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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, November 26th, 2020 - 7:00AM
Expires
Fri, November 27th, 2020 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE today and tomorrow. Human triggered large avalanches remain likely above 1000′ on all aspects. Avalanches can be triggered by people on a slope or remotely, from the side or below. They could fail near the ground, taking the entire snowpack down the mountain. Cautious route-finding and conservative decision-making are essential.

UPDATE, Friday Outlook:  Another system will impact the region Friday keeping avalanche danger elevated. Watch for changing conditions with new snow and wind. Natural avalanches are possible late Friday if this storm verifies.

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Thu, November 26th, 2020
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
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
  • Persistent Slabs
    Persistent Slabs
  • 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.
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 another foot of snow falling yesterday, accompanied by very strong easterly winds, we are in a spooky spot today and tomorrow. There is the potential for a person on a snowmachine or skis to trigger a large, destructive avalanche. Unfortunately lurking beneath all this new snow are two buried weak layers. The storm on November 20th buried a layer of surface hoar mid-pack and there are weak (sugary) facets at the base. Both these layers produced large human triggered avalanches on Sunday. We have even more snow load now. Avalanches could break anywhere from 1-5′ deep, maybe even deeper.

With better visibility today and tomorrow it will be tempting to push it. It is crucial to keep it mellow and give the snowpack time to adjust. Be on the lookout for signs of instability. Recent avalanches, whumpfing/collapsing and shooting cracks are all indications that the snowpack is stressed. Ease into terrain. Avalanches could be triggered remotely today, from the side or below. In addition, it might not be the first rider or skier on the slope that triggers the avalanche. There may be tracks already and the 5 person out tips the balance. Choose terrain wisely, have escape routes planned and watch out for other groups in the area. Sticking to low angle slopes, and out from under larger slopes, is a great way to enjoy the new snow and make it home for some Thanksgiving dinner.

Four feet of snow on the ground at 1000′ means there could be avalanches 4′ deep or even deeper. It also means there is lots of fun to be had in lower angle terrain today, out of harm’s way. DOT stake. 11.25.20

Weak snow at the base of the snowpack is one of the main concerns today. 

View of the Hippy Bowl remote triggered avalanche from 11.22.20. There is the potential for this type of avalanche today but even larger and more destructive.

 

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

Yesterday’s storm was ‘upside down’. Snowfall started with slightly colder temperatures and then warmed up creating an 8-10″ layer of heavier snow over lighter snow. Today there is the potential for the denser layer to behave like a slab.  In addition, the snowfall was accompanied by strong easterly winds. Keep your eyes peeled for areas winds have deposited snow, pillowed surfaces, stiffer wind packed snow and shooting cracks. Triggering a storm slab or wind slab avalanche in the upper layers of snowpack could step down and trigger a much larger avalanche that breaks in buried surface hoar mid-pack or in weak snow near the ground.

Snowpack @ 1000′ near moto-lot. The top layer shows Wednesday’s storm interface. The bottom layer shows buried surface hoar above a decomposing crust. 11.25.20

Weather
Thu, November 26th, 2020

Yesterday: Rain and snow throughout the day with rain/snow line around 500′. A foot of new snow (1.0″ SWE) fell across the advisory area. Winds were easterly 20-50 mph gusting into the 90s. Winds eased off in the in late afternoon. Temperatures were in the mid 20°Fs to mid 30°Fs. Overnight there were snow showers, easterly winds 10-20 mph with gusts into the 30s. Temperatures were in the high teens to high 20°Fs.

Today: Skies will be partly cloudy to mostly sunny and winds will be light and easterly. Temperatures will be in the high teens to high 20°Fs. Clouds will build in the evening with snow showers overnight and 2-5″ in the forecast. East winds 5-15 mph gusting into the 30s and temperatures in the 20°Fs.

Tomorrow: Skies will be partly cloudy and scattered snow showers are possible. Winds will be mostly calm and temperatures will remain in the 20°Fs. Another round of snow is on tap for Saturday. Fun quote from the NWS discussion this morning, ‘There is seemingly no end in sight to the supply of large deep storm systems traversing the Alaska region’. #snowtosealevelplease

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 31 7 1.0 44
Summit Lake (1400′) 31 6 0.6 17
Alyeska Mid (1700′) 31 13 0.9 48

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 21 NE 26 95
Seattle Ridge (2400′) 24 SE 10 85
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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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