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Mon, February 19th, 2018 - 7:00AM
Tue, February 20th, 2018 - 7:00AM
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger remains  MODERATE  above 1,000′ on all aspects. Human triggered slab avalanches 1-2′ thick, and potentially larger, remain possible. These are either shallow old wind slabs or larger slabs that break in weak layers deeper in the snowpack. Sunshine may warm southerly slopes today and could contribute to instability on these aspects.

The Summit Lake area has seen more avalanche activity over the past week. Take a look at the Summit Summary  HERE.  

Special Announcements

Hatcher Pass users:  
Dangerous avalanche conditions exist in this area. Several close calls have occurred over the past few days. Please see the  Hatcher Pass Avalanche Center forecast  before heading that way.  

Mon, February 19th, 2018
Above 2,500'
2 - Moderate
Avalanche risk
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
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
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

Persistent Slabs:  Despite the mostly quiet weather and lack of people triggering avalanches over the past week, triggering a slab avalanche up to 2′ or more remains our main concern for President’s Day. These slabs are becoming more difficult to trigger with time, but a large and unmanageable slide is still possible. Between 1 and 2′ below the surface is the Jan 21 layer of buried surface hoar with weak facets directly below. This layer continues to show signs of reactivity. Red flags may not be present before a slope releases and snow pits can be misleading. Simply the knowledge there are weak layers below your feet or snowmachine should not be forgotten. Add sunshine and calm winds in the forecast today, and that ‘likelihood of triggering’ could increase slightly – both from increased traffic (triggers) and from sunshine affecting Southerly facing slabs. The most likely place to find these are slopes that have not seen significant traffic this season. 

Sunshine:  It’s that time of year when we need to pay attention to the sun. On calm days the sun can heat up Southerly aspects enough to melt surface snow. This heating can also cause a slab sitting on a weak layer to become more reactive. Keep this in mind if you are enjoying Southerly aspects later in the day. 

Wind Slabs:  Watch for old wind slabs that could pop out from you on steep slopes. Steep rocky terrain where the slab is not supported from below are the most suspect. In general, most wind slabs are fairly well locked into place.

Cornices:  Don’t forget about giving cornices wide berth at all times. Also, sunshine can be a big factor in de-stabilizing these during the afternoon/evening hours.

Old lingering wind slabs and cornices may become easier to trigger/fail with daytime warming and direct sun. Image from Warm-up Bowl on Seattle Ridge, Saturday, 2/17.


Additional Concern
  • Deep Persistent Slabs
    Deep Persistent Slabs
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.
More info at Avalanche.org

As you plan your day, keep in mind that there are deeper persistent layers that could ‘wake up’ if you find the right spot above 3,000′ in the Alpine. At these high elevations, old weak layers of facets and buried surface hoar sit in the bottom half of the snowpack. This structure is most pronounced in places with a thin overall snow cover, such as the South end of Turnagain Pass and the Summit Lake area. 

Mon, February 19th, 2018

Overcast skies with light snow flurries were over the region yesterday. Most areas only picked up a trace of new snow, but a couple inches may be found at upper elevations. Ridgetop winds were generally light with moderate gusts from a Southerly direction (5-15mph) during the day and have decreased to light and variable overnight. Temperatures dropped from 30F to the upper teens along ridgetops late in the day yesterday. Valley bottoms have remained warm, in the 20’sF, with cloud cover limiting a nocturnal inversion.

For today, President’s Day, skies should clear this morning and a mostly sunny day is expected. Ridgetop winds look to remain light from the Northwest. Temperatures should warm to the upper 20’sF along ridgetops with daytime heating and valley bottoms to around 30F.  

Tomorrow, and through the mid-week, partly cloudy skies are forecast with the possibility for Westerly winds to increase on Wednesday before a chance for snow on Thursday. Stay tuned.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 30   trace   0   63  
Summit Lake (1400′) 26   trace   0   25  
Alyeska Mid (1700′) 27   trace   0.01   56  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 22   variable   5   One gust to 37  
Seattle Ridge (2400′) 25   ESE   12   27  
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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.