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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, November 20th, 2021 - 7:00AM
Expires
Sun, November 21st, 2021 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

Today’s avalanche danger is MODERATE above 2500′. It will be possible for a person to trigger an avalanche in steep, wind loaded terrain. Wind slabs that formed in the middle of the week are sitting on weak snow, and we may see another round of wind building fresh slabs today. Identify and avoid steep slopes with stiff snow on the surface, which will be most likely just below ridgelines, on steep rollovers, and in gullies.

The danger is LOW below 2500′, where the snow surface has been sheltered from the wind and it is unlikely a person will trigger a slab avalanche. Be aware of the potential for dry loose avalanches, with close to a foot of loose faceted snow at the surface.

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Sat, November 20th, 2021
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
1 - Low
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.
Recent Avalanches

Seattle Ridge: During the last two days we’ve seen a few natural wind slab avalanches releasing in wind-loaded start zones along Seattle Ridge. These look to have been less than a foot deep and around 50′ wide, some of which may have been big enough to bury or injure a person.

Turnagain Pass: We continue to see fresh glide activity in the area, with recent releases on Seattle Ridge, the Library, and Raggedtop, as well as many other slopes in the periphery of our core advisory area. These have been releasing on all aspects between 2,000′ and 3500′ elevation.

Fresh glide release below the Library. Photo Peter Wadsworth. 11.18.2021

Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • 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

Today our main concern is triggering an avalanche on steep, wind loaded terrain where a stiff slab of snow is sitting on top of weak facets. After a week of cold and clear weather, the upper 12-18″ of our snowpack has transitioned to weak, faceted snow. This includes a layer that has started to form above the Halloween crust at elevations up to about 3100′ and a layer of surface hoar that was buried during the Veterans Day storm, now about 6″ deep. At elevations above treeline, the wind event from the middle of the week loaded certain slopes, putting a stiff slab on top of that weak snow. In the past few days we’ve found this avalanche-prone setup on specific terrain features just below ridgelines and in upper-elevation gullies. Because these wind slabs are sitting on top of persistent weak layers, we can expect to see lingering instability a little longer than we’d typically expect for a wind slab problem. Winds are expected to bump up slightly during the day today, which means we might see a fresh round of slab building in those same upper elevation start zones.

Luckily this problem is fairly easy to recognize and avoid. Be on the lookout for stiff, punchy snow on the surface. You might recognize a wind loaded slope by looking for a smooth pillow of snow adjacent to more textured or scoured surfaces. These wind slabs can also give you some warning signs like shooting cracks or collapsing. As always, recent avalanche activity is one of the best indicators that it is possible to trigger an avalanche.

Two snowpits showing very similar structure on Tincan yesterday. This is consistent with what we have seen around Turnagain Pass in the past week. 11.19.2021

A pillow of wind loaded snow just off of the Sunburst ridgeline (foreground), and a cross-loaded gulley in the middle ground. 11.18.2021

Avalanche Problem 2
  • Glide Avalanches
    Glide Avalanches
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to 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 are still seeing glide avalanches release, including one avalanche on Seattle Ridge that occurred sometime between Thursday afternoon and Friday morning. All of the glide activity has been between 2,000′ and 3,500′ elevation. These avalanches are large and unpredictable. The only way to manage the problem is to limit the time you spend under glide cracks.

Glide avalanche that released sometime after 2 p.m. Thursday. 11.19.2021

Weather
Sat, November 20th, 2021

Yesterday:  High temperatures made it up to the single digits F, with overnight lows in the single digits to low teens below zero. Skies were clear, and winds were light out of the east.

Today:  Winds are expected to pick up slightly, blowing 5-10 mph out of the east with gusts to 15-20 mph. Skies will remain mostly sunny, with increasing cloud cover in the afternoon. Temperatures are expected to remain cold, with highs in the single digits to maybe 10 F.

Tomorrow:  Active weather moves back to the area tomorrow, with snow in the forecast. For now it is looking like Turnagain Pass will be on the low side of precipitation totals, with around 6″ expected. But there is some uncertainty in how the storm will develop and the high end of the estimates put Turnagain Pass at 12-18″ new snow. Stay tuned for more, and keep your fingers crossed! The good news is it is looking like we will see snow to sea level.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 1 0 0 42
Summit Lake (1400′) -6 0 0 10
Alyeska Mid (1700′) 4 0 0 38

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 6 E 6 15
Seattle Ridge (2400′) 7 SE 5 10
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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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