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 2nd, 2020 - 7:00AM
Expires
Thu, December 3rd, 2020 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE for all elevations.  In the last 48 hours there has been heavy snow, rain and strong winds. Human triggered slab avalanches are likely on slopes 30° and steeper and will increase in likelihood with elevation. We are at the tail end of a significant storm, which means cautious route-finding and conservative decision-making are essential today.

SUMMIT LAKE: The snowpack is generally thinner and weaker in the Summit Lake region and 1-2′ of snow fell during this storm. This means it may be even easier to trigger an avalanche on a mid-pack buried weak layer or near the ground.

SEWARD/LOST LAKE: This region saw heavy rain and snow yesterday and there is no snowpack information for this area yet. Extra caution is advised as more data is collected.

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Wed, December 2nd, 2020
Alpine
Above 2,500'
Considerable (3)
Avalanche risk
Treeline
1,000'-2,500'
Considerable (3)
Avalanche risk
Below Treeline
Below 1,000'
Considerable (3)
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
Considerable (3)
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

If venturing out today in the soggy mountains caution is advised. Monday night a natural avalanche cycle occurred in upper elevation terrain near Girdwood and in Portage. Yesterday there were avalanches observed running in the Moose Pass zone during the intense rain.  There were no reports of avalanches in Turnagain Pass or Summit Lake but we have not had much visibility to assess.  Depending on location, between 1-3′ feet of new snow has fallen at upper elevations. In addition, rain fell to around 2000′ (maybe higher). Turnagain Pass and Girdwood were spared the heavy rain Portage and Moose Pass south to Seward received. The most intense winds were Monday evening but remained steady yesterday at ridgeline stations, blowing from the east 20-30 mph and gusting into the 40s and 50s.

As I like to say there is a buffet of avalanche issues to think about today. Storm snow avalanches, such as wind slabs along ridgelines, storm slabs in the sheltered zones, cornice falls and wet loose (push-a-lanches) in the saturated snow should all be viewed as likely today. It’s a day to carefully evaluate terrain and consequences if an avalanche does release. We need to give the mountains time to adjust. Low angle slopes without steep slopes above are great ways to enjoy the new heavy snow without worry.

Red flags to watch for:
–  Recent avalanches, from today and was there any avalanche activity during the storm?
–  Whumpfing (collapsing) of the snowpack, sure sign to avoid avalanche terrain period.
–  Shooting cracks, likely to be seen near ridgelines where the wind has formed wind slabs.

Storm totals (beginning Monday night through 6am Wednesday morning)

  • Turnagain Pass at 1,880′:  2.4″ of water equivalent, roughly 2-2.5′ of snow above treeline.
  • Girdwood Valley at 1,700′:  1.5″ of water equivalent, roughly 1-2′ above treeline.
  • Portage Valley at sea level:  3″ of water equivalent, roughly 3′ of snow above treeline.
  • Summit Lake at 1,400′: 1.6″ of water equivalent, roughly 1-2′ of snow above treeline.

As the storm moves on and the temperatures cool the good news is the new snow should bond relatively quickly. From what we know, the pre-existing snow surface lacked any persistent type of snow grain and was mainly wind slab and settled powder. As always however, the snowpack is guilty until proven innocent and there are always surprises out there – we can’t get caught being over confident.

A wet view from the Seattle Ridge web cam yesterday afternoon.

Avalanche Problem 2
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and 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

We have been concerned about a skier or snowmachiner triggering a large destructive avalanche that fails near the ground on the weak October snow. We have not seen avalanche activity on this layer recently and in most of forecast area it is now buried more than 6′ deep. However, as the storm moves out and we get more visibility this week, we will be looking for signs of any natural avalanches on this layer and will continue to ask the question, ‘If I hit a shallow spot on my machine or skis could I still trigger a deep slab avalanche?’  Towards the southern end of Turnagain Pass and in Summit Lake the snowpack is shallower and potentially more sensitive to triggering an avalanche. In Summit Lake we have also found a layer of buried surface hoar in protected areas that may also be an issue. We have no snowpack information for the Lynx Creek and Johnson Pass area which is now open. This lingering ‘what if‘ about deep slabs is another great reason to keep it mellow today. Allow the snowpack time to adjust to all this recent loading.  As said above, we are in the guilty until proven innocent stage.

Large buried surface hoar in Summit Lake. 11.29.20. This weak layer may be an issue in protected areas there and in the Lynx Creek/Johnson Pass zone.

Weather
Wed, December 2nd, 2020

Yesterday: Skies were overcast with rain/snow showers throughout the day. Rain/snowline was approximately 2000′ and maybe higher. There was heavy rain at times in Seward and Portage. Winds were easterly blowing 15-25 mph gusting into the 40s and 50s. Temperatures were in the high 30°Fs and low 40°Fs at sea level and high 20°Fs in the Alpine. Overnight rain/snow showers, elevated easterly winds and warm temperatures continued.

Today: Skies will remain overcast and rain and snow showers continue today. As temperatures drop rain/snowline will come down from around 2000′ to sea level. Temperatures will start out in the high 30°Fs to mid 40°Fs at sea level and the high 20°Fs in the Alpine and cool to the the low 20°Fs and teens overnight. Winds will be easterly 10-20 mph gusting into the 30s and shift to the south/south west overnight and decrease.

Tomorrow: Skies are forecast to be partly cloudy with a chance of snow showers, temperatures in the teens and 20°Fs and light southwest winds. There is a pause in the stormy weather with snow potentially returning Sunday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 36 0 0.7 63
Summit Lake (1400′) 33 0 1.0 26
Alyeska Mid (1700′) 35 0 0.44 59

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 27 NE 22 55
Seattle Ridge (2400′) 30 E 12 24
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Riding Areas
Updated Tue, June 01st, 2021

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
It is packrafting and jetboat season!
Skookum Drainage
Closed
The Skookum Valley is closed to snowmachines. This closure occurs annually on April 1 as per the CNF Forest Plan.
Turnagain Pass
Closed
Closed as of June 1. 188 day season, that\'s a wrap!
Twentymile
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It is packrafting and jetboat season!
Seward District
Carter Lake
Closed
Closes May 1.
Lost Lake Trail
Closed
Closes May 1.
Primrose Trail
Closed
Closes May 1.
Resurrection Pass Trail
Closed
Closed for the 2020/21 winter season. Will be open for moto use in the 21/22\\\' winter season as per the CNF Forest plan.
Snug Harbor
Closed
Closes May 16th.
South Fork Snow River Corridor
Closed
Closes May 1.
Summit Lake
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Closes May 1.

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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.