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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, February 8th, 2016 - 7:00AM
Expires
Tue, February 9th, 2016 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

There is a  MODERATE avalanche danger at the upper elevations around Turnagain Pass for fresh wind slab avalanches. Slab avalanches up to a foot thick will be possible to trigger on slopes that have been, or are being actively, loaded by winds. At the mid-elevations, watch out for glide cracks and stay out from under them; two cracks have released in the past 36-hours. Lastly, give cornices a wide birth; these are large, looming and still breaking off.

*The avalanche danger in Girdwood Valley and Portage Valley may rise to CONSIDERABLE today due to increased precipitation (up to a foot of new snow is possible by this evening).  

Special Announcements

The Chugach National Forest Avalanche Information Center and Friends of the Chugach Avalanche Information Center are pleased to announce the launch of an observations program partnership with Hatcher Pass Avalanche Center.

By clicking the €œObservations € link on both  hatcherpassavalanche.org  and  www.cnfaic.org, users can now browse and submit Hatcher Pass observations. Data is mirrored on both sites giving Southcentral Alaska backcountry enthusiasts the ability to quickly browse recent snowpack and avalanche observations throughout the region.  Please  submit your own observations  and spread the word to others!  

Save the dates! Hatcher Pass Avalanche Center and Friends of the CNFAIC are also joining forces to support FREE avalanche education at Hatcher Pass and in Palmer. There will be a FREE companion rescue workshop on Saturday, February 13th, 10:30am-12:30pm, Hatcher Pass Gold Mint Lot and a FREE avalanche awareness class Wednesday, February 17th, 6:30-8pm, Palmer High School Library. Click  HERE  for more info!

Mon, February 8th, 2016
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
0 - No Rating
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
Travel Advice Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern. Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches Natural and human-triggered avalanches unlikely. Natural avalanches unlikely; human-triggered avalanches possible. Natural avalanches possible; human-triggered avalanches likely. Natural avalanches likely; human-triggered avalanches very likely. Natural and human-triggered avalanches certain.
Avalanche Size and Distribution Small avalanches in isolated areas or extreme terrain. Small avalanches in specific areas; or large avalanches in isolated areas. Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas. Large avalanches in many areas; or very large avalanches in specific areas. Very large avalanches in many areas.
Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.

Likelihood of Avalanches
Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.

Size of Avalanches
Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word Size (D scale) Simple Descriptor
Small 1 Unlikely to bury a person
Large 2 Can bury a person
Very Large 3 Can destroy a house
Historic 4 & 5 Can destroy part or all of a village
More info at Avalanche.org

After a short break between storms over the weekend, we have another small system over us today that is a bit more ‘blow than snow’. So far only a couple inches of snow has fallen at Turnagain Pass but up to 6″ in the Girdwood Valley; we are expecting another 2-5″ to fall through the day above 1,000′. Ridgetop winds are the big player today and have increased into the strong category from the East. Add these winds to the new snow, and the existing soft snow available for transport, and we have a wind slab avalanche problem. 

If you are headed out today, watch for areas of wind drifted snow and/or slopes that the winds are actively depositing snow. Traveling over wind drifts on gentle slopes and watching for cracks that shoot out from you is a great way to assess if these wind slabs are not bonding well and could avalanche if the slope is steep enough. Quick hand pits in these areas (stiffer snow over softer snow) are good tools as well. 

CORNICES:
Don’t forget, cornices are still falling and the weight of a person and/or snowmachine could help tip the balance. Give these guys a wider berth than you may think necessary along the ridgelines; many have grown to the point where it hard to determine where the ground ends and the cornice begins.

Photo Left: Northeast winds loading Southwest slopes late in the day yesterday, Magnum Ridge. Photo Right: Wind transport over Taylor Pass.
 

Avalanche Problem 2
  • Glide Avalanches
    Glide Avalanches
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.

Likelihood of Avalanches
Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.

Size of Avalanches
Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word Size (D scale) Simple Descriptor
Small 1 Unlikely to bury a person
Large 2 Can bury a person
Very Large 3 Can destroy a house
Historic 4 & 5 Can destroy part or all of a village
More info at Avalanche.org

Two new glide avalanches were seen yesterday in the forecast region. The first was on Pete’s South and the second was in Skookum Valley (side note: if you are not familiar with the names at Turnagain Pass check out this map – maps are found under the resource tab!). Avoiding and limiting time under glide cracks is essential as these avalanches will destroy anything in their path. Many many cracks still litter the mid-elevation band at Turnagain Pass and surrounding regions.

Pete’s South glide avalanche pictured below, before and after the crack released. Photos Tim Glassett. Additional photos can found HERE.
Left, Glide Crack 1:37pm Sat. 2/6                                                Right, Crack released and avalanched sometime before 11:40am Sun. 2/7
 

Skookum glide avalanche, this slide can easily be seen from the Seward Highway as you pass the Portage Valley road.

Additional Concern
  • 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

We have been concerned about a layer of weak older snow that sits roughly 2′ below the surface. This older snow is essentially last week’s surface that became loose and started to facet before it was buried by 2-3′ of storm snow Feb 3-5th. Over the weekend, we have found that this old snow/new snow interface has bonded very well – great news. Report from yesterday’s investigation HERE. However, we have little to no data for areas South of the Pass, such as the high elevations above the Johnson Pass trail or Summit Lake. Hence, there may be areas that did not bond well and if this is the case large and ‘connected’ avalanches will be possible. 

Weather
Mon, February 8th, 2016

Partly cloudy skies and light winds covered the region yesterday morning. By the early afternoon, a weak low-pressure system began to move in bringing cloud cover and a bump in Easterly winds (~25mph averages) along the ridgetops. Temperatures remained mild in the 20’sF at the mid-elevations.

Overnight, we have seen an inch of new snow at Turnagain Pass with up to 6″ in the Girdwood Valley. The rain/snow line is currently ~500′ and may  rise to 1,000′ by later today. We are expecting an additional 2-5″ at the higher elevations by 6pm. Winds will continue in the 20-30mph range from the East along the ridgetops and temperatures are expected to rise to the mid 30’s at 1,000′ and the mid 20’s at 3,500′.  

Tuesday looks to be a short break in weather before a much warmer and wetter system moves in from the South for Wednesday. We could see rain above 2,000′ with this next round of storms mid-week.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 28   1   0.1   109  
Summit Lake (1400′) 29   0   0   32  
Alyeska Mid (1700′) 30   6    0.5 87  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 22   NE    22 53  
Seattle Ridge (2400′) 24   –   –   –  
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This is a general backcountry avalanche advisory issued for Turnagain Arm with Turnagain Pass as the core advisory area. This advisory does not apply to highways, railroads or operating ski areas.