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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
Wed, December 18th, 2019 - 7:00AM
Expires
Thu, December 19th, 2019 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line

A MODERATE avalanche danger exists in the Alpine above 2,500′ for wind slab avalanches and cornice falls. Watch for fresh or lingering wind slabs from 1-3′ thick that may be possible to trigger just off ridgelines and in cross-loaded gullies. Cornices have grown and could break off easily- they deserve a wide berth.

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Wed, December 18th, 2019
Alpine
Above 2,500'
Moderate (2)
Avalanche risk
Treeline
1,000'-2,500'
Low (1)
Avalanche risk
Below Treeline
Below 1,000'
No Rating (0)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
Moderate (2)
Treeline
1,000'-2,500'
Avalanche risk
Low (1)
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
  • 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

Finally, the winds look to let up… After 11 days of sustained moderate to strong easterly ridgetop winds across our region, weather models show a slow decline beginning today. As we might expect, the snow cover has been significantly reshaped in the Alpine where it has been snowing for the past 11 days as well. Windward areas are scoured to the tundra and catchment zones have drifts over 10 feet deep. The good news is, much of the terrain we like to play on seems to still have a decent degree of snow cover.

Wind slab avalanches are the main concern today. Fresh slabs could be found from the overnight winds or in areas still seeing some wind loading today. On top on that, older wind slabs from yesterday or prior could still pose an issue in steep wind loaded terrain. What to watch for:

  • Wind loading patters- where has the snow been depositing and scouring.
  • Be extra wary of wind loaded steep slopes at the high elevations. This is the most likely place to trigger a wind slab.
  • Stiffer snow over softer snow- does the snow feel hollow under you?
  • Is the snow cracking around you? Do you hear/feel any whumpfing?

These are all signs a slab has been found and if the snow cracks and buckles with our weight, we’ll know it’s telling us it’s unstable. As always, before jumping onto a slope, it’s good to consider the consequences if an avalanche is triggered.

Active wind loading along Sunburst Ridge yesterday. Winds were blowing northeasterly and loading the SW side of the ridge.

 

Cornices:  If traveling along ridgelines, give cornices a wide berth, they have grown and could be teetering on the balance. Limit exposure under them as well, especially if there is a group of people above you along the ridge.

Small natural cornice fall yesterday, also along the SW face of Sunburst. Note the larger cornices further down ridge.  

Avalanche Problem 2
  • Glide Avalanches
    Glide Avalanches
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to 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

The glide avalanche cycle from last week has slowed down dramatically. There are a few cracks in the Turnagain area we know of. These are good to avoid being under, two in the Tincan Trees and one just above alder line under Common Bowl on the SW face of the ridge, pictured below. You never know when cracks appear – always keep a lookout for them as they can release into an avalanche at anytime.

Additional Concern
  • Persistent Slabs
    Persistent Slabs
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.
More info at Avalanche.org

As we head into a clear sky period and travel further off the road into the Alpine becomes possible, keep in mind there could be some lingering funny business deeper in the snowpack in shallow snow cover areas such as Summit Lake. Various layers of facets and crusts could still remain intact at the high elevations. If this is the case, the potential exists for a larger avalanche to be triggered that breaks deeper in the snowpack.

Also of note, in a pit dug at 3,500′ on Sunburst yesterday, we found the old Nov Veteran’s Day facets over the hard basal melt-freeze crust at Turnagain. The layer didn’t show signs of reactivity here, which is good news (yet still something we are tracking). We dug in a shallow zone to find it as the layer is now buried between 3-6′ deep and even more in places.

Weather
Wed, December 18th, 2019

Yesterday:  Partly cloudy skies with intermittent light rain below 1,500′ and light snow above were seen region wide. Only a trace to an inch of new snow fell above the treeline. Ridgetop winds were 15-25mph with gusts nearing 50mph from an easterly direction. Temperatures cooled slightly, ridgetops are in the low 20’sF and sea level is in the mid 30’sF.

Today:  Partly cloudy skies with an instability shower or two are on tap. Similar to yesterday, only an inch to a trace of snow is expected above 1,000′ in areas close to Turnagain Arm and Portage Valley. Light rain could be seen at sea level. Ridgetop winds look to slow down today to the 10-15 mph range with gusts in the 20’s. Temperatures should remain cooler with sea level in the mid-upper 30’sF and ridgelines in the low 20’sF.

Tomorrow:  High pressure is headed in to Southcentral bringing a clearing trend starting tomorrow (Thursday) and extending through the weekend. Cold and sunny weather is forecast and we will be watching the northerly winds to see if they materialize into a potential outflow event.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 32 trace trace 31
Summit Lake (1400′) 33 0 0 7
Alyeska Mid (1700′) 33 1 0.1 16

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 24 NE 22 49
Seattle Ridge (2400′) N/A* N/A* N/A* N/A*

*Seattle Ridge weather station is not reporting. We will get it back up as soon as possible.

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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.