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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sun, March 25th, 2018 - 7:00AM
Expires
Mon, March 26th, 2018 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line

A  MODERATE  avalanche danger remains above 1000′ where triggering a hard slab avalanche 2-4+ feet thick is possible. These could be found on all aspects and may be remotely triggered. Watch for old wind slabs along ridgelines and avoid cornices. Evaluate snow and terrain carefully.  

Below 1000′ avalanche danger is  LOW  where a stout surface crust has formed.  

Check out the most recent  Summit snowpack and avalanche summary  if you are headed South of Turnagain Pass.

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Sun, March 25th, 2018
Alpine
Above 2,500'
Moderate (2)
Avalanche risk
Treeline
1,000'-2,500'
Moderate (2)
Avalanche risk
Below Treeline
Below 1,000'
Low (1)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
Moderate (2)
Treeline
1,000'-2,500'
Avalanche risk
Moderate (2)
Below Treeline
Below 1,000'
Avalanche risk
Low (1)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Avalanche Problem 1
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    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 2-4+ feet thick remains a possibility. It has been over two weeks since significant snow fell and almost a week since a large scale Northerly wind event loaded slopes and increased avalanche hazard. Since then, generally quiet weather has occurred yet people are still able to trigger avalanches. Many of these have been remotely triggered from the side or below, such as on Friday when a skier remotely triggered a large avalanche while descending a low angle slope on Raggedtop Mountain in Girdwood Valley.

The problem is old buried weak layers from January. There are facets sitting on a slick melt/freeze crust at the mid elevations and facets mixed with buried surface hoar at the upper elevations. To add to this, strong Northwest winds that ended Thursday caused unusual loading patterns opposite the usual Easterly direction. This means the typical windward slopes with thinner weaker snow may be more loaded and ‘trigger spots’ may be lurking just below the surface in unexpected places. Observers over the last few weeks have found poor structure along scoured ridges and under sastrugi.

The tough thing is, our hard-pack snow surface appears safe and stable but this is not the case. Keep in mind that no obvious clues may be present until the slope releases. It may be the 10th skier or snowmachiner onto a slope that finds a thin part of the snowpack (a trigger point). Triggering a slab remotely from the side or below is possible. Take a moment to visualize the consequences if the slope does slide. 

  

Remotely triggered avalanche by a skier two days ago in the Girdwood Valley. This was on an Southeasterly aspect around 3,000′ on a lower shoulder of Raggedtop Mtn.

 

Two snow pits dug on Taylor Pass show one of the weak layers we are concerned about along with the difference in slab height in a short distance.

Additional Concern
  • Wind Slabs
    Wind Slabs
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.
More info at Avalanche.org

Wind Slabs: Easterly winds will be moderate to strong today, yet little snow is available for transport to form fresh wind slabs. However, this is something to be on the lookout for if you head into the hills. Additionally, old hard wind slabs may be lurking on a variety of aspects due to the unusual loading patterns this week. Smooth supportable surfaces where the snow is hollow sounding are suspect, especially if the slope is unsupported in steep rocky terrain. 

Cornices: Cornices are large in places and the sun and above freezing temperatures can make them more unstable. Give cornices plenty of space and limit exposure underneath them.

Taylor Pass on the left photo and Pastoral Glacier on the right. Winds last week stripped the majority of the soft surface snow away. You can see evidence of an old avalanche on the Pastoral Glacier.

 

Weather
Sun, March 25th, 2018

Sunny skies were over the region yesterday with ridgetop winds light from the West. Overnight, winds shifted to the East and have picked up to the 10-20mph range. Temperatures warmed to the mid 20’sF along ridgetops yesterday and in the mid 30’s at 1,000′ before dropping to the teens and single digits overnight.  

Today, clear skies are expected this morning before clouds begin to stream in associated with a low-pressure spinning in the Bering. Light snowfall is expected this afternoon through tonight bringing just a trace of snow to our forecast area. Ridgetop winds will continue to increase from the East through the day and should range between 20-30mph. Temperatures look to stay cool, in the upper teens along ridgelines and near 30F at 1,000′.

Tomorrow, Monday, we can expect light showers to continue that may dust the old hard surfaces with a inch or two of new snow. The middle of the work week looks a bit uncertain, but warmer temperatures and a chance for snow flurries is expected.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 25   0   0   79  
Summit Lake (1400′) 20   0   0   32
Alyeska Mid (1700′) 26   0   0   74  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 22   E   8   23  
Seattle Ridge (2400′) 24   ESE   7   19  
Observations
Recent Observations for Turnagain Pass
Date Region Location
05/06/20 Turnagain Avalanche: Pastoral Peak, north face
04/10/20 Turnagain Avalanche: Wolverine
04/10/20 Turnagain Observation: Eddies lookers right shoulder
04/09/20 Turnagain Observation: Bench Peak
04/04/20 Turnagain Observation: Tincan
04/04/20 Turnagain Observation: Pete’s North
03/26/20 Turnagain Avalanche: Tincan – Proper (SW facing)
03/26/20 Turnagain Avalanche: Seattle Ridge
03/25/20 Turnagain Avalanche: Sunburst Uptrack @ 2000′
03/24/20 Turnagain Observation: Turnagain – Road Observations
Riding Areas
Updated Fri, May 01st, 2020

Status of riding areas across the Chugach NF is managed by the Glacier and Seward Ranger Districts, not avalanche center staff. Riding area information is posted as a public service to our users and updated based on snow depth and snow density to prevent resource damage at trailhead locations. Riding area questions contact: mailroom_r10_chugach@fs.fed.us

Area Status Weather & Riding Conditions
Glacier District
Johnson Pass
Closed
Placer River
Closed
Skookum Drainage
Closed
Turnagain Pass
Closed
Closed as of May 1. Thanks for a fun, safe season!
Twentymile
Closed
Seward District
Carter Lake
Closed
Lost Lake Trail
Closed
Primrose Trail
Closed
Resurrection Pass Trail
Closed
Snug Harbor
Closed
South Fork Snow River Corridor
Closed
Summit Lake
Closed

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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.