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Issued
Mon, January 28th, 2019 - 7:00AM
Expires
Tue, January 29th, 2019 - 7:00AM
Forecaster
Graham Predeger
Avalanche risk The Bottom Line

There remains a CONSIDERABLE avalanche danger above treeline throughout Turnagain Pass.   Forecasters and observers continue to find a layer of surface hoar buried last week (MLK day) that was responsible for several human-triggered avalanches on Saturday.   Given the nature of this problem, avalanches have the potential to be triggered from significant distances away from a slope and may be quite wide and connected.  

If winds kick up from the East today, fresh wind slabs may add an additional stress onto this known weak layer.    Cautious route finding and conservative decision-making will be essential for a safe day of mountain travel.  

SUMMIT LAKE / JOHNSON PASS:    Dangerous avalanche conditions exist in these regions south of Turnagain Pass. Strong wind, a poor snowpack structure and recent avalanche activity all point to an unstable snowpack.

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Mon, January 28th, 2019
Alpine
Above 2,500'
3 - Considerable
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. 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
  • 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

It doesn’t take much digging right now to find and easily identify the buried layer of surface hoar (buried on MLK day) responsible for a number of human-triggered avalanches on the motorized side of Turnagain Pass on Saturday.  We found this same layer to be widespread on Sunburst yesterday above 2,000’ (and up to our high point at 3,400’).  It shows up in a snowpit wall or snowmachine trench as a thin grey line 12-24” below the surface on all aspects.  Though no obvious red flags were present during yesterday’s tour, the poor structure and widespread nature of this persistent weak layer has our hackles up.  Thin spots in the slab (near rocks or wind scoured areas) are more likely trigger points and should be avoided. 

 There is no mistaking the MLK buried surface hoar layer in snowpits right now.  As seen on Sunburst yesterday buried 12-16″ below the surface.

Snowpack tests over the last two days have all showed high propagation potential on the MLK layer.  With such a connected weak layer across multiple aspects and elevation bands, any avalanche initiated has the potential to be quite wide.  As seen on Saturday, these avalanches can be triggered remotely from the bottom of a slope or even an adjacent ridge. 

Maintain your situational awareness of where you are on slope in relation to avalanche terrain, any objective hazards (terrain traps) and other groups today.  This is the type of avalanche problem that can very quickly affect an adjacent, unsuspecting party.  Cautious route-finding and conservative decision-making is critical when dealing with known persistent weak layers.

Long shooting crack and subsequent cornice fall triggered by snowmachiners on the ridge near the Seattle Ridge weather station on Saturday Jan.26th.  photo: Iron.iii.Oxide

Wind slabs: If forecasted winds kick up in the 20-40mph range from the east today, expect fresh wind slabs to form on lee aspects. This additional weight will push the MLK layer closer to its tipping point.

Cornices: Give these backcountry bombs a wide berth while traversing ridges and limit exposure underneath.  A cornice fall today on a slope greater than 30 degrees may trigger a slab avalanche below. 

South of Turnagain – Johnson Pass/Summit Lake zone: Areas south of Turnagain Pass generally harbor a thinner, weaker snowpack with multiple weak layers present, including the MLK buried surface hoar layer.  An avalanche triggered in these zones could step down into deeper weak layers, with the potential for large avalanches.  See video here  of two separate and reactive weak layers in a January 26th snowpit in the Silvertip area south of Turnagain Pass, that clearly demonstrates the step down potential. 

Weather
Mon, January 28th, 2019

Yesterday:   Mostly cloudy skies and flat light dominated the eastern Turnagain arm region.   Ridgetop winds were light from the east with the occasional gust into the 30’s with no measurable precipitation.   Temperatures were consistent in the low 20’s at ridgetops (Sunburst at 3880′) and mid to high 30’s at sea level (Portage).

Today:  East winds are expected to bump up into the 20-40mph range as a low moves up Cook Inlet.   We may squeeze out 2-5″ of snow under overcast skies with a rain/ snow line around 500′.   Temperatures look to be in the mid-30’s at 1,000′ and mid-20’s at ridgetops.

Tomorrow: Tonight and into tomorrow unsettled weather continues.   Precipitation type and rain/ snow line will be the big question.   Generally though, temperatures look to be cooling throughout the work week as the unsettled pattern continues.   Think #snowtosealevel

*Seattle Ridge weather station was heavily rimed and the anemometer (wind sensor) was destroyed.   We are currently working to replace it.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 32    0 0   58  
Summit Lake (1400′) 28   0    0 20  
Alyeska Mid (1700′) 32   trace   .01   44  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23   ENE   13   44  
Seattle Ridge (2400′) 27   *N/A   *N/A   *N/A  
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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.