Turnagain Pass Avalanche Forecast RSS

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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Fri, December 17th, 2021 - 7:00AM
Expires
Sat, December 18th, 2021 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

The avalanche danger is MODERATE since it is possible to trigger an avalanche 3-6′ deep on a deeply buried layer of weak, sugary facets. This layer is becoming more stubborn as we move farther out from the last major loading event, which makes it possible that a person could trigger an avalanche after there are multiple sets of tracks on a slope. These avalanches can be triggered from above, adjacent to, or below steep terrain. The only way to effectively manage a problem like this is to avoid steep terrain. In addition to the deep slab problem, it will be possible to encounter lingering wind slabs up to a foot deep that formed on top of weak snow over the past few days that can remain reactive today. Be on the lookout for stiff, punchy, or hollow-feeling snow near ridgetops, gullies, or upper-elevation convexities.

PORTAGE VALLEY: Gap winds will once again be stronger in the Portage Valley than they will be in the core advisory area. In addition to the the problems mentioned above, sustained winds at 10-15 mph with gusts in the 30’s will make it possible to trigger fresh wind slabs today.

SUMMIT LAKE: The snowpack in the Summit Lake area is thinner and more suspect than the core advisory area. This will make buried weak layers more sensitive to human triggers, making it easier to trigger a slab avalanche 2′ deep in steep terrain.

SEWARD: Like the Portage Valley, winds will be stronger in Seward than they will be in the core advisory area. Fresh wind slabs forming today will be up to a foot deep and more sensitive to human triggers.

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Fri, December 17th, 2021
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
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    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

With a quiet day of weather on tap for today, the main concern will be that deeply buried layer of weak, faceted snow that plagues us throughout the advisory area. This layer is buried 3-4′ deep on average towards the north end of Turnagain Pass and in the Girdwood area, just barely 3′ deep on the south end of the pass, and even shallower in Summit Lake. The most likely place to trigger a deep slab avalanche is where the snowpack is the thinnest, so given that spatial trend it is important to be aware of potentially more dangerous conditions as you move further south in the advisory area.

In addition to changes in reactivity with snow depth, there are a few other things to key in on with this tricky layer. First, deeply buried weak layers tend to stop showing any of the classic warning signs even when they are still reactive, including the way the react in stability tests (see video below- or linked here). That makes it hard to recognize potentially dangerous conditions. All you can rely on is recognizing poor structure (i.e. a stiff slab on top of weak snow), which we know we have throughout the advisory area. The other dangerous characteristic of this deep slab problem is how wide it can propagate. We have seen this snowpack lead to very large avalanches, with crowns wrapping around large terrain features and sometimes wrapping around corners connecting different aspects. The deep and stiff slab can also propagate up into low-angle terrain above steeper slopes, and it can be triggered from low slope angles below steeper slopes. All of these challenges can be managed by smart terrain use. Hedge your bets by avoiding the big, steep slopes that can produce unsurvivable avalanches for now. Problems like this do improve with time, but it takes some patience. Luckily we still have plenty of winter left.

Large natural avalanche on the west face of Comet Peak, likely from the 12/9-12/10 natural cycle. This shows what our current snowpack is capable of. Avalanches like these are becoming more difficult to trigger, but the possibility remains. Photo courtesy of Chugach Powder Guides. 12.15.2021

Avalanche Problem 2
  • Wind Slabs
    Wind Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    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

With the exception of a few zones outside of our forecast area (Portage, Seward), we are not expecting strong enough winds to build fresh slabs today. But we did see some slopes growing surface hoar prior to the last wind event, which will make it possible that some lingering wind slabs can still be reactive to human triggers today. Be on the lookout for stiff, punchy, or hollow snow at upper elevations near ridgelines, in gullies, or on the downslope side of convexities. Unlike our deep slab problem, these older wind slabs will likely give you warning signs like shooting cracks or collapsing. A relatively small wind slab avalanche could turn into a serious problem if it steps down to the deeper weak layer mentioned in problem 1 above.

 

(If the video below doesn’t load you can find it on our YouTube channel or this link)

Weather
Fri, December 17th, 2021

Yesterday: With high temperatures in the upper teens to mid 20’s F, it was feeling tropical yesterday! Winds were 10-15 mph out of the east with gusts to 30 mph for most of the day, switching to the west and dropping to around 5 mph overnight. A trace to 2″ of snow fell, with snow to sea level. Skies were fully cloudy.

Today: As of 6:00 a.m. temperatures are in the low to mid 20’s F, and are expected to drop to the low to mid teens during the day. Skies are mostly cloudy, but visibility should improve with partly to mostly sunny skies by mid day. Westerly winds are expected around 5 mph in Girdwood and Turnagain Pass, and closer to 15-20 mph in Seward and the Portage Valley. No measurable precipitation is expected today.

Tomorrow: Temperatures are expected to hover in the mid teens tonight through tomorrow, and we might see a trace of snow overnight. Expect a slight bump in westerly winds tomorrow, with sustained ridgetop winds around 10-15 mph. Skies will once again be mostly cloudy in the morning, with decreasing cloud cover during the day.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 23 0* 0* 72
Summit Lake (1400′) 18 0 0 21
Alyeska Mid (1700′) 21 1 tr 43

*No measurable precip at the Snotel station, but 1-2″ observed from the field at the pass yesterday.

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 15 E* 8 32
Seattle Ridge (2400′) 18 SE 5 20

*Winds shifted to the west at 2:00 a.m.

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Riding Areas
Updated Sun, November 27th, 2022

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
Scheduled opening Dec 1st per Forest Plan, contingent upon adequate snow depth for forest resource protection.
Placer River
Closed
Scheduled opening Dec 1st per Forest Plan, contingent upon adequate snow depth for forest resource protection.
Skookum Drainage
Closed
Scheduled opening Dec 1st per Forest Plan, contingent upon adequate snow depth for forest resource protection.
Turnagain Pass
Closed
The Forest has issued a closure order for Turnagain Pass due to inadequate snow cover for resource protection. Conditions will be monitored daily. Between 16-20” of snow exists at the parking lot. The scheduled opening would have been the Wednesday before Thanksgiving per Forest Plan.
Twentymile
Closed
Scheduled opening Dec 1st per Forest Plan, contingent upon adequate snow depth for forest resource protection.
Seward District
Carter Lake
Closed
Scheduled opening Dec 1st per Forest Plan, contingent upon adequate snow depth for forest resource protection.
Lost Lake Trail
Closed
Scheduled opening Dec 1st per Forest Plan, contingent upon adequate snow depth for forest resource protection.
Primrose Trail
Closed
Scheduled opening Dec 1st per Forest Plan, contingent upon adequate snow depth for forest resource protection.
Resurrection Pass Trail
Closed
Closed to motorized use for the 2022/23 winter season per Forest Plan. Open next season.
Snug Harbor
Closed
Scheduled opening Dec 1st per Forest Plan, contingent upon adequate snow depth for forest resource protection.
South Fork Snow River Corridor
Closed
Scheduled opening Dec 1st per Forest Plan, contingent upon adequate snow depth for forest resource protection.
Summit Lake
Closed
Scheduled opening Dec 1st per Forest Plan, contingent upon adequate snow depth for forest resource protection.

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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.