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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
Sat, February 29th, 2020 - 7:00AM
Expires
Sun, March 1st, 2020 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line

The avalanche danger remains a scary MODERATE today and may increase to CONSIDERABLE overnight with new snow and wind on the way. Today, the main concern is triggering a large and dangerous slab avalanche that breaks in weak snow 3-6 feet deep. The likelihood is slowly decreasing, but the possibility remains. Other avalanche issues are triggering lingering wind slabs from yesterday’s northwest winds and cornice falls. Once the snow begins to accumulate overnight, new wind slabs and sluffs will be likely pending new snow amounts.

SUMMIT LAKE TO SEWARD REGION: The likelihood for triggering a large slab avalanche is higher due to a weaker snowpack and wind effect yesterday. Extra caution is advised.

A Winter Weather Advisory has been issued for Eastern Turnagain Arm from 6pm this evening to 6am tomorrow. 

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Sat, February 29th, 2020
Alpine
Above 2,500'
Moderate (2)
Avalanche risk
Treeline
1,000'-2,500'
Moderate (2)
Avalanche risk
Below Treeline
Below 1,000'
Moderate (2)
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
Moderate (2)
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

This Leap Day morning, clouds are oozing in ahead of a low-pressure system that will skirt by to the south tonight. Snowfall is slated for this afternoon and should pick up overnight giving us a shot for 4-8+” of new snow by tomorrow morning. Ridgetop winds are on the same schedule, blowing light from the east today until increasing to the strong category after the sun sets. With new weather coming in, it will not only be important to pay attention to this, but also not to forget about the deep slab problem lurking below.

Today’s main concern still centers around the weak faceted snow from January hidden 3-6 feet below our feet. It has been almost 5 days now since the last known deep slab avalanche was triggered. That said, for a ‘persistent’ deep slab problem, that really isn’t that long. The message remains: a person could find just the wrong spot and trigger one of these deadly slabs. Things to keep in mind:

  • The snowpack can feel ‘stable’ and no signs of instability may be present before a deep slab releases.
  • Areas with a shallower snowpack are more concerning for triggering a large slab.
  • Areas with little traffic so far this year are also more likely for triggering a deep slab.
  • It can be the 10th or 15th person on a slope before it avalanches.
  • These slabs can be triggered remotely, from the bottom, top or side.
  • Sticking to the low angle terrain (slopes 30° and less) with nothing steep above us are ways to avoid this hazard.

Great photo from Magnum ridge of the SW face of Sunburst and the large slab avalanche that naturally occurred 9 days ago (Feb 19/20th). Roughly 1.5 to 2 feet of snow has fallen since the avalanche yet the crown line is still quite remarkable. Photo taken 2.28.20 by Billy Finley.

 

More avalanche eye candy from Billy Finley. This shot is of the north face of Cornbiscuit and shows the propagation of a very large slab avalanche that occurred naturally also 9 days ago, Feb 19/20th during the end of the Feb 18-20 storm cycle. 

Avalanche Problem 2
  • 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

The northwest wind yesterday escaped much of Turnagain Pass, but looked to have impacted portions to the south, such as Summit Lake and areas closer to Seward. As always, keep an eye out for areas with wind drifted snow and loaded slopes. Feel for stiff snow over softer snow and hollow feeling snow, all signs of a wind slab. With the January weak layer below, a wind slab or cornice fall has the ability to trigger a deep slab avalanche.

As new snow and wind picks up this evening, new fresh wind slabs will likely greet us tomorrow morning and pending on how much snow falls, these could be 1-2′ thick and quite touchy. Sluffs and shallow slab avalanches composed of the new snow are also likely and will depend on the new snow amounts. A layer of surface hoar sits on the snow surface currently, which can keep the new snow from bonding easily. Something we’ll be paying attention to as the storm progresses.

Cornices:  As always, give these features extra room and limit exposure under them.

Surface hoar coats the top of the snowpack from peaks to valleys at the moment. 

Weather
Sat, February 29th, 2020

Yesterday:  Sunny skies with light to moderate northwesterly winds were over the region. Temperatures were chilly, in the 0-15°F range and have dropped to the minus single digits in low-lying valleys overnight.

Today:  Partly cloudy skies becoming cloudy ahead of a storm system moving in this evening. Snowfall is expected to start in the afternoon and continue into tomorrow. Between 4-8″ of snow is expected with favored areas (such as Portage Valley) seeing closer to 10″ by tomorrow morning. Ridgetop winds will be 10-15mph from the east today before increasing to the 20-25mph range overnight. Temperatures will stay in the teens and rise to the 20’s°F at most locations.

Tomorrow:  Snowfall should slowly tapper off tomorrow as the system moves out. An additional 2-4″ is expected through the day. Ridgetop winds switch more southwesterly and look to remain in the 15-20mph range. Temperatures are forecast to rise close to 32°F at sea level and the 20’s°F along ridgelines.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 15 0 0 76
Summit Lake (1400′) 8 0 0 28
Alyeska Mid (1700′) 13 0 0 82

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 9 SW 7 21
Seattle Ridge (2400′) 10 N 5* 10*

*Seattle Ridge wind speed and direction estimated from past 12 hours of data.

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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
Closed
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Carter Lake
Closed
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Closed
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Closed
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Closed
Snug Harbor
Closed
South Fork Snow River Corridor
Closed
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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.