Turnagain Pass Avalanche Forecast RSS

Archives
Issued
Sat, November 28th, 2020 - 7:00AM
Expires
Sun, November 29th, 2020 - 7:00AM
Forecaster
Wendy Wagner
High Avalanche Danger
Travel in avalanche terrain is not recommended. Avoid being on or beneath all steep slopes.
The Bottom Line

The avalanche danger is HIGH at all aspects and elevations. Heavy snowfall has been over the region for the past 24 hours and will continue through today. Upwards of 3-4′ of snow has fallen and is overloading a weak snowpack. Large natural and human triggered avalanches are likely.

*Travel in avalanche terrain is NOT recommended. This means sticking to low angle slopes and staying well away from larger slopes and runout zones.

A Winter Weather Advisory has been issued by the National Weather Service for Portage Valley and Turnagain Pass.

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Sat, November 28th, 2020
Alpine
Above 2,500'
High (4)
Avalanche risk
Treeline
1,000'-2,500'
High (4)
Avalanche risk
Below Treeline
Below 1,000'
High (4)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
High (4)
Treeline
1,000'-2,500'
Avalanche risk
High (4)
Below Treeline
Below 1,000'
Avalanche risk
High (4)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Recent Avalanches

Many natural avalanches likely occurred during yesterday’s first pulse of heavy snowfall; that was between 1.5-2.5 feet of snow from yesterday morning through midday! This photo below is a natural avalanche in upper Girdwood Valley in the Crow Creek area. With such little visibility, it was difficult to assess the extent of natural avalanche activity.

Natural avalanche in the upper Girdwood Valley caught on camera in action yesterday. 11.27.20. Photo: George Creighton

Before yesterday’s storm, Thursday offered a brief break in weather and blue skies. Skiers along Sunburst ridge noted a large natural avalanche that likely occurred at the tail end of Wednesday’s (11.25) storm and strong winds. This slide began as a wind slab that stepped down to the ground to take the entire snowpack.

Wind slab avalanche that stepped down into the facets at the bottom of the snowpack on north facing Sunburst around 2,500′. The wind slab portion can be seen in far upper right corner of photo. 11.26.20. Photo Nick D’Alessio

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

Today’s RED FLAGS are ‘Rapid Loading’ and ‘Recent Avalanches’. It’s simple right now. Let the mountains do their thing while we stay away from, and out from under, steep slopes.

Too much snow too quick = large and dangerous avalanches.

This is all because the Thanksgiving parade of storms continues. After an exciting last week of heavy snowfall and large avalanches, another system rolled in yesterday morning as mentioned above. Add to that another foot of snow fell overnight and up to 10″ more could fall through the day. Furthermore, the easterly ridgetop winds are howling – blowing in the 40-50mph range with gusts over 80mph.

Snowfall totals in the past 28 hours (estimated for mid and upper elevations):
– Turnagain Pass:  3-4 feet (3″ of water equivalent)
– Upper Girdwood Valley:  2.5-3 feet (2.5-3″ of water equivalent)
– Summit Lake:  ~1.5 feet (1″ of water equivalent)

Storm snow avalanches have, and are likely, to continue to occur naturally. These will be in the form of wind slabs, storm slabs and cornice falls. Slabs could be anywhere from 2-6′ thick depending on the amount of new snow and wind loading. All of which could generate a significant amount of snow and send debris well into valley bottoms.

Deep snow immersion:  With so much snow be aware, even in areas in the trees (i.e., Tincan Trees) a small slope can push snow over you, making it hard to escape if you get caught up and head down. Additionally, falling into tree wells or other hollows in the snow can cause great problems as snow can fill in over you and make escape difficult to impossible without help. If thinking of heading out today, be sure to watch your partners!

Too much snow..?! A skier wallows in yesterday’s storm snow in low anger terrain near Sunburst ridge.

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

Making the avalanche problem worse is that the existing snowpack is unstable to begin with. There are weak sugary facets at the bottom of the pack (basal facets) and a foot or so above this is a layer of buried surface hoar in some areas. Both these layers could fail with the overloading of new snow, creating a much larger avalanche. The basal facets are looking to be the most reactive of these layers and we are headed into what could be a future tricky deep persistent slab avalanche issue.


Note the amount of new snow over a shallow old snowpack. This pit is showing the basal facets that sit at the bottom. The codes (ECTP14, etc) are snow science speak meaning the snowpack is unstable. 

CNFAIC new forecaster Andrew Schauer points at the facets under the snowpack on Seattle Ridge Thursday (11.26).

Weather
Sat, November 28th, 2020

Yesterday:  Heavy snowfall was seen from Girdwood to Seward. Turnagain Pass saw 2-2.5′ with 1-1.5′ in the Summit Lake area. Winds were moderate from the east along ridgetops (~20mph, gusting ~40mph). Temperatures were near 32F at 1,000′ and 20F along ridgelines. Rain/snow line was near sea level.

Today:  After 12-14″ of new snow overnight, heavy snowfall continues this morning, which should taper off by the afternoon. An additional 8-10″ is expected through the day at Turnagain Pass. The rain/snow line could creep up to 200′ at times. Winds are forecast to remain strong from the east (40-50mph and gusting ~80mph) before lessening this evening. Temperatures should stay near 32F at 1,000′ and 20F along ridgetops.

Tomorrow:  Cloudy skies with scattered snow showers are expected for Sunday. Winds look to decrease to the moderate range from the south and east. Looking forward, snow showers are expected to continue through the week ahead with an unsettled weather pattern continuing. Let it snow!

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 28 25 2.5 ~68
Summit Lake (1400′) 27 7 0.7 25
Alyeska Mid (1700′) 28 13 1.4 63

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 20 E 25 81
Seattle Ridge (2400′) 24 *N/A *N/A *N/A

*Seattle Ridge anemometer (wind sensor) is rimed and not reporting data.

Observations
Recent Observations for Turnagain Pass
Date Region Location
11/27/20 Observation: Sunburst
11/27/20 Turnagain Observation: Eddies
11/26/20 Observation: Seattle Ridge
11/26/20 Turnagain Avalanche: Sunburst
11/26/20 Turnagain Observation: Tincan Trees
11/25/20 Turnagain Observation: Turnagain Pass -Moto side
11/24/20 Turnagain Observation: Seattle Ridge Up-Track
11/24/20 Turnagain Observation: Eddies
11/24/20 Turnagain Observation: Sunburst
11/24/20 Turnagain Observation: Sunburst West Bench
Riding Areas
Updated Thu, November 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
Open
Turnagain Pass open to motorized use as of Wednesday 11/25.
Twentymile
Closed
Seward District
Carter Lake
Closed
Lost Lake Trail
Closed
Primrose Trail
Closed
Resurrection Pass Trail
Closed
Closed for the 2020/21 winter season
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.