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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
Tue, February 4th, 2020 - 7:00AM
Expires
Wed, February 5th, 2020 - 7:00AM
Forecaster
Ryan Van Luit
The Bottom Line

Today the avalanche danger is CONSIDERABLE above 1,000′.  Human triggered wind slab avalanches are likely on steep slopes with previous or active wind loading.  If triggered, these wind slabs have the potential to overload buried weak layers and initiate a larger slab avalanche. Where wind slabs are not present, the persistent weak layers 2-3′ down are becoming more stubborn, but it still remains possible for a person to trigger a large slab avalanche on these layers.

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Tue, February 4th, 2020
Alpine
Above 2,500'
Considerable (3)
Avalanche risk
Treeline
1,000'-2,500'
Considerable (3)
Avalanche risk
Below Treeline
Below 1,000'
Moderate (2)
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
Moderate (2)
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

Since yesterday morning we’ve had several hours of wind near 20mph with gusts to 47mph at the Sunburst weather station.  Winds as low as 12-15mph can transport low density snow.  Wind slabs continue to form in wind exposed areas throughout the region.  Where wind slabs exist on steeper slopes, it could be easy and likely for a human to trigger an avalanche.  Although we have a warming trend, the temperatures in the Alpine have remained below freezing helping maintain loose snow.  Our 3″ of new snow over night in addition to the snow from last week is still unconsolidated in most places and easily moves around with the variable winds.

It can be difficult to predict which slopes have formed wind slabs because the region has experienced winds from many different directions.  It’s essential to look for clues where wind slabs are forming, or have already formed.  If you see the winds depositing snow onto the lee of ridges or gullies, that’s a clear indicator a slab could be forming.  Scoured exposed areas can indicate loading to the leeward side.  Where visibility is low or you can’t see your entire route, remain aware of the snow beneath you –  you could be entering a wind slab if you feel stiffening of the snowpack, see rippling on the surface, see shooting cracks, or sense hollow drum-like sounds in the snow.  Choose routes with intention and don’t hesitate to adjust your plan.

Steady winds over the previous 30 hours capable of transporting loose snow.

 

This image is looking toward the northeast aspect of Magnum.  The visible scouring suggests easterly winds cross loaded many gullies on Magnum.  Winds from this direction could have easily loaded the common northwest uptrack.  2.3.2020  Photo: CNFAIC Archive

 

Cornices:  Cornices continue to form and could be touchy to human trigger as they build with new snow and wind transport.  As always, give cornices a wide margin.

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

Our snowpack structure contains weak layers formed before the storms last week.  We’re particularly keeping an eye on the MLK Jr facets and the NYE crust.  The incremental loading and wind transported snow naturally triggered avalanches last week – the MLK Jr facets and NYE crust were the suspected weak layers.  Our testing shows these layers are becoming more stubborn but it remains possible for a human to trigger a large avalanche in these layers 2-3′ down in the snowpack.  With time and our current warming trend, the chances increase for the top 2′ of snow to consolidate and become more cohesive.  These conditions could create a more distinct slab above these weak layers.

Additionally, If a wind slab is triggered, it could step down to initiate one of these layers deeper within the snowpack.

Loose Snow Avalanches:  On steep slopes with unconsolidated snow, watch for easily triggered loose snow sluffs.

Weather
Tue, February 4th, 2020

Yesterday: Cloudy skies with scattered snow showers. Winds were easterly 10-20 mph gusting into the 40s in the morning then decreased into the evening. Temperatures were in the mid to high 20’s. Snow accumulation overnight 1-3″.

Today:  Mostly cloudy skies with a high near 30°F and low around 23°F. Winds out of the Southeast from 5 -20 mph. New snow with accumulation of 2-4″ possible.

Tomorrow:  Cloudy skies with a high near 32°F and lows in the teens. Intermittent snow showers expected throughout the day and into the evening adding 1-3″ of snow.  Winds expected to be from the east 5 – 10 mph becoming light and variable into the evening.

PRECIPITATION 24-hour data (6am – 6am)
Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 28 2 0.2 58
Summit Lake (1400′) 28 1 0.1 20
Alyeska Mid (1700′) 28 4 0.31 56

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 18 ENE 16 47
Seattle Ridge (2400′) 23 ESE 11 26
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Updated Mon, October 26th, 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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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.