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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, February 2nd, 2016 - 7:00AM
Expires
Wed, February 3rd, 2016 - 7:00AM
Forecaster
Graham Predeger
Avalanche risk The Bottom Line

Even though our snowpack is mostly stable in the Turnagain pass region, there are a few outliers that are keeping the danger at MODERATE today as both cornices and glide avalanches continue to act in unpredictable manners.   Fast moving, high volume sluffs and the potential for shallow, fresh wind slabs will also add to today’s concerns.

South of Turnagian Pass harbors a different snowpack structure where we found buried surface hoar responsible for a very large avalanche on Saturday in the Groundhog Creek drainage (Johnson Pass area).   If travelling in these areas recognize that we’ve got a persistent weak layer problem in addition to those listed below.

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Tue, February 2nd, 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
  • Cornice
    Cornice
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Cornice
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
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 was impressive to stand on top of a ridge yesterday and look around at literally thousands of tracks littering the Turnagain pass area, and to know that is only a fraction of what was skied, hill-climbed and boot-packed last weekend.  Observations yesterday are pointing toward generally stable snow in the heart of the core area, with a couple of exceptions. 

Cornices are growing larger by the day and we have seen several fail naturally over the last few days. This is very indicative that many of these are reaching their tipping point and can fail naturally or be sped along by a skier or snowmachiners added weight.  With any luck these will fail mid-storm or overnight but often times it is a rise in temperature or direct sunlight that weakens the bonds and promotes failure.  Simply avoid time spent below cornices and when travelling along a corniced ridge, stay back much farther than you think necessary to maintain that added margin of safety. 

A cornice failure may cause a slab to pull out on the slope below, even in a relatively stable snowpack.  This appeared to be the case on Lipps, last Saturday.  

Large cornices hang over the SW Face of Magnum.  The horizontal track traversing below the cornice is an example of a very poor route decision.  The much safer option would have been to travel along the shaded ridge, well back from the edge.  photo credit: Josh Varney.

Avalanche Problem 2
  • Dry Loose
    Dry Loose
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Dry Loose
Dry Loose avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.
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

In sheltered terrain and mid-slope, where winds have not effected the surface, the snow continues to loosen and facet out.  Sluffs will be fast and have the potential to reach high volume in the alpine.  Don’t get caught off guard by this in steep, alpine terrain.  Getting taken out by your sluff can have very serious consequences in some of the complex and challenging terrain that we’ve seen people travelling to over the last several days.

WIND SLABS:

With 6-8” of loose, unconsolidated snow available for transport above ~2500’, shallow wind slabs could be quick to build and prove tender today.  Keep an eye out for active pluming off ridges as a sure sign of wind slab formation.  Expect any wind slabs forming today to be shallow (6-12”) but tender.

A few wind plumes yesterday were seen transporting the loose, surface snow into shallow wind slabs on Sunburst.

Additional Concern
  • Glide Avalanches
    Glide Avalanches
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.
More info at Avalanche.org

The more you look around, the more glide cracks you see in Turnagain pass and surrounding areas.  Glide cracks continue to release here and there, without any real discernable pattern.  Best practice continues to be to limit your time spent exposed to glide cracks.  This takes some doing right now, considering the minefield of glide cracks that Turnagain pass currently is.

This glide crack on Cornbiscuit partially released sometime between Saturday afternoon and Sunday afternoon.

Weather
Tue, February 2nd, 2016

Yesterday proved to be quite a pleasant day in the backcountry with partly cloudy skies, temps in the low 20’s at ridgetops and winds in the low teens (mph) from the East.  

As fond memories of last weekend’s sunshine and exceptional snow fade into this workweek, we’ll usher in another period of unsettled weather beginning today and ramping up a bit tomorrow.   We can expect a few more clouds today along with an inch or two of snow above about 800′.   Temps are cool this morning but will be warming through the day and may reach the mid-30’s at 1,000′ by this afternoon.   Ridgetop winds are expected to be in the 15-30mph range from the East.

Tomorrow another front will begin to impact our area with potentially a bit stronger winds and more precipitation than what we see today.    

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′)  26  0  0 92  
Summit Lake (1400′)  24  0 0    27
Alyeska Mid (1700′)  28  0 0    70

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 19    ENE 11   34  
Seattle Ridge (2400′)  22  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.
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