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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, April 21st, 2018 - 7:00AM
Expires
Sun, April 22nd, 2018 - 7:00AM
Forecaster
Heather Thamm
The Bottom Line

The avalanche danger will remain  MODERATE  due to above freezing temperatures below 2000′ where triggering a wet avalanche is possible on steep slopes.  Triggering a storm slab up to a foot deep will be possible in the alpine, where winds will be actively transporting new snow as a storm moves into our area this afternoon. Cornices are unstable and could release without warming.  

Portage Valley: Avalanche danger in Portage Valley will rise to CONSIDERABLE today due to heavier rain and snow forecasted for that area. Natural dry snow avalanches are possible along ridgelines and wet snow avalanches below.  *Remember there are avalanche paths that can run over popular hiking areas such as Byron Glacier trail.  

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Sat, April 21st, 2018
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
  • 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

In the upper elevations, above 2500’, where drier snow exists Easterly winds 20-30mph will be actively transporting snow. In the afternoon up to 5” of new snow is possible and newly forming slabs could grow up to 12” thick on leeward features. It’s also possible that someone could trigger an older storm slab due to a slick bed surface that was buried earlier in the week. On Thursday several small human triggered avalanches were reported on Eddies and in Main Bowl on Seattle Ridge following a storm that ended Thursday morning. These slabs were pretty shallow 6-10” and the bed surface was an old melt/freeze crust. Spring time warming and sun also contributed to instability that day. Today it will be important to monitor how much new snow falls and how much snow is being transported by the wind. Shooting cracks will a clue that this new snow is tender. In Portage Valley where precipitation will be heavier natural storm slab avalanches will be possible. 

This small pocket on a W aspect of Eddies was triggered by the first skier. The third skier also triggered a small pocket adjacent to this avalanche. See photo below. 

 

A slick melt freeze crust was present prior to the storm that ended Thursday morning and one of the skiers involved reported feeling the crust bed surface.

 

Avalanche Problem 2
  • Wet Loose
    Wet Loose
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Wet Loose
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

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

Triggering a wet avalanche will be possible below 2500’ today where rain and wet snow will be falling. Overcast skies, light rain and above freezing temperatures have kept the snowpack wet in the mid and lower elevation bands all week. Today scattered rain showers and up to 0.3” rain/snow water equivalent is possible by this afternoon with an additional 0.5” of rain by late evening. Coastal areas will see higher amounts. Evaluate the snowpack for wet, soupy and non-cohesive snow. In places where rain is falling, avoid steep terrain if your skis or snow machine start to trench into the snow. In the mid elevations where wet snow is falling on a slick crust, wet loose avalanches will be proportional to how much snow falls today. If you’re out later into the evening wet loose avalanches could become unmanageable in a terrain trap or on a larger slope. 

In Portage Valley where precipitation will  be heavier today, be aware of popular hiking areas with avalanche terrain above like Byron Glacier trail. Natural wet avalanches are possible in this area. The photo below was take yesterday of lower elevations of Maynard Mountain above Portage Lake.

Additional Concern
  • Cornice
    Cornice
Cornice
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
More info at Avalanche.org

Cornices are starting to fall. New snow and wind will be adding additional stress today and through the weekend. Give them plenty of space and limit exposure time underneath. 

Weather
Sat, April 21st, 2018

Yesterday scattered showers were observed, but only trace amounts of precipitation were recorded in Girdwood and Turnagain Pass. Portage Valley recored 0.14 € of snow water equivalent in the last 24 hours. Winds were light from the East increasing to Moderate in the afternoon, 10-20mph with gusts up to 30mph. Temperatures remained above freezing in the mid and lower elevations with highs near 40F during the day. Ridgetop temperatures were in the mid 20F’s to low 30F’s. Rain/snow line was around 1500′ overnight.  

Scattered rain showers will continue across our region with heavier precipitation near coastal areas later in the day. Up to 0.3 € of rain is possible by this afternoon, this means 0-5 € of snow in upper elevations. Temperatures in the mid and lower elevations will remain above freezing with rain/snow line around 2000′. High’s will be in the mid 40F’s near sea level. Tonight an additional 0.5 € rain is forecasted, 3-6 € of snow in upper elevations. Easterly ridgetop winds will be 15-30mph with gusts in the 40F’s.  

Overnight rain showers will continue with a front intensifying precipitation by Sunday morning. Tomorrow expect rain in the lower elevations and snow above 2000′. Winds will be 20-40mph from the East. A series of Low pressure systems will continue to impact our region through next week. The long term forecast shows a deepening low pressure that may bring intense rain and snow into the Gulf of Alaska by Tuesday/Wednesday.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 36   0    0 64  
Summit Lake (1400′) 36   0    0 22  
Alyeska Mid (1700′) 35   0   .07   59  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 26   E   8   30  
Seattle Ridge (2400′) 30   ESE   15   31  
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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

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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.