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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Thu, December 7th, 2017 - 7:00AM
Expires
Fri, December 8th, 2017 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line

Avalanche danger is expected to increase to HIGH by this afternoon as another round of rain, snow and wind moves in. Around 10-15″ of snow falling today in the Alpine and 1″ of rain below 1,500′ will continue to overload a very unstable snowpack.  Natural avalanches are expected and human triggered avalanches will be very likely. As this stormy weather persists, travel in avalanche terrain is NOT recommended. This means avoiding slopes 30 degrees and steeper, including runout zones.

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

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Thu, December 7th, 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
  • 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

Temperatures are climbing today ahead of another round of precipitation. In the upper elevations above treeline, we are looking at 10-15″ of moist snow to fall with another 5-8″ tonight. Below 1,500′ up to an inch of rain is expected today with another .5″ tonight. The rain/snow line should creep up higher that we have seen with this storm cycle – up to and possibly over 1,500′. The forecast snow amounts are not exceptional for a HIGH danger day, but what is exceptional is the very unstable nature of our existing snowpack.

There is a very reactive layer of facets sitting under the storm snow from the past week, which is anywhere between 18 and 36″ below the surface. By tonight we should have another foot or so of new snow to add to this slab, making it anywhere from 2-4′ in thickness. In the event these snow amounts verify, avalanches occurring today could be larger than we have seen so far and send debris into valley bottoms. Additionally, they have the potential to propagate very wide and around terrain features on multiple aspects. Warmer temperatures will also add stress to the snowpack and enhance likelihood of triggering. 

Just how reactive are these facets? On Tincan, two avalanches were remotely triggered by a group of people yesterday from hundreds of feet away and on different sides of the ridge. What happened was a group skied/rode down Tincan’s Common Bowl (photos below) and re-grouped up on a bump below. While the group was on this ‘bump’ it appears they triggered a collapse and very large ‘whumpf’ that was heard from others in the area. Subsequently, an avalanche released ~600′ away. And, this is the even more impressive part, the collapse traveled over the ridge and released another avalanche on the North side. 

Photo of tracks in Tincan’s Common Bowl. Party skied one at a time, grouped up below on knoll, triggered a collapse from knoll that propagated hundreds of feet triggering two separate avalacnhes on different aspects. (Photo: Brad Cosgrove)

Photo of avalanche that released on the North side of Tincan a thousand feet away or more from the group who triggered the collapse.

Close up on the avalanche on the Southwest aspect and under the CFR ridge. (photo: Gabe Gibbs)

 

Once again, with a snowpack as precarious as this, this is not the time to be traveling into avalanche terrain. As the snow piles up on top of the facets, the avalanches are becoming larger. They are also likely to become a bit more stubborn to trigger, which makes for a very scary snowpack and hard to manage avalanche problem. Very conservative choices are needed for safe days in the backcounty right now. Sticking to slopes under 30 degrees with nothing steeper above you, or next to you, is key. 

 

Snowpack on Sunburst. The current problem, facets below the slab clearly seen. 

Avalanche Problem 2
  • Wind Slabs
    Wind Slabs
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.
More info at Avalanche.org

Winds from the East have begun to increase this morning and are expected to blow 25-45mph with gusts into the 60’s or more through tonight. This will continue to load leeward slopes and cross load snow into gullies. Cornices will also continue to build along ridgelines. Naturally or human triggered wind slab avalanches or cornice falls are likely, but what is more of a concern is the potential for one of these to trigger a much larger and dangerous persistent slab avalanche as discussed above. 

Weather
Thu, December 7th, 2017

Yesterday was a break between storms. Skies were mostly sunny and winds were light and Easterly around 5mph. Temperatures dropped slightly at sea level to the upper 20’s and at the upper elevations to the low 20’s F. Overnight, clouds and warmer air has streamed in allowing temperatures to rise a few degrees (low 30’s F sea level and upper 20’s along the ridgelines). Winds have increased overnight from the East and are blowing 25-35mph with gusts up to 50mph.  

Today, we can expect winds to continue increasing to 35-45mph with gusts over 60mph from predominantly the East. Temperatures will continue climbing to ~38F at 1,000′ and 30-32F along the ridgelines. An inch of rain is forecast by 6pm tonight below 1,500′ and 10-15″ of snow above 1,500′. Another .5″ of rain is expected tonight along with 5-8″ of snow in the Alpine.

Tomorrow, this ‘pulse’ of moisture looks to shift East giving us a slight break in precipitation intensity before the fire-hose of moisture points more in our direction late Saturday. In general, this active pattern of warm, windy and wet weather is expected to continue into next week.  
 

*Center Ridge SNOTEL is reporting erroneous temperature data. See  Turnagain Pass DOT weather station  for accurate temperature at 1000′.

**Seattle Ridge anemometer is rimed.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) *28   0   0   30  
Summit Lake (1400′) 23   0   0   11  
Alyeska Mid (1700′) 29   0   0.05   21  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 24   ENE   17   50  
Seattle Ridge (2400′) 26   n/a**     n/a**     n/a**    
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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