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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Tue, February 21st, 2017 - 7:00AM
Expires
Wed, February 22nd, 2017 - 7:00AM
Forecaster
Heather Thamm
The Bottom Line

There is a MODERATE avalanche danger on all aspects above 1,500′ where triggering a  slab avalanche 2-3′ thick is possible on slopes steeper than 35 degrees and could have high consiquences. In addition there are a handful of other avalanche concerns to be aware of including fresh shallow wind slabs, cornice fall and loose snow point releases. Also its worth mentioning that a glide crack has opened up on Seattle Ridge and continues to creep open on Repeat Offender. Be aware of it and limit time below if you are near the uptrack.  

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

In Summit Lake, Girdwood, and near Johnson Pass triggering a deeper more dangerous avalanche near the ground is still possible but will be hard to trigger. If headed to Summit Lake please check out the weakly summary HERE.

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Tue, February 21st, 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’s been almost a week since the Valentines Day storm dumped 2-3’ of heavy snow on top of a weak surface (surface hoar and/or near surface facets.) On Saturday several human triggered avalanches occurred on Seattle Ridge, and luckily there were no reports of anyone being caught. Poor visibility over the last two days has made travel into the alpine challenging and has limited traffic and observations in this zone. The good news is with time, the slab has been adjusting to the weak layer below, making it more difficult to trigger. However the consiquences remain high, meaning it is still possible to trigger an avalanche large enough to bury, injure or a kill a person. 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 a 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. The trickiest part about our current snowpack is how difficult it is to assess due to the spacial variability of both the weak layer, the slab, and even the bed surface in some places. Some steep solar aspects (S-SE) may have a slick sun crust bed surface/buried surface hoar combo. Be aware that snow pits and stability tests may not be representative of the actual slope you are trying to assess.  

 This snowpit from yesterday on a West aspect of Taylor Pass shows all of the weak layers described in both the Persistent and Deep Slab poor structure. This location did not produce any conderning results, but its still good example of the difficulty in assessing these avalanche concerns. 

 

Human triggered avalanche that was reported second hand on Saturday in Seatte Creek on JR’s, Southern aspect. If anyone has info or details about this avalanche please submit an observation HERE. Photo taken on Monday, 2/20/17 by Bryan Pfaender. 

 

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

WIND SLABS: There is 3-6” of very light weight fluffy snow on all aspect throughout our region and ridgetop winds 5-15mph are in the forecast today. If this is the case expect small isolated wind slabs to be easy to trigger on leeward features. Be aware that Northwest winds can funnel through some terrain in Turnagain Pass in more of a Southwest direction, causing an unusual loading pattern.  Although these slabs are not expected to be very dangerous, don’t let one catch you off guard in a the wrong place. Watch for blowing snow and identify steep features where the snow may be loading. 

LOOSE SNOW:  Sluff will also be easy to initiate on steep features and could be fast moving

CORNICES that grew last week could still be teetering on the balance and could break further back than expected

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 winds are expected to keep the surface cool today, be aware of solar warming that may trigger 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 as well as some areas in Turnagain Pass. Last week’s storm cycle tested these layers and only a few avalanches that we know of broke in them (Girdwood Valley, Portage 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
Tue, February 21st, 2017

Yesterday temperature dropped into the single digits. Skies were obscured and snow flurries left several inches of fluffy low density snow throughout our region. Ridgetop winds were calm. Overnight a similar pattern continued.

Today skies will start out mostly clear becoming cloudy be this afternoon as a front moves into our region. Ridgtop winds are expected to increase 5-15mph from the NW throughout the day with winds increasing to the mid 20’s from the SE by this evening. Temperatures are also expected to warm up into the teens F by the afternoon. Snow flurries are in the forecast and 1-3 € of snow expected by this evening.  

Tomorrow night into the Thursday a second warm front is expected to bring more snow and warmer temperatures. There is much uncertainty as to how much accumulation will occur in Turnagain Pass zone.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 11    trace 0   67  
Summit Lake (1400′) 12   trace    0 31  
Alyeska Mid (1700′) 12   1   .1   65  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 3   –   0    2
Seattle Ridge (2400′) 6   n/a   n/a   n/a  
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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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