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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, February 3rd, 2021 - 7:00AM
Expires
Thu, February 4th, 2021 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is MODERATE today above 1000’. Triggering a slab avalanche on a layer of weak snow, buried 1-2’ deep, is possible in steep terrain that has seen prior wind effect. Assess the snowpack as you travel, identify areas of concern and evaluate terrain consequences. In addition, give cornices a wide berth, limit time spent under glide cracks and watch your sluff.

The avalanche danger is LOW below 1000’.

SUMMIT LAKE: This area was more impacted by the recent NW wind event. Observers saw natural wind slab activity on Colorado Peak on Monday, including an avalanche that stepped down to the ground.

LOST LAKE/SNUG/SEWARD: With recent snow, strong winds and buried weak snow, large avalanches may be triggered by a person on skis or snowmachine in these areas as well.

Wed, February 3rd, 2021
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

Am I traveling in terrain that is harboring a slab over a buried weak layer? This is the question of the day. With the few inches of low density snow that fell overnight it will be harder to see evidence of prior wind-loading. While typical wind slabs generally heal soon after a wind event. When the slabs form over a persistent weak layer, like surface hoar or facets, they can become a persistent slab issue. This is unfortunately what we are dealing with now. A weak layer of surface hoar and near surface facets was buried last week by 4-18″ of snow.  The most snow (12-18″) fell in Placer, Portage and Whittier with much smaller amounts (4-6″) at the southern end of Turnagain Pass and to the north in Crow Creek. Girdwood and the northern end of the pass got 8-12″.  The snow was followed by light to moderate winds which formed wind slabs in some terrain and there were a number of human triggered avalanches reported over the weekend. While the weak layer may be found on most slopes in the area, the wind effect was variable. The key factor to determining stability today will be whether the wind formed a slab of stiffer snow above the weak layer or not and how deep is the slab. Ski and track penetration and snow stiffness will help answer these questions. Is the snow loose and sugary or stiff and maybe supportable? The most suspect areas are loaded slopes at higher elevations.  Remember there was loading from a variety of directions as winds shifted from east to west. Avoid steep terrain with stiffer snow and look for signs of instability like cracking, whumpfing. On the flip side, keep in mind that signs of instability may not be present as time passes with a persistent weak layer in the snowpack. However, the potential to trigger an avalanche will still be there. There is lots of fun, soft snow to enjoy in non wind affected terrain.

Loose snow avalanches (sluffs): Sluffs are likely to be high volume and fast running. They could entrain the new snow as well as the facets that sit below it and could have serious consequences if they carry you into terrain traps like cliffs, trees, or rocks.

Cornices: Large cornices are peeling away from ridgelines and cracks are opening. Be sure to give them plenty of space along the ridge and minimize the amount of time you spend below them.

 

Snow pit near the Squirrel Flats uptrack on Sunday, 1.31.21. The buried weak layer was easy to spot but the snow above wasn’t quite stiff enough to be a slab yet. Photo: Graham Predeger. In places with wind effect there is a slab over the buried weak layer. 

A layer (small slab) of wind affected snow failing on the layer of buried near surface facets, Fresno Peak – Summit Lake, 2.2.21.

 

 

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

Glide cracks exist across the forecast area. Remember it is important to limit time spent underneath them. Glide avalanches are totally unpredictable, not triggered by people and are the entire snowpack sliding at the ground. This type of avalanche could be large and unsurvivable if you happened to be in wrong place when one releases. If you see recent glide activity please let us know.

Weather
Wed, February 3rd, 2021

Yesterday: Skies were overcast and temperatures climbed from the single digits to the mid teens as the day progressed. Winds were calm becoming easterly and light overnight. Light snow showers started in the early evening and continued overnight with 1-2″ of accumulation to sea level.

Today: Skies will be mostly cloudy in the morning becoming partly sunny in the afternoon with some valley fog possible.  Temperatures will be in the high teens to mid 20°Fs and winds will be light and westerly. Overnight skies will be partly cloudy with temperatures in the teens to single digits and light west winds continue.

Tomorrow: Mostly sunny skies with temperatures in the teens and mid 20°Fs and light NW winds. The next shot of snow looks to be Friday into Friday night.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 14 1 0.1 123
Summit Lake (1400′) 11 2 0.2 44
Alyeska Mid (1700′) 14 2 0.15 111

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 13 E 8 18
Seattle Ridge (2400′) 11 SE 4 12
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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.