Turnagain Pass RSS

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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, November 26th, 2022 - 7:00AM
Expires
Sun, November 27th, 2022 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE above 2500′. Fresh wind slabs 1-2′ deep created by NW outflow winds are likely for a person to trigger and possible for natural avalanches. With very light snow on the surface, it will not take very strong winds to transport the snow and build fresh wind slabs. Common areas to find wind slabs are along ridgelines, convex features, and cross loaded gullies. Larger avalanches up to 2-4′ deep are possible on a layer of sugary facets on the ground.

Between 1000′ and 2500′ the avalanche danger is MODERATE. At the upper end of this elevation band the conditions may be more similar to the alpine, so be on the lookout for active wind transport and areas that the winds over the past 24 hours could have created wind slabs. Below 1000′ the avalanche danger is LOW.

Special Announcements

Hatcher Pass: Check out the forecast for Hatcher Pass if you are headed to the Talkeetna’s

New this season: we are adding the avalanche problem rose to our icons. We’ve posted a quick guide on how to use it (and its limitations) HERE.

Sat, November 26th, 2022
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
Travel Advice Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern. Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches Natural and human-triggered avalanches unlikely. Natural avalanches unlikely; human-triggered avalanches possible. Natural avalanches possible; human-triggered avalanches likely. Natural avalanches likely; human-triggered avalanches very likely. Natural and human-triggered avalanches certain.
Avalanche Size and Distribution Small avalanches in isolated areas or extreme terrain. Small avalanches in specific areas; or large avalanches in isolated areas. Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas. Large avalanches in many areas; or very large avalanches in specific areas. Very large avalanches in many areas.
Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • Aspect/Elevation
  • 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.
Aspect/Elevation of the Avalanche Problem
Specialists develop a graphic representation of the potential distribution of a particular avalanche problem across the topography. This aspect/elevation rose is used to indicate where the particular avalanche problem is thought to exist on all elevation aspects. Areas where the avalanche problem is thought to exist are colored grey, and it is less likely to be encountered in areas colored white.
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

Fresh winds slabs being created by the NW outflow winds are the story of the day. These outflow winds tend to be strongest along gaps in the terrain that connect the interior air with the coast, like along Turnagain Arm. At upper elevations you are likely to find 20-35 mph winds and fresh winds slabs that have been forming since yesterday morning. When the winds started to pick up yesterday there was about a foot of soft snow on the surface which could create wind slabs up to 1-2′ deep. Due to the low density of the surface snow it won’t take extreme wind speeds to cause significant wind transport. The most likely location to find wind slabs are along high elevation ridgelines, convex roll overs, and cross loaded gullies.

Mountain winds are influenced by the shape of the surrounding terrain, so just because the overall wind direction is out of the NW doesn’t necessarily mean you will only find wind slabs on SE aspects. Paying attention to the texture of the snow surface and stepping out of the skin track to feel the snow surface can help you identify the local wind direction and locate areas likely to harbor fresh wind slabs. Using small test slopes can be a great way to check for wind loading and shooting cracks in fresh wind deposits. Safe travel practices, like travelling one at a time in avalanche terrain and always spotting your partners, can help to minimize the exposure of your group.

Active wind transport along the southern end of Seattle Ridge (Peak 4940) yesterday. Photo 11.25.22 from Andy Moderow

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
  • Aspect/Elevation
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    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.
Aspect/Elevation of the Avalanche Problem
Specialists develop a graphic representation of the potential distribution of a particular avalanche problem across the topography. This aspect/elevation rose is used to indicate where the particular avalanche problem is thought to exist on all elevation aspects. Areas where the avalanche problem is thought to exist are colored grey, and it is less likely to be encountered in areas colored white.
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

At upper elevations a layer of facets on the ground is still a cause for concern because we are still dealing with a relatively shallow early season snowpack. On Sunburst yesterday the overall snowpack depth was highly variable and triggering this buried persistent weak layer from a thin spot in the snowpack is possible. Our test results did not indicate that the layer was very reactive yesterday, but there have been observations of reactive tests on this layer within the past couple days (ob here). If an avalanche releases on this layer it could be very large and destructive. The last example of a possible avalanche on this deeper weak layer was observed in the Girdwood valley outside our forecast zone on 11.24.22 (ob here).

There have also been some observations of a layer of facets failing on top of the melt freeze crust that formed prior to the new snowfall this week. We have not observed any avalanches on this layer yet, but it is a layer that we will be tracking. With cold and clear weather expected for the next week the interface between the crust and the new snow may develop into a weak layer.

We observed a failure at the interface of the new snow and melt freeze crusts. There was no propagation in this snowpit but others have observed more reactive test results. Photo 11.25.22

Weather
Sat, November 26th, 2022

Yesterday: Snowfall tapered off in the early morning hours on Friday with about 4″ of very low density snow falling across the forecast area. Clouds started clearing out in the morning as winds speeds increased. Heavy flagging on the high peaks indicated strong winds, however some lower elevation areas were more sheltered from the NW winds. Temperatures gradually decreased throughout the day are in the teens to low twenties at upper and mid elevations this morning.

Today: Moderate to strong winds out of the NW persist throughout the day today. Expect averages of 20-35 mph and gusts of 40-50 mph in exposed upper elevation terrain. Temperatures will continue to drop throughout the day and should be in the single digits to teens by the end of the day. Skies should stay clear during the cold and windy outflow period.

Tomorrow: Sunday should be very similar to Saturday, with clear skies, cold temperatures, and wind. Wind speeds are expected to decrease slightly on Sunday with averages in the 15-20 mph range and gusts of 30-40 mph in exposed upper elevation terrain. Temperatures should remain in the single digits before dropping further on Monday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 24 0 0 27
Summit Lake (1400′) 23 0 0 15
Alyeska Mid (1700′) 24 0 0 22
Bear Valley (Portage) (132′) 31 0 0 –

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 13 NW 5 34
Seattle Ridge (2400′) 19 NW 12 30
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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.
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