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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Mon, February 20th, 2017 - 7:00AM
Expires
Tue, February 21st, 2017 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line

There is a MODERATE avalanche danger on all aspects above 1,500′ for triggering a slab avalanche 2-3′ thick. The new snow from last week is still trying to bond with the old snow surface and  triggering a large, and potentially very dangerous, slab is possible. Other avalanche concerns are cornice falls and glide avalanches; there is an opening glide crack on Seattle Ridge that threatens popular terrain underneath. If the sun shines today, calm winds may allow the surface to warm enough for loose snow sluffs in steep southerly terrain.

Below 1,500′ there is a LOW danger where triggering an avalanche is unlikely due to a snowpack consisting of hard crusts.

*If skies clear up today and travel is possible to the upper elevations, remember cautious route-finding is recommended with this avalanche problem.

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Mon, February 20th, 2017
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
  • 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

It has been roughly 4 days since the Valentine’s Day Storm exited the region and 6 days since the peak of the storm. Essentially, that has given the mountains some time to adjust to the new load (2-3+’ of dense snow) but, since all that storm snow fell on a weak surface, instability remains. Today we will be in a “scary moderate” regime where the probability of triggering a 2-3′ slab is lower, but the consequence can be high. This is the major concern for the day and is a tricky problem considering some slopes show better bonding than others.

The weak layer is a thin layer of buried surface hoar and near surface facets. It is deep enough (2-3′) that in most areas digging a snowpit is the only way to assess it. Furthermore, finding a representative and safe place to dig a pit is also a challenge. Be aware that no red flags may be present (whumphing or cracking in the snow) and the pack could have a general “it seems fine to me” feel before someone finds the trigger point. Trigger points are often where the slab is thinner, near rocks or scoured areas. Also keep in mind, these slabs can break above you, release after several tracks are on a slope and be triggered remotely. 

 

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 that grew last week could still 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 the sun comes out, warming may increase the potential for them to break.  

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

 

GLIDE AVALANCHES:

There is a new glide crack above the flats along Seattle Ridge, just looker’s left of the up-track and Repeat Offender slide path. Avoid hanging out under this crack and any others you may see – these release without warning and are very destructive.

 

SUNSHINE?

Although cooler temperatures are moving in, the winds should be very light to calm today. This may allow Southerly slopes to warm enough for sun triggered loose snow avalanches in steep Southerly aspects. 

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

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 them (Girdwood Valley, Portage Valley and Summit Lake). These layers will be tough for people to trigger, but possible in shallow snowpack zones. The more likely case is where an avalanche occurring in the upper layers of the pack has the potential to step down and release the entire snowpack. If this does happen the volume will be large and could run long distances. 

Weather
Mon, February 20th, 2017

Obscured skies with a few patches of broken cloud cover filled the region yesterday. Light snow flurries fell in the afternoon and evening with only a trace in most areas, the exception was Girdwood Valley that picked up 3-4+” of very low density snow. Temperatures were in the mid 20’sF below treeline and in the upper teen’s F in the Alpine. Ridgetop winds were calm and with the light cloud cover created a ‘greenhouse’ effect that increased the temperature slightly during the day. Overnight, winds backed to the Northwest bringing in cooler temperatures with a very light flow, near 10F at the high elevations.

Washington’s Birthday (President’s Day), we can expect the cool Northwest flow to remain over the area with temperatures dropping to the single digits in the Alpine and the teens below treeline. Rigetops winds associated with this will be light, 5-10mph. Skies should be partly cloudy with clearing in some areas. A possible instability shower may add a trace of snow to some locations.  

Tuesday looks to be another break in weather with mostly clear skies while Wednesday a frontal system moves in with a chance for snow.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 18   trace   0   69  
Summit Lake (1400′) 19   trace   0   32  
Alyeska Mid (1700′) 19   3-4   0.2 67  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 12   E   2   7  
Seattle Ridge (2400′) 15   Rimed   Rimed   Rimed  
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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
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Turnagain Pass
Closed
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Twentymile
Closed
Closed.
Seward District
Carter Lake
Closed
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Lost Lake Trail
Closed
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Primrose Trail
Closed
Closed.
Resurrection Pass Trail
Closed
Closed. Will be open for the 2019/20 season pending adequate snow cover.
Snug Harbor
Closed
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South Fork Snow River Corridor
Closed
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Summit Lake
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