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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, November 30th, 2021 - 7:00AM
Expires
Wed, December 1st, 2021 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger remains CONSIDERABLE at elevations above 1,000′ due to continued stormy weather. Roughly 8 – 12″ of new snow has fallen with strong east winds, and more is on the way. Natural wind slabs and cornice falls are possible and these are likely to be triggered by people. These avalanches, or people alone, could also trigger a larger slab that breaks in weak snow 2-3 feet deep, creating a larger avalanche. Cautious route-finding and conservative decision-making is key for travel in the backcountry.

On slopes below 1,000′, the danger is MODERATE, where small avalanches could be triggered in the new snow and a larger avalanche from above could send debris into low elevations.

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Tue, November 30th, 2021
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
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. 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
  • Wind Slabs
    Wind Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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

The weather front that was only supposed to blow for one day (yesterday) and drop less than a foot of total snow, has been upgraded. The system is peaking this morning and forecasted snowfall totals have been increased to 12 – 18″ by tomorrow. Things are looking on track to hit that 18″ mark and snowfall as of 6am is in the 8-10″ range for both Girdwood and Turnagain Pass. Portage has around a foot and sadly, Summit Lake is being left out again with only what looks to be a trace.

The snow has been accompanied by strong east winds, beginning 30 hours ago. We can bet that wind slabs and cornices have been forming during that time and will continue to do so today as the wind should remain near 15-25mph with stronger gusts. To add to increased instability, check out the rapidly rising temperatures at Sunburst – over the last 30 hours temperatures have climbed their way from -4F to 20F… Mid elevations are seeing conditions as warm as 28F. That’s almost tropical.

Wind Slab avalanches and Cornice Falls are the main concerns today. These have likely been releasing naturally along ridgelines and it should be easy for us to trigger them if we get in the wrong spot. Remember that 2′ of fluffy powder from last weekend? The wind probably didn’t have much trouble blowing it around and this will only add to the size of new slabs. Furthermore, warming temperatures can make the snow ‘stickier’, aiding in wind slab and cornice development. These slabs are likely to be fairly soft and in the 1-3′ foot range pending the amount of sustained wind. They may also have the ability to overload those buried weak layers, which is a real concern discussed in Problem 2.

Storm slabs could also be forming in areas seeing 10″ or more of new snow. The storm is coming in ‘upside down’, meaning warmer snow over colder weak snow. This can create a slab/weak layer combo in the new snow. Storm slabs will only be as deep as the new snow, and should vary with location and elevation.

Similar to yesterday, if heading out be sure to:

  • Watch for, and avoid, slopes that have been, or are being, wind loaded
    • New wind slabs are likely to avalanche easily
    • They could step down to buried weak layers, creating a larger avalanche
  • Watch for stiffer snow over softer snow, cracking in the snow around you, or collapsing (whumpfing)
  • Sticking to lower slope angles below treeline (below ~2,500′) and out of the wind will drastically reduce the chance for triggering an avalanche.

 

Graphics courtesy of the National Weather Service Anchorage Forecast Office.

 

Sunburst weather station (3,812′ elevation) at 6am, Nov 30th.

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

Triggering an avalanche breaking in weak snow, now hidden 2-3′ below the surface, is also a concern. This would be a larger avalanche than your typical wind slab and likely more dangerous. Whether or not this new load of snow, especially wind loaded slopes, will be enough to trigger these weak layers naturally, is a question. Add a person to the equation and the likelihood increases.

These are those layers of buried surface hoar and facets that formed in early to mid November and were causing grief last week when they were buried by 2-3′ of light snow. We know from last Saturday, when there were several remotely triggered slabs that occurred on Seattle Ridge and one remote triggered slab on Max’s in Girdwood Valley, that they are reactive in places.

If you are headed to those treed areas out of the wind, we still need to keep our guard up and look for any signs the loose snow is ‘slabbing up’. Warming temperatures alone could do this in areas that had no instability before. Both cracking and whumpfing, along with feeling the new and older snow with your boot or ski pole, to see if it has become stiffer or more cohesive will be good ways to assess the zone you are in.

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

Let us know if you’ve seen any glide avalanches lately. The last known glide crack that released was on Lipps SW face, sometime in the past week. These are clearly slowing down, but still, if you see a crack it’s never a good idea to hang out under it. They are unpredictable as to when they avalanche.

Weather
Tue, November 30th, 2021

Yesterday:  Cloudy skies with light snowfall in most areas was seen. Many sensors are offline or rimed, but it appears 8-10″ has fallen in Girdwood and Turnagain Pass, with more in Portage. Winds have been strong from the east for just over 24 hours, blowing along ridgetops in the 20-30mph range with gusts near 50mph. Temperatures have climbed to near 20F along ridgelines and in the mid to upper 20’sF at mid elevations.

Today:  Moderate snowfall through the day should add another 4-6″ with and additional 2-3″ overnight (snow to sea level). Ridgetop winds look to stay elevated in the 15-25mph range with gusts to 40. Temperatures should be leveling off in the 20’s today before falling back to the teens tonight.

Tomorrow:  This storm system looks to start moving out tomorrow with tapering snowfall, lighter east winds and cooling temperatures. Skies may start to clear somewhat, but that’s more likely to happen on Thursday and the end of the week when a NW flow sets up with cooling temperatures.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 23 8-10* 0.7 68-70*
Summit Lake (1400′) 18 0 0 11
Alyeska Mid (1700′) 22 8-10* 0.7 N/A*

*Snowfall and snow depth for Center Ridge are estimated due to the sensor being rimed. Alyeska snow depth sensor is being maintained.

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 15 NE 24 45
Seattle Ridge (2400′) 18 SE 10 19
Observations
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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.