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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
Sun, February 23rd, 2020 - 7:00AM
Expires
Mon, February 24th, 2020 - 7:00AM
Forecaster
Ryan Van Luit
The Bottom Line

We have issued a Special Avalanche Bulletin through the National Weather Service for the Turnagain Pass area and surrounding mountains.

Today the avalanche danger is CONSIDERABLE at all elevations from Girdwood, though Turnagain Pass to Seward. Steady northwest winds throughout the day are expected to create wind slabs that could release naturally or likely triggered by a person. It remains possible for a human to trigger a large and deadly slab avalanche 3-6′ thick on slopes 30 degrees or steeper. Conservative decision making and route-finding continues to be essential if headed into the backcountry.

SUMMIT LAKE TO SEWARD REGION: Expect the avalanche danger to remain elevated due to strong winds and recent precipitation.

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Sun, February 23rd, 2020
Alpine
Above 2,500'
Considerable (3)
Avalanche risk
Treeline
1,000'-2,500'
Considerable (3)
Avalanche risk
Below Treeline
Below 1,000'
Considerable (3)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
Considerable (3)
Treeline
1,000'-2,500'
Avalanche risk
Considerable (3)
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
  • 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

Cold temperatures and moderate to strong northwest winds are expected today from 20-25+mph and will be capable of transporting surface snow.  With all this fresh snow, we can expect to find new touchy wind slabs around 1-2′ thick. In areas seeing snow pluming off the peaks and ridges, natural wind slab avalanches are possible. This is most likely south and north of Turnagain Pass in areas such as Summit Lake, Lost Lake and Girdwood.  Additionally, there still could be older wind slabs or storm slabs lingering from last week’s storm.  If a wind slab or other slab releases in the top few feet of the snowpack, it could ‘step down’ and trigger a much larger slide that breaks in deep weak layers; more on that below.

Cornices:  These have grown substantially over the past week and could easily be triggered. If making it to the ridgelines, be sure to give these features a very wide berth.

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

3-5 feet of new snow accumulated from 2/18-20 and has added to forming a thick deep slab over weak faceted snow from January.  The storm cycle this week naturally triggered many steeper slopes to avalanche.  Many steep slopes didn’t avalanche, and may not take much load to initiate.  If todays clear skies draw us into the mountains, it’s easiest to avoid these issues by remaining in the flats or on slopes 30 degrees or less with nothing steeper above.

The natural cycle we had a glimpse of Thursday afternoon proves that the snowpack isn’t stable. Many of the avalanches had crowns 5-8′ thick, were rated ‘very large’ and stepped down into buried weak snow. In shallower snowpack zones such as Lynx Creek or Summit Lake, it could be much easier to initiate a large slab. Yesterday a party felt many large whumpfs in Summit Lake.

If you are headed out into avalanche terrain today, things to remember:

  • Between 3-7′ below your snowmachine or feet sits various layers of weak sugary faceted snow
  • Very large avalanches could be triggered that take out the entire slope
  • Yes, many slopes have already slid, but these could have portions still intact and waiting for a trigger
  • There is no way to know how close a slope is to avalanching. It could be the first person or the 15th person that hits the trigger point.
  • Remote triggering is possible (initiating a collapse in the weak layer from the top, side or bottom on the slope resulting in an avalanche)

 

This slab avalanche was naturally triggered sometime in the range of 2/18-20 on a southwest facing aspect of Sunburst.   This slope is frequented by skiers.  What will it take to initiate the adjacent slopes?  2/22/2020 . Photo:  Kellie Okonek


Pictured here is one slab of many naturally triggered avalanches that occurred earlier in the week. Many steep areas that haven’t avalanched could still be near the tipping point.  2/22/2020 . Photo: E. Roberts

Weather
Sun, February 23rd, 2020

Yesterday: Skies were partly cloudy with a few snow flurries but no accumulation. Ridgetop winds were out of the east in the 10-15mph range with gusts to 25mph. Temperatures were mostly in the teens °F  to single digits in the evening.

Today: Isolated snow showers are expected in the morning but mostly sunny today, with a high near 12°F and a low of -11°F.  Winds will be out of the northwest and expected to be 20 to 25 mph.

Tomorrow: Sunny skies are forecast with a high near 16°F and low near 0°F. Winds are expected to be from the Southwest at 5 mph, then shifting to easterly in the evening as  clouds roll into the region.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 15 1 0.1 83
Summit Lake (1400′) 12 0 0 30
Alyeska Mid (1700′) 15 1 0.14 90

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 7 VAR 5 14
Seattle Ridge (2400′) 13 *N/A *N/A *N/A

*Seattle Ridge anemometer (wind sensor) is rimed over and not reporting.

Observations
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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
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.