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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Thu, March 8th, 2018 - 7:00AM
Expires
Fri, March 9th, 2018 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line

The avalanche danger will rise to CONSIDERABLE today and to HIGH tonight as a potent storm moves into Western Prince William Sound. Heavy snowfall combined with strong wind will create dangerous avalanche conditions by this evening. Natural and human triggered avalanches will become likely. Elevated caution is advised today as snow begins to accumulate and travel in avalanche terrain is NOT recommended tonight.  

*The National Weather Service has issued a SPECIAL WEATHER STATEMENT*

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Thu, March 8th, 2018
Alpine
Above 2,500'
High (4)
Avalanche risk
Treeline
1,000'-2,500'
High (4)
Avalanche risk
Below Treeline
Below 1,000'
Considerable (3)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
High (4)
Treeline
1,000'-2,500'
Avalanche risk
High (4)
Below Treeline
Below 1,000'
Avalanche risk
Considerable (3)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Avalanche Problem 1
  • Storm Slabs
    Storm Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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

A strong storm beginning this morning will bring up to a foot of new snow today and an additional 1-2 feet tonight. This will rapidly load slopes and if the storm verifies, large natural avalanches will be likely along with likely human triggered slides. These avalanches will be large and could run far into runout zones. Areas closer to the coast, such as Portage and Placer valleys will see the majority of the snow, but heavy snow is also expected in Girdwood Valley, Turnagain Pass, Moose Pass and Seward. The Summit Lake area is likely to see lesser amounts (16-20″ storm total compared to 3+’ of storm total for the other zones). 

Storm snow avalanches will be the primary concern as the day progresses. Avalanche danger is directly related to how much new snow has fallen combined with increasing winds loading slopes. The new snow will fall on a few inches of light powder that sits over hard wind affected surfaces. This set-up creates a slick surface for the new snow and bonding is expected to be poor. It is a day to be mindful of changing conditions and leaving the mountains before sun down. 

Types of storm snow avalanches that are expected to be occurring naturally tonight:

–  Wind slabs:  1-3 feet of new snow combined with strong Easterly winds = wind slabs up to 4-6′ thick 
–  Storm slabs:  Soft slabs are expected in areas out of the wind due to rapid loading and poor bonding with the old surface
–  Loose snow sluffs:  Sluffing in the new snow on steep slopes is expected
–  Cornice falls:  Fresh cornices are expected to build and break off during the storm, often this will trigger a wind slab or storm slab below
 

Forecaster excercise:  Imagine 2-3+ feet of new snow by tomorrow morning, with wind, falling on the slopes pictured below. We are powder starved and in need of snow, but sticking well out of avalanche terrain (including runout areas) as the storm sets in will be key.

Seattle Ridge, with the up-track on right of photo

 

Southwest face of Sunburst

 

Magnum Ridge 


Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
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

New snow and wind loading could overload old weak layers buried in the existing snowpack. In the case this occurs, or a storm related avalanche ‘steps down’ into a deeper weak layer, avalanches could be very large and send debris well into valley bottoms. Another reason to let the mountains sit during the storm. Areas on the South end of the Pass and the Summit Lake area are most suspect for having deeper weak layers release.

Weak faceted snow on a crust sits about a foot below the surface in the mid-elevations in the Summit Lake region. An example of weak layers in the pack that could become overloaded and release during the storm

Weather
Thu, March 8th, 2018

Partly sunny skies with valley fog was over the region yesterday. Ridgetop winds were light from the North and temperatures were in the teens along ridgelines and in the mid 20’sF at 1,000′. Overnight, winds shifted to the East and are increasing along with temperatures as a small, but strong, low pressure develops South of Seward.  

Today, Thursday, a quick and potent storm will move in with heavy snowfall and strong wind. Snow has just begun this morning and by 6pm this evening should bring 8-14″ (.5-.7″ water). From 6pm to 6am tomorrow, another 12-24″ (1-1.5″ water) is expected. Favored areas will be Placer and Portage Valleys with Summit Lake likely seeing the least in these ranges. Ridgetop winds will be Easterly in the 30-50mph with stronger gusts. Temperatures should remain close to 30F at sea level (#snowtosealevel) and in the low 20’sF along ridgetops.

Quote from the NWS’s Special Weather Statement:

Depending on the track the low center takes, the road system from
Seward to Portage could see snow accumulations anywhere from one
to three feet through Friday evening. Heaviest snowfall is
expected to be from Moose Pass southward.

Friday, this system will slowly exit and light snow showers are expected. Ridgetop winds are forecast to remain strong from the East and South however. Periods of light snowfall should continue through the weekend as a Southerly flow keeps pumping moisture into our area.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 22   trace   0.05   67  
Summit Lake (1400′) 20   0   0   29  
Alyeska Mid (1700′) 23   trace   0.05   59  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 15   NE   5    25
Seattle Ridge (2400′) 20   SE 5     21  
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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

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
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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.