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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
Thu, March 5th, 2020 - 7:00AM
Expires
Fri, March 6th, 2020 - 7:00AM
Forecaster
Aleph Johnston-Bloom
The Bottom Line

The avalanche danger remains MODERATE today. Northwest outflow winds will continue to impact the region. Watch for winds actively moving snow and wind effect on the snow surface. Wind slab avalanches around a foot thick will be possible to trigger on freshly loaded slopes. Additionally, triggering a very large avalanche breaking in weak snow 3-6 feet deep remains a concern region-wide.

SUMMIT LAKE TO SEWARD REGION:  Winds are will continue to be strong in this area and a CONSIDERABLE danger applies again today. Check Seward and Summit Lake observations from yesterday. Natural wind slabs are possible and human triggered slabs are likely. Watch for blowing snow and loading slopes. Extra caution is advised.

 

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Thu, March 5th, 2020
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
  • 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

Heading out today to enjoy some sunshine? The outflow winds are forecast to continue through the day. Paying attention to what the wind is doing and has done in the terrain you are traveling will be crucial. Observing the outflow winds (west/northwest) in the forecast area yesterday was pretty interesting and as often the case with this wind pattern, the impact was not straightforward.  Pluming was observed in the mountains around Girdwood but there was not much wind at Alyeska. Winds were channeled through the Arm and into Placer Valley but wind speeds were light at the Seattle Ridge weather station. Sunburst weather station reported mostly light northwest winds but observers on Sunburst felt the wind coming through the Pass from the south.  Looking at the south end of Seattle Ridge, near the Johnson Pass north trail head, the pluming was obvious. A natural wind slab was observed in motion near the Hope Y mid-day. From there south to Lost Lake the wind transport was notable. As Wendy mentioned yesterday many of our ridgetop weather stations don’t pick up the northwest flow direction very well.  AKRR MP 43 weather station that sits near Bench Peak, above and to the west of Grandview, does seem to capture these winds. Steady winds in the 20s with gusts into the 40s were recorded in the late afternoon and overnight.

Wind Slabs: Other than witnessing active wind loading and slabs developing while you are on slope, pay attention to surface conditions indicating previous wind loading. Be on the lookout for stiff snow over softer snow, shooting cracks and drifting patterns near ridgelines and in gullies. Also keep in mind that a layer of buried surface hoar may be sitting under wind slabs, making them easier to trigger and possibly propagate further above or to the side than expected.

Wind on peak 4,940 (South end of Seattle Ridge across from Johnson Pass north TH ) at 4:30pm. 3.4.20. Photo: Nick D’Alessio

AKRR MP 43 weather station data.

Cornices:  Small fresh cornice falls from winds today are possible in areas seeing significant wind. Most likely outside of the forecast zone.

Loose snow avalanches:  In areas out of the wind, sluffs are possible in steep terrain.

Sun effect:  The wind and cold temperatures should overrule any sun effect today.

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

You might ask yourself, “Why are they still talking about the January facets in March?”  Unfortunately that is the nature of a deep persistent issue. Those facets are a lingering concern, a nagging what if.  These layers are buried between 3-6 feet deep and there is still a chance of a person could trigger a very large and dangerous avalanche. As time goes by, the likelihood of triggering is decreasing but we haven’t completely it ruled out. In areas outside our forecast zone to the south, with a shallower snowpack, you are more likely to find one of these large slabs. Also, areas with little traffic this season are more suspect. Don’t forget to use good travel protocol and consider consequences when choosing terrain.

Weather
Thu, March 5th, 2020

Yesterday: Skies were mostly clear and temperatures ranged from the single digits above 0°F to single digits below 0°F. Winds were westerly 5-15 mph with gusts into the 20s in areas sheltered from the outflow. In areas more impacted by this wind pattern winds were westerly 15-25 mph with gusts into the 40s. Overnight temperatures rose just slightly and outflow winds continued to be gusty.

Today: Skies will be clear today with temperatures in the single digits to low teens. The outflow winds are forecast to continue today 10-20 mph with gusts into 30s and ease off this evening. Look for the westerly winds to be more pronounced through Turnagain Arm and south of Turnagain Pass. Tonight will be mostly clear with temperatures around or just below 0°F.

Tomorrow: Increasing clouds, temperatures in the teens and low 20°Fs and light east winds. Skies will be mostly cloudy overnight with a chance of snow showers. There is snow in the forecast for the weekend but the timing and details are still uncertain. Stay tuned!

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 5 0 0 73
Summit Lake (1400′) 4 0 0 32
Alyeska Mid (1700′) 3 0 0 83

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) -5 WNW 8 24
Seattle Ridge (2400′) -1 W 4 17
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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
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Closed
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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.