation snowpack there are a variety of weak layers in the mid pack and near the ground. Because of this poor structure, there is still a chance of triggering a deep slab if you find the wrong spot. The most likely trigger spots are in thin areas in the snow cover, often near rocks, or where the slope rolls over. 

Weather
Thu, February 1st, 2018

Yesterday the clouds moved out and the skies became clear again. Temperatures were in the teens at upper elevations and single digits in the valleys. Winds were light and easterly. Overnight they picked up a little from the SE blowing in consistently in the teens. Temperatures dropped below 0F in the valley bottoms and were in the single digits to low teens at ridgelines stations.

Today will be clear and sunny. Temperatures will be in the single digits to teens with an inversion still in place. Winds will be easterly 5-15 mph gusting into the 20s.  

This sunny, cold weather is forecast to continue into the weekend. There is a chance of snow showers Monday afternoon and a “gradual uptick in temperature.” Stay tuned to see what actually happens with the blocking high that is dominating our weather.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′)  18  0  0  62
Summit Lake (1400′)  3  0  0  17
Alyeska Mid (1700′)  11  0    0    51

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  12 NE  6 20  
Seattle Ridge (2400′)  12 variable  7   18  
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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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