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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Tue, December 18th, 2018 - 7:00AM
Expires
Wed, December 19th, 2018 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line

The avalanche danger is MODERATE above 1,000′. Soft slab avalanches composed of Sunday’s storm snow will be possible to trigger on slopes steeper than 35 degrees. These are most likely to be shallow, 6 to 12″ thick, and found near ridgelines and over rollovers.  Additional concerns: watch your sluff in steep terrain  and avoid traveling underneath  glide cracks.

SUMMIT LAKE:    There are more developed weak layers near the ground; increasing the chance a person could trigger a larger slab avalanche. Choose terrain carefully.  

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Tue, December 18th, 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
  • 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

Snowfall over the past several days continues to deepen our early winter snowpack and provide some excellent surface conditions. Girdwood Valley has been the most favored for snowfall and has seen over a foot in the past 48 hours, while Turnagain Pass has seen 6-10″. The Summit Lake zone has seen roughly 6-8″. Very little wind effect during this time has been observed and avalanche activity has been relegated to small pockets of storm slab, soft wind slab and loose snow sluffs. For today, another 1 to 3″ of light snow is forecast and will add to these ‘surface instabilities’.

If skies open enough for travel to the upper elevations today, keep an eye on the surface conditions. Watch for stiffer snow over softer snow, cracking in the snow and whumpfing (collapsing). The Girdwood Valley could harbor some foot thick pockets of storm slab or wind slab due to the higher recent snowfall amounts. Also, watch your sluff. Sluffs should start getting larger with the cooler temperatures. 

 

A small storm/wind slab pocket was triggered yesterday in Tincan’s Common Bowl. Thank you to Drew Petrie for sending in his photo.

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

As we have been mentioning for a while now, in thinner snowpack zones such as Summit Lake and Crow Pass, we are tracking buried layers of facets, crusts and buried surface hoar that sit 1-3′ under the snow surface. These layers are most prevalent in the mid-elevations (2000’ – 2700’). Snow pit data and a lack of avalanche activity has been pointing to an unlikely chance for an avalanche releasing in these deeper layers. However, as we push out into more and more terrain, we need to keep in mind the snowpack could have some surprises lurking. 

Snow pit on Tenderfoot, in the Summit Lake area, from Sunday shows buried surface hoar deep 2′ below the surface that will react with a lot of force. 

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

Glide cracks are moving. We have had several reports that cracks are opening. Glide cracks we know about are on Sunburst’s SW face under the weather stationSW face of Tincan Proper, Gold Pan area (behind Cornbiscuit/Magnum) and a crack that did release in the Johnson Pass area. These cracks can release at any moment. They are not associated with human triggers and the best way to manage the hazard is to avoid being on or beneath slopes with cracks. 

Weather
Tue, December 18th, 2018

Yesterday:    Partly cloudy skies with some valley fog were over the region. Intermittent flurries overnight has added a trace to an inch of snow at Turnagain Pass and 1-2″ of snow in the Girdwood Valley. Ridgetop winds were easterly yesterday, 5-10mph, and have been light and variable overnight. Temperatures remain near 20F along ridgelines and the upper 20’sF at sea level.

Today:   Mostly  cloudy skies with a chance for 1 to 3″ of cold, low-density snow are on tap today. Ridgetop winds should be northerly in the 5-10mph range. Temperatures will remain winter-like, in the 20’sF at sea level and in the teens along ridgelines. Tonight, we may see a bump in snowfall rates with another 2-4″ of low-density snow possible with this light northerly flow.  

Tomorrow:   Light snow showers should cover the region Wednesday. Models are showing a low-pressure forming just south of Seward that looks to shift winds back to the east and pump in a bit of moisture. Snow numbers for tomorrow look to be in the 3-6″ range.  

*Seattle Ridge anemometer (wind sensor) is rimed over and not reporting.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 21   trace   0   36  
Summit Lake (1400′) 20    0 0   15  
Alyeska Mid (1700′) 21 1-2″   .1 – .2   27  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 19   E   5   21  
Seattle Ridge (2400′) 20   *no data   *no data     *no data    
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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