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Turnagain Pass Avalanche Forecast RSS

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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Wed, January 24th, 2018 - 7:00AM
Expires
Thu, January 25th, 2018 - 7:00AM
Forecaster
Aleph Johnston-Bloom
The Bottom Line

The avalanche danger is MODERATE  on all aspects above 1,000′. Triggering a wind slab in leeward terrain is possible today. Watch for changing conditions. 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 protected from the wind and be aware of cornice falls along ridgelines. There is a  LOW  danger below 1,000′ where triggering a slab avalanche is unlikely but sluffs are possible.

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Wed, January 24th, 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
  • Wind Slabs
    Wind Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    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

NW winds are forecasted to pick up today, blowing 15-25 mph with gusts into the 30s. There is light, soft snow available for transport. Watch for wind slabs building on leeward slopes. Avoid steep slopes that looked pillowed and pay attention to changing conditions. Wind flow from this direction can be stronger over Seattle Ridge. It also can be channeled through Turnagain Pass from the South and winds near Taylor Pass can be much stronger. Look for flagging (snow blowing off of ridges), surface snow getting stiffer, cracking and drifting. An observer yesterday triggered a small pocket of wind slab in steep, rocky terrain. There is more snow in Girdwood so expect slabs to be larger. Keep in mind; if winds are stronger than forecasted, natural avalanches may occur. 

Cornices grew during the storms last week. As always, give these features a wide berth and remember they can break further back than expected. A cornice fall at the high elevations could trigger a large avalanche on the slope below.

Snow available for transport today. 

Avalanche Problem 2
  • 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

We know the snowpack structure above 3000′ is poor, there is a dense 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 in the advisory area. Triggering a deep slab is becoming more and more 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. High peaks, that see wind, can also be thinner and it is more likely to find the trigger point for a deep slab where some of the slab has been stripped away. 

Key points that keep in mind about our current snowpack:

  • We have a ‘low probability, high consequence’ situation at the upper elevations for deep slab avalanches. If you do trigger an avalanche it could be huge and unsurvivable.
  • Obvious signs of instability are not likely to been seen before a deep slab is triggered (such as whumpfing and cracking) 
  • Remote triggering is possible
  • 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 above 3000′ on Sunburst.

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

On slopes that are protected from the wind expect loose snow avalanches (sluffs) in steep terrain. The additional snow we picked up over the past couple days, 6-10″ in Girdwood and 3-5″ at Turnagain landed on already loose surface snow. The surface snow has been quick to sluff on steep slopes and the new snow just added to the volume of the sluffs. As the sluffs grow larger they could definitely catch a person by surprise, especially on steep, committing terrain.  Observers are reporting that the sluffs are running long distances and are large enough to have a powder cloud. Winds blowing today could initiate these off of the top of ridgelines.

Weather
Wed, January 24th, 2018

Yesterday was mostly to partly cloudy with a thick fog layer and very light snow showers. Winds were light and shifted from east to west.  Temperatures were in the teens. Overnight temperatures dropped into the single digits. Winds remained light.  

Today skies will be partly cloudy to mostly clear and sunny. Temperatures are forecasted to remain in the single digits and may drop below 0F at upper elevations. Winds will be from the NW 15-25 mph gusting into the 30s. Tonight temperatures drop into the negative with the lows ranging from -6F to -19F. Winds continue from the NW 15-25 mph.  

The cold temperatures continue tomorrow with a chance of snow and NW winds. Temperatures look to warm up into the teens on Friday as a low moves into the Gulf and brings snow showers. However, the overall pattern looks to be mostly cold and dry for the weekend into next week.  

*Seattle Ridge anemometer is rimed over and not able to collect wind data.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′)  14  trace 0  56
Summit Lake (1400′)  13 0   0    15
Alyeska Mid (1700′)  13  1.3 0.07    49

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  11  E-W 4   10  
Seattle Ridge (2400′)  10 *n/a    *n/a *n/a  
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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

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Closed as of May 1. Thanks for a fun, safe season!
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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.