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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sun, December 31st, 2017 - 7:00AM
Expires
Mon, January 1st, 2018 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line

The avalanche danger is expected to rise to HIGH this afternoon in the Alpine due to heavy snowfall and increasing wind.  Pay attention to changing conditions.  The  NWS has issued a Blizzard Warning from Portage Valley to Turnagain Pass. *Travel in avalanche terrain is NOT recommended above treeline and careful snowpack evaluation and cautious route-finding is required below treeline.  

This morning there is a CONSIDERABLE  avalanche danger at all elevations above 1,000′ for storm snow avalanches. New snow overnight is landing on a weak surface and soft slab and wind slab avalanches, up to a foot or more thick, are expected to be easy to trigger. Additionally, above 3,000′  large avalanches breaking near the ground may occur due to new snow overloading a poor snowpack structure.  The avalanche danger may rise to  CONSIDERABLE  below 1,000′ where debris from avalanches above may run.

**Click  HERE  for the Saturday Summit Lake Summary.

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Sun, December 31st, 2017
Alpine
Above 2,500'
High (4)
Avalanche risk
Treeline
1,000'-2,500'
Considerable (3)
Avalanche risk
Below Treeline
Below 1,000'
Considerable (3)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
High (4)
Treeline
1,000'-2,500'
Avalanche risk
Considerable (3)
Below Treeline
Below 1,000'
Avalanche risk
Considerable (3)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Avalanche Problem 1
  • Storm Slabs
    Storm Slabs
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.
More info at Avalanche.org

On this last day of 2017, after a 2 week dry spell, snow is falling again in the mountains. The storm is slowly filtering in and is expected to favor the Portage Valley area, as is common, along with the Southern Kenai mountains near Seward. Snowfall rates are around 1/2 inch per hour this morning and so far Girdwood Valley has picked up 6-8″ of snow at the mid and upper elevations and Turnagain Pass around 5-7″ of new snow. 

Snowfall is expected to increase through the day. By this evening an additional 10″, and up to 20″, could fall in favored locations. Another 5 – 10″ could fall tonight. This heavy snowfall combined with strong winds will dramatically increase the avalanche danger. In the Alpine, where snowfall and wind are most pronounced, natural avalanches will be expected. Near and below treeline, winds may be less active and soft slab avalanches in the new snow should be expected. The new snow is falling on a weak surface made of facets and surface hoar. This will make it hard for the new snow to stick to the slopes. If you are thinking of heading out today keep these things in mind:

–  Soft storm slab avalanches may be easy to trigger in the trees where over 8″ of snow has fallen
–  Keep a close eye out for how much snow has fallen, perform hand pits to assess the old/new snow bonding, remember the more snow the larger the avalanche
–  Avoid large slopes, their runout zones and know the terrain you are traveling in if choosing to go into the backcountry
–  Wind slab, soft slab and cornice falls are likely to be occurring naturally during the storm in the Alpine, travel in these upper elevations is not recommended
–  Large deep slab avalanches are possible in the Alpine
 

 Photo: near surface facets capped with surface hoar is what is now underneath the new snow. This ‘set up’ will inhibit bonding with the new snow and encourage avalanches during this storm cycle, and possibly beyond.

 

Stormy weather moved in yesterday on Turnagain Pass. Photo is from the top of treeline on Tincan.

 

Avalanche Problem 2
  • 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

In the alpine, above 3,000’, rapidly loading slopes may awaken a large and dangerous deep slab avalanche. At these elevations, a hard slab, 3-5+ feet thick, is sitting on top of weak sugary snow (basal facets) near the ground. Observations over the last few weeks indicate this poor structure is widespread across our region in the alpine elevations. As new snow increases the load over this snowpack structure during the upcoming storms, there will be the potential for large natural avalanches. Between storms, human triggered deep slab avalanches will be possible. This is a high consequence avalanche problem that is impossible to outsmart and can take a long time to heal. Keep this in mind as breaks between storms may allow for travel to the Alpine. 

Weather
Sun, December 31st, 2017

Overcast skies, light snow flurries and strong winds filled the region yesterday as a low-pressure system moved in. Ridgetop winds have been from a generally Easterly direction and averaging 25-45 mph with gusts up to 89mph. Snowfall has been slow to arrive and as of 6am roughly 5-8″ has fallen at the mid and upper elevations from the Girdwood Valley to Turnagain Pass. Temperatures have risen with the storm and are in the mid 20’sF in the Alpine and near 30F at 1,000′.  

Today, the storm is expected to peak with an additional 10-20″ of snowfall (1″ water equivalent) before decreasing tonight (5-10″ forecast and ~.6″ water equivalent). The Easterly winds are expected to continue in the 25-45mph range with stronger gusts. Temperatures should hover in the mid 20’sF in the Alpine and near 30F at 1,000′. Snow should fall to sea level through today and into the New Year.

New Year’s Day another low-pressure moves in on the heels of our current storm. Expect the Easterly ridgetop winds to ramp up during the day along with light snow flurries and overcast skies. Temperatures may begin to rise as well as this storm system is warmer. Stay tuned and keep your fingers crossed that the rain/snow line remains low!

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 28   5   0.4   35  
Summit Lake (1400′) 24   0   0   11  
Alyeska Mid (1700′) 26   6   0.3   31  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 16   NE   31   89  
Seattle Ridge (2400′) 20   SE   23   51  
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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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