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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Mon, December 11th, 2017 - 7:00AM
Expires
Tue, December 12th, 2017 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line

The  avalanche danger is HIGH  due to heavy rain, wet snowfall and strong wind.  A very potent warm and wet storm has brought rain near the top of treeline and possibly higher. This has triggered a natural wet avalanche cycle in the Treeline elevation band and up to 2,700′. Above this, natural storm snow avalanches are likely occurring in the Alpine. Today,  natural avalanches will remain likely and human triggered avalanches very likely.  Travel in avalanche terrain is NOT recommended.  

Below 1,000′ there is a  CONSIDERABLE  avalanche danger for debris running into these lower elevations from avalanches releasing above.

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Mon, December 11th, 2017
Alpine
Above 2,500'
High (4)
Avalanche risk
Treeline
1,000'-2,500'
High (4)
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
High (4)
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
  • Wet Loose
    Wet Loose
Wet Loose
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
More info at Avalanche.org

Rain falling on snow has initiated a widespread avalanche cycle in the mid elevation band. The Girdwood Valley saw 2.4″ of rain in the past 24 hours below ~2,200′ and subsequently significant wet slab and wet loose avalanche activity. Wet avalanches were reported up to 2,700′ in elevation, which is just above the rain/snow line and where very wet snow is falling. This trend is expected to continue today. There is not much known about the activity yet at Turnagain Pass where around 2″ of rain fell or at Summit Lake with 1″ of rain, but we can expect wet avalanche activity in these areas as well. The rain/snow line has been fluctuating between 2,200′ and 2,500′ in general. Yes, this is soaking the meager snowpack between 1,000′ and 2,500′ and allowing the alders to bounce back up. 

Image: A look at Tincan Ridge at Turnagain Pass being wetted down up to the top of the trees. Once these storm cycles cease, the upper elevations should look much different, the lower will hopefully remain snow covered!

Avalanche Problem 2
  • 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

In the Alpine, wet and heavy snow is falling, which becomes drier with elevation. From our rainfall totals at the snow stations, we can expect that during the past 24 hours 1-2+’ of snow has accumulated. Cornices are building and the weak pre-existing snowpack continues to be loaded. Storm snow avalanches that include storm slabs, wind slabs and cornice breaks all can be expected today at the high elevations. What can also be expected is the snow may stick fairly well and instead of avalanche immediately as a storm snow problem, it may overload and tip the balance of the weak snow sitting near the ground and create a very large avalanche, taking most of the snowpack with it. 

Again, the message is simple for today, avoid avalanche terrain. This means steering clear of slopes 30 degrees and steeper with nothing steeper above or near you. Also, be extra cautious of evaluating runout zones.

Weather
Mon, December 11th, 2017

Heavy rain and overcast skies filled the region yesterday. Rain/snow line looks to have been between 2,200′ and 2,500′ depending on location. Around 2-2.5″ of rain fell while 1-2+ feet of heavy wet snow fell at the upper elevations. Much less precipitation fell inland on the Kenai (Summit Lake area). Winds have been strong from the East, averaging 20’s-30’s mph with gusts in the 70’s. Temperatures are way too warm – low to mid 40’s at sea level and up to 32F along the 3,000′ ridgelines.

For today, Monday, we can expect this very warm system to move out and precipitation rates diminish. Around .3″ of rain is expected today below 2,900′ and 1-3″ of heavy snow above. Temperatures should remain very warm, in the mid 40’s at sea level and 32F along the ridgelines. Winds from East and Southeast will remain moderate to strong, in the 20’s – 30’s mph with much stronger gusts. Slightly cooler temperatures are expected tonight along with .5-.6″ of rain below 2,100′ and 3-6″ of snow above as another system moves in.

For Tuesday and heading into the week, the active warm and stormy weather pattern with continue with the next storm moving in tonight. However, “by the end of the work week there is high confidence for the anticipated pattern change” says the NWS, meaning a return to cooler temperatures and a break from the incessant stormy weather.

*Center Ridge and Summit Lake SNOTEL stations stopped reporting at 9pm and 11pm respectively. Values below are estimates.
**Reported 24-hour water at the RWIS station at Turnagain Pass (1000′ elev.)  and RWIS Summit Lake (1300′ elev.)

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) *35   0 (rain)     **1.9   *25
Summit Lake (1400′) *35   0 (rain)     **1 *10  
Alyeska Mid (1700′) 35   0 (rain)   2.4   15  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 28   ENE   36   77  
Seattle Ridge (2400′) 31   SE   28   65  
Observations
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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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