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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sun, February 19th, 2017 - 7:00AM
Expires
Mon, February 20th, 2017 - 7:00AM
Forecaster
Aleph Johnston-Bloom
The Bottom Line

The avalanche danger is rated  CONSIDERABLE  due to a thin layer of weak snow 2-4+ feet below the surface. Triggering a slab avalanche breaking in this layer and taking out the top few feet of snow will be likely on slopes steeper than 35 degrees at elevations above 1000′. These slab avalanches may be triggered remotely (from the side, above or below) as well as propagate across entire bowls. Cornice falls are also a concern and will have the potential to trigger a large slab below. If the sun comes out, solar radiation may trigger wet loose sluffs as well as increase the likelihood for triggering a large slab.

Below 1,000′ the danger is  LOW  due to snowpack composed of hard crusts.

**The avalanche conditions we are dealing with this President’s Day Weekend have the potential to be unsurvivable and call for cautious route-finding and conservative decision-making. These avalanches can release when there are no red flags present and several tracks are already on a slope.

Summit Lake:   An unstable snowpack persists in the Summit Lake area, make sure and check the Saturday Summary  HERE.  

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Sun, February 19th, 2017
Alpine
Above 2,500'
Considerable (3)
Avalanche risk
Treeline
1,000'-2,500'
Considerable (3)
Avalanche risk
Below Treeline
Below 1,000'
Low (1)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
Considerable (3)
Treeline
1,000'-2,500'
Avalanche risk
Considerable (3)
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
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

There was one confirmed human triggered avalanche yesterday on the buried surface hoar that is the main concern this weekend. A snowmachiner on Seattle Ridge triggered a 1.5-4′ deep avalanche on a small WNW slope at 3000‘.  The snowmachiner was on the slope and it broke above him but he was able to ride off to the side. This incident really illustrates the potential of this layer and the reason to avoid steep terrain and large or high consequence slopes (ones that slide into terrain traps). The layer of surface hoar and/or near surface facets buried under the Valentine’s Day Storm has been found in almost every snow pit that has been dug throughout the advisory area since the storm. Observations from Friday continue to confirm that the layer is reactive and triggering a slab avalanche on the steeper slopes is likely. On Wednesday, there were several remotely triggered slabs on this layer in the Girdwood Valley.  This could be a tricky situation since the pack may ‘seem like it feels fine’ and people could get away with riding/skiing steeper terrain before a slope breaks. Slopes that saw higher traffic last weekend could remain intact while an adjacent slope slides. Triggering this set-up in spots where the slab is thinner will be more likely and adds to the overall spookiness. In one spot the layer could be a foot below the surface and in another 4′ deep, the slab being connected across the slope and the weak-layer being fairly uniform is why the avalanche danger remains elevated. 

What to keep in mind if headed to the mountains:

  1. These slabs can be triggered remotely, from the ridge, the side or below
  2. In terrain that saw high traffic last weekend, this layer will likely be more stubborn to trigger, but don’t let that lure you into thinking the pack has stabilized
  3. No obvious signs on instability, or red flags, may be present before a slab is triggered
  4. This is the kind of avalanche that could have several tracks on the slope before someone finds the right trigger point (thin areas of the slab). 

What can we do? Terrain management is the key for hedging our bets. This is simply sticking to lower angle slopes, under 35 degrees with nothing steeper above you.

Snowmachine triggered avalanche on Seattle Ridge that occured yesterday. Photo: Bryan Pfaender

The February 9th buried surface hoar layer (with some preserved stellars) was the culprit in this avalanche. Photo: Bryan Pfaender

Solar effects from SUNSHINE??

If the sun shines and winds are calm, solar radiation can be significant this time of year. Wet/damp loose sluffs on steep South aspects are likely when we do get this first shot of warmth. Additionally, the warming surface snow can enhance the likelihood of triggering a slab. Avoiding sun baked slopes this weekend will be recommended. These Southerly aspects likely have a crust under the new storm snow with either buried surface hoar or near surface facets on top – creating a perfect slab/weak layer/bed surface set up.

Surface warming on Seattle Ridge yesterday. 

 

Avalanche Problem 2
  • Cornice
    Cornice
Cornice
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
More info at Avalanche.org

Cornices have grown and changed shape following the Valentine’s Storm. These could be teetering on the balance and could break further back than expected. If one does fall it may trigger a slab avalanche below, potentially creating a very dangerous situation if a person is involved. Yesterday there were a few places where chunks of cornice had released. If the sun comes out again today, warming can loosen these beasts and increase the potential for them to break. Remeber these often break much further back than expected. 

 

 

Cornice hanging over steep terrain on Wolverine ridge. Note the slab that was triggered looker’s right. 

Another concern that needs to be avoided is travel under glide cracks. A new one was observed yesterday on Seattle Ridge.  Remember these release without warning. 

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

Lurking at the bottom of the snowpack are various layers of facets with varying degrees of strength. In the Summit Lake zone and some areas in Girdwood Valley and Johnson Pass depth hoar has been found. Last week’s storm cycle tested these layers and only a few avalanches that we know of broke in the deeper layers (Girdwood Valley, Portage Valley and Summit Lake). These layers will be tough for people to trigger, but possible in shallow snowpack zones. More likely is the case where an avalanche occurring in the upper layers of the pack will have the potential to step down and release the entire snowpack. If this does happen the volume will be large and could run long distances. The possibility of these large avalanches is another reason for conservative terrain choices. 

 

Weather
Sun, February 19th, 2017

Yesterday was mostly sunny with some scattered clouds across the area. Easterly winds were calm and temperatures were in the low 30Fs to mid 20Fs in the valleys and low 20Fs to teens in the Alpine. Overnight temperatures dropped a little and winds remained calm.  

Today will be partly to mostly cloudy and there is a chance of snow throughout the day. Winds are forecasted to remain light and easterly shifting to the north this evening. Temperatures should be similar to yesterday and then cool a bit more tonight.  

Tomorrow will partly cloudy and stay a little cooler.  An “active trend” is on tap for the coming week. Stay tuned for details.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′)  26 0   0   71  
Summit Lake (1400′)  24 0   0 31  
Alyeska Mid (1700′)  27 0 0 64  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 17   ENE   6    20
Seattle Ridge (2400′) 20 rimed   rimed   rimed  
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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
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Closed
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Turnagain Pass
Closed
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Carter Lake
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Closed
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Summit Lake
Closed
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