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Issued
Sun, February 11th, 2018 - 7:00AM
Expires
Mon, February 12th, 2018 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is currently MODERATE and expected to rise to CONSIDERABLE  late tonight and tomorrow in conjunction with snowfall and strong wind. For the daylight hours today, wind slabs up to a foot thick along with lingering larger slabs up to 2′ thick will be possible to trigger on slopes with wind deposited snow. Watch for changing conditions and in the event snowfall begins earlier than expected, avalanche danger will begin to rise.

Sun, February 11th, 2018
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
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. Dangerous avalanche conditions. 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
  • 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

One more breezy overcast day is on tap before snowfall begins to fall in earnest tonight. We could see light snowfall add a couple inches through the day, but the bulk of the expected storm snow (6-12″) should fall late tonight through Monday morning. Keep in mind, avalanche danger will rise along with snowfall. The new snow will fall on a weak surface and storm snow avalanches should be expected if this storm verifies. 

Prior to tonight’s snow, our main concern for triggering an avalanche remains with slopes that have seen, or are seeing, wind loading. Ridgetop winds have been moderate to strong over the past 48 hours from a generally East direction. These have scoured some ridges and loaded leeward slopes with the existing loose snow available for transport. Small pockets of wind slabs could be triggered on these leeward slopes and these should be relatively easy to identify (smooth pillowed surfaces and stiffer snow over softer snow). More concerning however, is the possibility of finding a wind slab that is sitting on buried surface hoar 1-2′ deep. There are multiple layers of buried surface hoar  that sit in the top 2′ of the snowpack, which may now be overloaded by wind deposited snow. These types of slabs could be larger than expected and break deeper than your ‘typical’ wind slab. If you head out today, watch for how the winds are transporting the snow and avoid wind loaded slopes. Additionally, slabs may range from soft or hard and have the potential to break above you once committed to a steep slope. 

Wind scouring along the South end of Seattle Ridge yesterday, looking SW from the Eddies ridge (photo: Allen Dahl) 

 

Wind transport just above the trees on Eddies (video: Allen Dahl). Winds are not only loading certain slopes and scouring others, but creating wind crusts as well.


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

Our snowpack structure is generally poor and weak snow can still be found in the bottom portion of the snowpack at the higher elevations, above 3,000′. Shallow snowpack zones, such as on the South end of Turnagain Pass, Summit Lake and portions of the Girdwood Valley are more pronounced. Although triggering an avalanche breaking deeper in the pack is unlikely, a chance does remain, as a party found the right trigger spot last weekend in the Twin Peaks area. It is worth keeping this poor structure in mind and considering a conservative approach to avoid large steep terrain with shallow snow cover or exposed rock. 

Weather
Sun, February 11th, 2018

Overcast skies filled the region yesterday with windy conditions near and above the trees. Ridgetop winds averaged 15-25mph with gusts to 40mph from a generally East direction. Temperatures were on the warm side, low 20’sF in the Alpine and 30F at 1,000′. A few flurries fell overnight with around an inch of snow recorded at mid-elevations in Girdwood Valley and Turnagain Pass.

Today, a quick moving warm(ish) storm is moving into the area. Snowfall is expected to be light through the daylight hours, with around 1-3″ accumulation, then intensify this evening through Monday morning, adding another 6-12″. Ridgetop winds are forecast to shift Southeasterly and remain in the 10-20mph range until tonight when they bump up into the 25-35mph range with the heavier snowfall. Temperatures are expected to rise to the mid 20’sF along ridgetops and the mid 30’sF at sea level, this will bring a rain/snow mix to sea level and possibly raise the snow line to ~500′-1,000′.  

Monday, light snowfall is expected as the system moves out with a possible 1-3″ accumulation. Ridgetop winds look to remain moderate to strong with a shift to Westerly bringing cooler temperatures. Storm total by Monday evening is looking to be around 6-12″ of medium density snow at the mid to upper-elevations.  

NWS graphic for the Monday storm:

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 27   1   0.06   61  
Summit Lake (1400′) 28   0   0   19  
Alyeska Mid (1700′) 29   1   0.1   52  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 21   NE   13   41  
Seattle Ridge (2400′) 25   SE   21   41  
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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.