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Issued
Fri, December 10th, 2021 - 7:00AM
Expires
Sat, December 11th, 2021 - 7:00AM
Forecaster
John Sykes
High Avalanche Danger
Travel in avalanche terrain is not recommended. Avoid being on or beneath all steep slopes.
Avalanche risk The Bottom Line

The avalanche danger will remain HIGH today because we received up to 2′ of new snow in the past 24 hours, with most of that falling in the last 12 hours. Large human triggered avalanches 2-4′ deep will be very likely in the new snow, especially in areas that received recent wind loading. It is likely that avalanches in the new snow could step down to weak layers 4-6′ deep creating very large and destructive avalanches that could run into the valley bottoms. Travel in avalanche terrain is not recommended.

PORTAGE/PLACER VALLEYS: These areas also saw heavy snowfall over the past 24 hours which fell on top of a weak existing snowpack. Avalanches at the interface of the new snow and the old snow surface are very likely and very large avalanches on buried weak layers are likely.

SNUG HARBOR/LOST LAKE: Weather stations in Seward and Snug Harbor are showing up to a foot of new snow in the past 24 hours, which is a big load on top of a weak existing snowpack. Conservative terrain selection and awareness of steep slopes above you are essential.

Special Announcements
Fri, December 10th, 2021
Alpine
Above 2,500'
4 - High
Avalanche risk
Treeline
1,000'-2,500'
4 - High
Avalanche risk
Below Treeline
Below 1,000'
4 - High
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

We have received 12 to 24″ of new snow in the past 24 hours, with the majority of that falling in a 12 hour period starting in the mid-afternoon yesterday. Turnagain pass saw the most snow with around 24+”, Girdwood 17″, and Summit Lake 2-4″. That is a big load to add to the snowpack in a short amount of time which could result in avalanches up to 2′ deep at the boundary between the new snow and old snow surface. The temperatures stayed below freezing throughout this storm so we can expect these storm slabs to exists on all aspects and elevations today. As the storm moves out this morning the potential for natural avalanches will decrease while human triggered avalanches will remain very likely.

In addition to the new snow we have a deeply buried layer of facets that has been reactive to human triggering (see problem 2). An avalanche triggered in the storm snow has the potential to step down to this deeper layer and produce a much larger avalanche.

Wind Slabs: In addition to the new snow load we also had high winds yesterday, with sustained ridge top winds in the 50’s mph and gusts into the 80’s during the initial pulse of heavy precipitation. Areas exposed to the wind could have deeper and stiffer slabs and produce avalanches 2 to 4′ deep.

Full on storm day yesterday afternoon as we exited Eddies

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

A layer of weak faceted snow buried 4 to 6′ deep has been producing a lot of large human triggered avalanches in the past week (recent observations: here, here, here) . The consequences of triggering an avalanche on this layer are increasing with every additional snow load we get on top of it. The rapid addition of a new load in the past 24 hours onto this buried persistent weak layer could make it more reactive to human triggering today. This is a scary snowpack setup which has shown that it can be triggered by a human and is capable of remote triggering.

Snowpack discussion from Eddies on 12.9.21

Weather
Fri, December 10th, 2021

Yesterday: Conditions were stormy with moderate to heavy snow throughout the day and high winds. A total of 12-24″ of new snow fell down to sea level.  Winds were primarily out of the east ranging from 20-35 mph with gusts into the 80’s. From the afternoon to early evening ridge top winds were sustained in the 50’s with gusts into the high 70’s to 80’s. Temperatures stayed below freezing, ranging from the teens to high 20’s.

Today: The precipitation should slow down today and winds will dye down as well. An additional 1-4″ of snow could throughout the day. Winds will range from 5-20 mph and the wind direction will shift to north and northwest as the precipitation slows down this morning. Temperatures will also start to drop throughout the day moving from the teens to single digits.

Tomorrow: We are entering another period of cold, dry, and calm weather for the next few days. The coldest days look like Sunday and Monday with lows well into the negatives. Winds should be light out of the northwest during this cold period.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 26 23 1.9 89
Summit Lake (1400′) 22 2 0.2 25
Alyeska Mid (1700′) 25 17 1 58

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 15 ENE 28 83
Seattle Ridge (2400′) 18 NA NA NA
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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.