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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, January 25th, 2018 - 7:00AM
Expires
Fri, January 26th, 2018 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is  MODERATE  on all aspects above treeline. Fresh wind slab avalanches, up to a foot thick, could be found and triggered on leeward slopes. In addition, triggering a very large slab avalanche between 3 and 8 feet thick is still possible above 3,000′. Loose snow avalanches (sluffs) should be expected on steep slopes and be aware of cornice falls along ridgelines. There is a  LOW  danger below 2,500′ where triggering a slab avalanche is unlikely but sluffs are possible.

Special Announcements

 

Thu, January 25th, 2018
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
1 - Low
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
  • 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 Northerly winds that were forecast to hit the area yesterday only bumped up slightly along most ridgelines. Stations reported averages from the North and West in the 10-15mph before settling down last night. This is good news for limited wind slab development. However, areas like Portage Valley, Seattle Ridge and Crow Pass are suspect to have seen stronger winds and subsequent new wind slabs. We had a report of two small slab avalanches (believed to be new yesterday and likely wind slabs) that released on Maynard Mountain in Portage Valley.

With plenty of loose surface snow to blow around, be on the lookout for wind slabs. Textured surface snow, rounded and pillow-like surfaces and feeling for stiffer snow over softer snow are all signs the winds have visited. Cracking in the snow around you will be a sure sign you have found a wind slab. Any fresh slabs are expected to be shallow, a foot or less thick, and could be touchy. These are most concerning if they knock you somewhere you don’t want to go, like over cliffs.

Avalanche Problem 2
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.

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

Unfortunately, the snowpack structure above 3000′ is poor. Specifically, there is a hard slab (3-8 feet thick) sitting on a variety of weak layers in the mid pack (including buried surface hoar) and old November facets near the ground. Deep persistent slab avalanches remain a concern on these upper elevation slopes. Triggering a deep slab is becoming difficult, but is still possible. The most likely trigger spots are thin areas in the snow cover, often near rocks, or where the slope rolls over. South of Turnagain Pass the snow cover is thinner, triggering a slab in this area could be more likely as well. Remember, this is a ‘low probability, high consequence’ situation and no Red Flags are likely to been seen (such as whumpfing and cracking). Remote triggering is possible and there may already be tracks on the slope. This issue can simply be avoided by sticking to terrain below 3000’ (which is a good portion of terrain at Turnagain) or choosing low-consequence terrain in the Alpine.

Snowpack structure at 3,600′ on North facing Tincan Ridge below. We found the poor structure in our pit, meaning the weak layers are there, but they were either hard to get to fail or did not want to propagate. This is a good sign, but as long as these weak layers are present, the chance a person can trigger a deadly deep slab remains despite snow pit results…

 

Additional Concern
  • Dry Loose
    Dry Loose
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.
More info at Avalanche.org

Watch your sluff. Cold temperatures are ramping up the ‘faceting process‘ in the top foot of the snowpack and sluffs are getting bigger and faster day after day. *Once we do get a new load of snow on this very weak snow, you can imagine just how hard that new snow will be to stick (today’s good riding is tomorrow’s avalanche producer…).

These skier/boarder initiated sluffs on Tincan Proper ran to valley floor in some cases with small powder clouds.


Weather
Thu, January 25th, 2018

Very cold and mostly clear skies were had yesterday. Ridgetop winds were variable blowing on Seattle Ridge into the teens from the North and only light from the West on Sunburst. Temperatures were in the single digits to the minus single digits at all elevations. No precipitation has been recorded for two days.  

Today, another cold and partly sunny to cloudy day is on tap. A chance for a few snow flurries is possible with up to an inch of accumulation by this evening. Temperatures could climb from the negative single digits, where they sit this morning, to the teens at the lower elevations – but the upper slopes will remain in the -10 – 0F range. Ridgetop winds are forecast to be light from the North and West in the 5-15mph range.

This evening and into Friday, Easterly winds will pick up and bring warmer temperatures (teens) and a chance for 2-4″ of snowfall.  

* THANK YOU to the snowmachiners that cleared off Seattle Ridge weather station yesterday!!  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 7   0   0   57  
Summit Lake (1400′) 3   0   0   15  
Alyeska Mid (1700′) 5   0   0   50  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) -4   W   4    10
Seattle Ridge (2400′) -2   NNE   8     18  
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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.