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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, December 7th, 2019 - 7:00AM
Expires
Sun, December 8th, 2019 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line

The avalanche danger will rise to CONSIDERABLE above 1,000′ today as a warm storm brings up to a foot of new snow with strong ridgetop winds. Another 5-8″ of snow is expected tonight. Natural avalanches composed of the new snow are possible, such as wind slabs, storm slabs and cornice falls, and human triggered avalanches are likely. Additionally, a larger avalanche that breaks in old weak snow near the ground may be possible in the high elevations above 3,000′.

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Sat, December 7th, 2019
Alpine
Above 2,500'
Considerable (3)
Avalanche risk
Treeline
1,000'-2,500'
Considerable (3)
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
Considerable (3)
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

New snow, wind and rain at sea level is on its way. This morning the storm is just moving in. Girdwood has picked up 3″ of new snow at the mid-elevations (rain at sea level). Turnagain Pass is still waiting for precipitation to start and is 34 degrees along the road at the Pass itself. There is a chance wet snow, and not rain, will fall at this elevation as our rain/snow line is forecast for 1,000′-1,500′. Avalanche danger will be related to how much snow falls. If the forecast verifies, there should be close to a foot of new snow above 1,500′ by this evening and another 5-8″ by tomorrow morning. (Don’t forget to click the ‘weather’ tab next to the ‘Avalanche Forecast’ for more on mountain weather!)

Storm snow avalanches can be expected today, either releasing on their own in areas with high snowfall rates or easily triggered by people. These will most likely be in the form of wind slabs just off ridgelines and/or in gullies that have been cross-loaded. Slabs could be anywhere from 6″ to 2′ thick. Cracking in the snow around you will be a sure bet you have found a slab and a sign to stick to low angle slopes, 30 degrees and less with nothing steeper above you.

Also keep in mind, soft slab avalanches on slopes out of the wind will be possible to trigger where a foot or more of new snow falls. Quick hand pits to test how the new snow is bonding with the old snow are a great tool on a day like today. Cornices will be growing with warm temperatures, wind and snow and should be very easy to break off.

*We hope it doesn’t, but if the rain line creeps higher than expected, wet avalanches will be possible in the mid-elevations. For example, along the east face of Seattle Ridge.

 


Natural sluffs along Sunburst’s SW face just under the weather station yesterday. How well will the new snow bond to this old snow surface is the question moving forward. (Photo: Billy Finley).

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 snowpack on high elevation slopes over 3,000′ remains a concern due to a layer of faceted weak snow over a hard crust near the bottom of the snowpack. With a new load of snow being added today/tonight and another round of heavier snow Monday, we will be watching to see if this weak layer becomes overloaded and results in larger avalanches that break near the ground. This is also something to pay attention to if traveling up to these higher elevations.

Weather
Sat, December 7th, 2019

Yesterday:  Partly cloudy skies with moderate ridgetop easterly winds were over the region as our next storm system slowly moved in. A trace of snow fell at Turnagain Pass overnight and 2-3″ in the upper Girdwood Valley. Temperatures rose to the upper 30’s F at sea level and the mid 20’s F along ridgelines yesterday and that is where they are sitting this morning.

Today:  Cloudy skies, strong ridgetop winds and snowfall are expected as the first round of several warm storms hits us. Today, we are expecting 1″ of rain at sea level (2″ in Portage area) with a rain line up to 1,000-1,500′. This equates to ~10-12″ of snow above 1,500′. Tonight, another .75″ of rain (1″ in Portage) is forecast with a rain line close to 1,500′, which should bring another 7-10″ of snow in the higher elevations. Ridgetop winds will be easterly in the 35-50mph range and temperatures will remain warm, near 40F at sea level and upper 20’s F along ridgetops.

Tomorrow:  This first storm moves out tomorrow morning and models are showing a brief break in weather before a warmer and wetter system moves in for Sunday evening into Monday. Right now it’s looking like Girdwood and Turnagain Pass will see 2-3″ of rain at sea level (2-3 feet of snow in the Alpine) with Portage area seeing almost double this amount. Stay tuned!

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 30 1 0.1 15
Summit Lake (1400′) 27 1 0.1 10
Alyeska Mid (1700′) 28 3 0.25 18

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 20 NE 15 49
Seattle Ridge (2400′) N/A* N/A* N/A* N/A*

*Seattle Ridge anemometer (wind sensor) is rimed over and the temperature sensor is not functioning. A new temperature sensor is arriving soon and we hope to get it up on the next clear day.

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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.