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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, December 10th, 2019 - 7:00AM
Expires
Wed, December 11th, 2019 - 7:00AM
Forecaster
Aleph Johnston-Bloom
The Bottom Line

The avalanche danger above 2500′ is CONSIDERABLE. Human triggered avalanches are likely and naturals are still possible. The Alpine received heavy snow with strong winds yesterday and the snowpack needs time to adjust. Below 2500′ where most of the precipitation fell as rain the danger is MODERATE. There is no snow below 1000′ and NO RATING.

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Tue, December 10th, 2019
Alpine
Above 2,500'
Considerable (3)
Avalanche risk
Treeline
1,000'-2,500'
Moderate (2)
Avalanche risk
Below Treeline
Below 1,000'
No Rating (0)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
Considerable (3)
Treeline
1,000'-2,500'
Avalanche risk
Moderate (2)
Below Treeline
Below 1,000'
Avalanche risk
No Rating (0)
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

Yesterday was an impressive weather event with 2.5-3.5 inches of water falling within 24 hours and sustained easterly winds 30-40 mph gusting into the 80s. With unseasonably warm temperatures the rain/snow line was somewhere above 2500′. This means upper elevation terrain saw 2-3′ of snow. Being well within the 24-48 hours after the storm, human triggered avalanches are likely today in the Alpine. Warm snow fell over colder snow potentially forming storm slabs. Expect cornices to have grown and wind slabs to still be touchy in leeward terrain. Look for signs of instability and pay attention to surface conditions. Where did the strong winds deposit all the snow?

Below 2500′ it may be possible to trigger some type of wet avalanche in the saturated snowpack. There were a couple of different crust layers in this elevation band prior to the storm, these may act as a sliding surface if they did not deteriorate in the rain. Watch for push-a-lanche conditions (snow easily moving from your skis and entraining snow below) in steep terrain. If skies clear at all and/or temperatures cool a crust will form and any remaining hazard will decrease.

One hour precipitation graph from the base of Alyeska. Courtesy of Mesowest/MADIS.

Rain runnels on Tincan prior to the storm starting on Sunday. What will it look like today???

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

The facet/crust combination near the base of the snowpack in the Alpine may have been overloaded by yesterday’s storm or teetering on the brink.  We will be looking for evidence of natural avalanches failing close to the ground in the upper elevation terrain and investigating whether or not this layer remains reactive or goes dormant after the additional snow load. Keep this in mind if traveling above 2500′.

Weather
Tue, December 10th, 2019

Yesterday: Obscured skies and heavy precipitation through most of the day. Easterly winds 20-40 mph gusting into the 80s. Temperatures ranged from the high 20Fs/low 30Fs at ridgetops to the high 40Fs at sea level. Precipitation eased off around 8pm and winds speeds decreased to teens and 20s. Temperatures remained steady overnight.

Today: Mostly to partly cloudy skies with a chance of rain/snow showers. Temperatures will be in the high 30Fs to low 40Fs at sea level and in the high 20Fs to low 30Fs at upper elevations. Winds will be easterly 10-20 mph with gusts into the 30s.  Overnight temperatures will cool slightly and there is a chance of rain/snow showers.

Tomorrow: Mostly cloudy with rain and snow showers. Temperatures will be in the mid to high 20Fs at ridgetops and in the mid 30Fs to low 40Fs at sea level. Winds will be easterly 15-25 gusting in 30Fs. The overall pattern for the week looks to be cloudy with rain/snow showers and mild temperatures.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 36 1 2.9 19
Summit Lake (1400′) 36 0 2.3 7
Alyeska Mid (1700′) 36 0 3.24 17

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 27 NE 38 107
Seattle Ridge (2400′) NA* SE 24 53

*Seattle Ridge temperature sensor is not functioning.

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Riding Areas
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

Area Status Weather & Riding Conditions
Glacier District
Johnson Pass
Closed
Placer River
Closed
Skookum Drainage
Closed
Turnagain Pass
Closed
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.