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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, November 22nd, 2020 - 7:00AM
Expires
Mon, November 23rd, 2020 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE at all elevation bands. In the Alpine and Treeline, triggering a slab avalanche is likely. Avalanches may be triggered remotely. Avalanches that release in higher elevation terrain could send debris into the lower elevation (below 1000′) runout. Cautious route-finding and conservative decision-making are essential.

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Sun, November 22nd, 2020
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
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
  • 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

Viva La Niña! Winter is off to a promising start. However, caution is advised. There is a long winter ahead. Don’t let early season stoke get the best of you! Around 2′ of snow (2.0″ of SWE) has fallen in the mid to upper elevation terrain in Girdwood Valley and Turnagain Pass since Friday afternoon. Unfortunately this snow fell on weak surface snow (small facets and surface hoar) and there is weak snow near the ground (more on this in Avalanche Problem 2). In addition, there were strong sustained winds Friday evening into Saturday morning. Observers yesterday reported all the signs of instability. There were both natural and human triggered avalanches, large whumpfs and shooting cracks. This included avalanches large enough to bury a person in the Tincan Trees.

Light snow is falling this morning but there is a bit of a break forecasted between storms today. Don’t let a window of improving visibility lure you into bigger terrain. Human triggered avalanches are likely today. With the current snowpack structure there is the potential for avalanches to be triggered remotely. It’s a day to carefully evaluate terrain and consequences if an avalanche does release and pay attention to other groups around you. With more snow and strong winds expected again tonight and tomorrow, this is the time to be patient, the snowpack will need time to adjust to all this loading.

As a reminder, here are the Red Flags to look for:
– Recent avalanches
– Whumpfing (collapsing) of the snowpack
– Shooting cracks

New snow easily failing on the weak old snow interface. 11.21.20. Photo: Andy Moderow

Multiple avalanches on Tincan. 11.21.20. Photo: Tully Ward-Hammer

Avalanche Problem 2
  • 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

As mentioned above there is weak snow at the base of the snowpack. This means that not only are we concerned about an avalanche failing at the new snow/old snow interface but there is the potential for an avalanche to fail in the facets (sugar snow) at the ground. One of the large natural avalanches observed yesterday on Tincan Proper is believed to have run to the ground. We have been mentioning poor snowpack structure as a concern for a couple weeks and now there is an even bigger slab on top of the weak October snow. Triggering this set-up could produce a large and deadly avalanche. This type of avalanche (Persistent Slab) could be triggered remotely (from above, below or from the side of the slope). This set-up could be an issue for days or weeks to come and should be front and center when choosing what terrain to ride.

Slab over old weak snow on Seattle Ridge. 11.13.20. This now has another 1-2′ of snow on top.

 

 

Weather
Sun, November 22nd, 2020

Yesterday: Skies were cloudy with periods of heavy snow above 800′ and rain falling at sea level in the morning. Skies became broken in the afternoon. Winds were easterly 20-40 mph in the early morning gusting into the 60s and eased off late morning. Temperatures were in the low 30Fs at sea level to low 20Fs in the Alpine. Overnight skies were cloudy with snow showers, light winds and temperatures in the 20Fs to low 30Fs.

Today: Snow showers this morning becoming partly cloudy this afternoon. Light east winds and temperatures in the high 20Fs. Snow showers starting again in the late evening and east-northest winds picking up overnight. Temperatures in the mid 20Fs to low 30Fs.

Tomorrow:  Snow likely with east-northeast winds 25-35 mph gusting into the 40s and temperatures in the 20Fs to low 30Fs.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 30 10 1.0 34
Summit Lake (1400′) 29 2 0.2 11
Alyeska Mid (1700′) 31 10 1.12 37

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 21 NE 21 68
Seattle Ridge (2400′) 25 SE 9* 26*

*Seattle Ridge weather station is likely rimed and under reporting.

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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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