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Issued
Wed, December 15th, 2021 - 7:00AM
Expires
Thu, December 16th, 2021 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger remains MODERATE for all aspects and elevations. Triggering a large slab avalanche, breaking in buried weak layers, is still possible. A slab like this could be 3-5′ feet deep and be triggered remotely (from below, the side, or the top of a slope). Additionally, triggering a lingering wind slab is also possible in areas that saw strong winds on Sunday. Pay close attention to any signs of instability, know your terrain and safe spots, and watch your partners.

SUMMIT LAKE: Strong NW winds on Sunday have likely formed stiff wind slabs that sit on a shallow and unstable snowpack. Human triggered slab avalanches are possible and these could break in buried weak layers 1-2′ deep.

SNUG HARBOR/LOST LAKE: Strong NW winds also hit the central and southern Kenai on Sunday. Old hard wind slabs sitting on an already unstable snowpack can be expected. A cautious mindset is recommend as large slab avalanches could be triggered by people.

Special Announcements
  • AK DOT&PF:  There will be intermittent traffic delays today, December 15, for Avalanche Hazard Reduction Work on the Seward Highway near mileposts 38 to 36 at the Wye Junction. Motorists should expect delays of up to 45 minutes between 10:00 am and 12:00 Noon. Updates will be posted on at 511.alaska.gov.
Wed, December 15th, 2021
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
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. 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
  • 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

Triggering a deep slab avalanche, a deep persistent slab avalanche to be correct, is our main concern.

With a couple days of quiet weather behind us and a mostly sunny day on tap, the snowpack is finally getting a chance to adjust to the loading from the past few weeks. Unfortunately for us, we have those buried weak layers to deal with and they don’t heal up in a few days. In fact, it can take weeks or more. These are the same buried weak layers of facets and buried surface hoar that have been responsible for almost all the avalanche activity over the last month, including the several close calls on Dec 2nd and 3rd when two skiers were caught and one snowmachiner was buried. This last storm (12-26″, 6 days ago) also overloaded these layers, producing large natural avalanches.

How reactive are the layers now? There has not been a lot of traffic to test them, but avalanche science says, with time and being buried over 3′ deep, they should be more difficult to trigger. This is the scary MODERATE scenario… slabs are becoming harder to trigger, but can be large and have high consequences.

Here are the main characteristics of a deep persistent slab:

  • Difficult to trigger – stubborn
  • Capable of producing very large avalanches that could propagate across terrain features
  • No signs of instability before a slope releases
  • Can be triggered after many people have been on a slope
  • Can be triggered remotely (from the side, top of below)
  • The slab itself is more than 3′ deep and is composed of hard snow

The weak layers are roughly 3-5′ deep in Girdwood and Turnagain Pass, but are much shallower to the south of the pass and in Summit Lake (south of the forecast area). They are most concerning in the 1,500 – 3,000′ elevation band where the Halloween crust sits under them and can exacerbate the issue. Hence, open steep slopes in the trees can be just as dangerous, or more so. It’s a tricky situation and many of us are simply avoiding the issue all together by sticking to slopes 30 degrees or less.

If you are considering stepping into larger/steeper terrain, be mindful of this potential. It’s also good to know slopes that have seen little to no traffic this season are more suspect. Watching your partners closely, only exposing one person at a time, and having an escape route planned are all ways to help hedge your bets. Also, thinking about the consequences if the slope does slide; will the debris pile up deeply in a terrain trap (bad) or spread out (also bad, but better chance for rescue)?

Schematic of a deep persistent slab. (avalanche.org)

Wind Slabs:  Lingering wind slabs from Sunday’s winds are likely to be found in exposed terrain and especially in areas on the periphery of the forecast zone – south end Settle Ridge, Summit Lake, etc. Watch for the classic signs of wind loading, cracking in the snow around you and stiff snow over softer snow. Wind slabs could trigger a deeper slab, creating a much more dangerous situation.

Photo from Eddie’s looking toward Kickstep by Andrew Schauer. Andrew’s comment: “Daydreaming of better stability. Getting big ‘You can look but you can’t touch’ vibes right now.” 12.13.2021

Weather
Wed, December 15th, 2021

Yesterday:  Mostly sunny skies were over the region with light north to westerly winds. Temperatures remained in the minus single digits with some areas climbing their way into the positive single digits.

Today:  Another sunny day is on tap with what looks like somewhat warmer temperatures by the afternoon. However, an inversion is in place this morning with valley bottoms in the -20 to -5 range and ridgetops around 5-10F. Ridgetop winds will remain from the NW where they are forecast to be 5-10mph with some areas seeing 15mph.

Tomorrow:  A weak weather system is headed in for tomorrow with increasing easterly winds late tonight and snowfall beginning Wednesday morning. At this point models are only suggesting 1-3″ of snow with this event. The east winds with the event should be in the 20-30mph range. Temperatures are expected to climb back to a respectable 10-20F range.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) -1 0 0 73
Summit Lake (1400′) -12 0 0 23
Alyeska Mid (1700′) 1 0 0 46

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 5 W 7 18
Seattle Ridge (2400′) 7 N 3 6
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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.