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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, February 26th, 2018 - 7:00AM
Expires
Tue, February 27th, 2018 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE  above treeline due to 6-12″ of new snow expected with wind. Natural storm snow avalanches will be possible and human triggered avalanches likely. The danger will be MODERATE in sheltered areas in the treeline band and below where loose snow sluffs composed of the new snow are expected. Debris could run far as these will be dry and fast running slides if the storm verifies. Additionally,  old weak layers deeper in the pack may be triggered, creating a larger avalanche.

Special Announcements

Dangerous avalanche conditions are expected in areas seeing new snow such as Hatcher Pass and the Western Chugach. See the Hatcher Pass Avalanche Center Facebook page – avalanche danger has increased to CONSIDERABLE. Image below is from the National Weather Service.


Mon, February 26th, 2018
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
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
  • Storm Slabs
    Storm Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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

Storm snow avalanches will be the main concern as a quick moving system is impacting the region. Snowfall totals so far are 4+” in Girdwood Valley and 1-2″ at Turnagain Pass. Another 6-8″ is expected through the day (to sea level), yet Turnagain Pass may only see a few more inches. The snow is very low-density and should sluff easily on steep slopes, both naturally as well as human triggered. The moderate to strong winds with the storm have a Southerly component, which is unusual and could create wind slabs in unusual places. Wind slabs could be up to a foot thick and will depend on how much snow has fallen. If found, they are likely to be soft and easy to trigger. In areas seeing more than 6″ of new snow, and definitely areas with 10″ or more, watch for soft storm slabs. 

It is a day to pay attention to how much new snow has fallen and what the winds are doing. If you are in an area with only a few inches of new snow, the main concern will be finding an old wind slab or triggering an avalanche in deeper weak layers of the pack (discussed below). Quick hand pits will be a good tool for assessing new snow amounts. They are also good for determining if there is stronger snow over weaker snow – signs that a wind or storm slab is present.

 

Image below is from Fresno Ridge yesterday (Summit Lake area). This small pocket triggered on a test slope in the trees shows Friday’s new snow overloading a faceted weak layer sitting on a crust. Be aware of old weak layers that may become overloaded with today’s new snow. (Photo: Chis McNeil)


Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.

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

The snowpack is very weak in general and we need to keep in mind that larger slides breaking in persistent weak layers could occur; even though most of today’s activity will be relegated to the new snow. The new load from today is adding to the weight from Friday and earlier in the week. This is incremental loading and can slowly overload weak layers. Furthermore, new snow avalanches have the potential to step down to these layers. In the upper elevations a layer of buried surface hoar from Jan. 21st continues to show signs of reactivity and in the mid-elevations a layer of facets over a melt-freeze crust is suspect. 

Deep Persistent Slabs:  At the high elevations above 3,000′, deeper persistent layers could ‘wake up’ if the wrong spot is found. Old weak layers of facets and buried surface hoar sit in the bottom half of the snowpack. This structure is most pronounced in places with a thin overall snow cover, such as the South end of Turnagain Pass, the Summit Lake area and Crow Pass. 

Photo below is from the mid-elevation slopes of Cornbiscuit (Korn/Biskis) yesterday where a layer of very weak facets sit on a crust below the February snow. 


 

Weather
Mon, February 26th, 2018

Sunny skies were over the region yesterday. Ridgetop winds were 15-25mph from the Northwest before backing off around noon. Temperatures were in the teens at most locations. Overnight a low pressure has moved in from the West bringing light snowfall. Between 2-4″ of snow has fallen as of 6am (.1 and .2″ of water equivalent). Winds shifted to Easterly with the snowfall and are blowing Southerly 10-20mph.  

Today, snowfall will continue as the low pressure moves Eastward. We are expecting an additional 4-8″ of low-density snow (.3-.4 water equivalent) through the day before the system moves out this evening. The storm track is favoring the Hatcher Pass and the Front Range (Talkeetna Mtns, Western Chugach), where upwards of a foot of snow is forecast. Ridgetop winds should remain Southerly in the 10-20mph range before turning Westerly and remaining around 15-20mph overnight. Temperatures will be in the upper 20’sF at sea level and lower 20’s along ridgetops.  

For Tuesday, clearing skies and windy, cold conditions are expected. Strong ridgetop winds should bring in cold air (single digits) from the Northwest. Stay tuned as to how these winds develop on tomorrow’s forecast.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 25   1-2″   0.05   68  
Summit Lake (1400′) 20   1-2″   0.1   30  
Alyeska Mid (1700′) 20   4″   0.2   63  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 11   Variable    12 33  
Seattle Ridge (2400′) 17   Southerly     16   58  
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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.