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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, December 15th, 2019 - 7:00AM
Expires
Mon, December 16th, 2019 - 7:00AM
Forecaster
Ryan Van Luit
Avalanche risk The Bottom Line

Today the avalanche danger is CONSIDERABLE in the Alpine where 1-3′ of snow has fallen over the last two days.  The primary concern is wind slab and cornice development in the lee of ridges and gullies where snow is likely being transported by recent winds ranging from 10-40 mph. Stay alert for signs of instability.  At Treeline the danger is MODERATE where triggering a wind slab or storm slab is possible.

Special Announcements

Headed to Hatcher Pass? Don’t forget to check hpavalanche.org and their Facebook page!

Join CNFAIC Forecaster Aleph Johnston-Bloom at Blue & Gold Boardshop tomorrow night, Dec 16th, 7:00-8:30 for a FREE evening avalanche discussion on patterns in Alaskan avalanche accidents with practical takeaways to use this season. There will also be an avalanche gear demo outside in the snow(weather permitting).

Sun, December 15th, 2019
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
0 - No Rating
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

No new avalanches observed.

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

Over the last two days 1-3′ of new snow has fallen in the Alpine and the winds have been sustained, blowing 10-40 mph.  This has likely created wind slabs 1-4′ deep in the lee of ridges and gullies. Yesterday at Tincan above 2500′, observers found snow depths varying from 3-9′ deep and obvious wind effect. Hand pit results were mixed. Some indicated good bonding between layers of snow and some produced easy shears with freshly formed wind slab.

If traveling to the higher elevations in search of dry snow today, caution is advised. Things to be on the lookout for:

  • Watch for wind loading patterns and areas with wind deposited snow.
  • Feel for stiffer snow over softer snow.
  • Is there any cracking in the snow around you? Hollow feeling snow?
  • Quick hand pits can be a good way to assess the top foot or so of snow. Remember, the weak interface may be lower than this and could be missed.

Storm slab avalanches:  Areas out of the wind could harbor slab avalanches that are simply made up of the storm snow that has not bonded yet with the older snow surface. Again being aware of any red flags in areas with new snow.

Cornices: Continue to develop –  give them plenty of space to prevent triggering from above, and limit exposure when traveling in the runout.

Wind transport on Pete’s South, Dec.14.

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

Glide cracks and glide avalanches remain a concern, especially in the Girdwood valley where many glide avalanches observed Thursday and Friday.  With limited visibility we’re still unsure how widespread this issue remains across the advisory area.  Yesterday, observers found evidence of a glide avalanche covered by some new snow and glide cracks at Tincan.  Glide avalanches are highly unpredictable. Give cracks a wide berth while traveling.

Glide activity on Tincan, 12.14.19. Photo: Ray Koleser

Additional Concern
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.
More info at Avalanche.org

In the Summit Lake area and other shallow snowpack zones that have seen less new snow, we are concerned about weak faceted snow that sits under the new snow from the past week. This is above 2,500′, where the snow is dryer. Before the storm cycles, faceted snow existed at the base of the snowpack and other layers of facets/crusts in areas. How these layers have adjusted over the past week of stormy weather and increasing snow load is something to pay attention to if headed to these areas.

 

Weather
Sun, December 15th, 2019

Yesterday: Mostly cloudy skies with temperatures ranging from 26-43*F, intermittent light rain up to 2000′, with intermittent snow above 2500′.  Light to moderate ENE winds.

Today:  Cloudy to mostly cloudy skies with temperatures ranging from 33-41*F at sea level.  Steady rain below 2000′ and snow accumulation of 2-6′ above 2500′. Light to moderate winds out of the ENE.

Tomorrow: Partly to mostly cloudy skies, temperatures from 30-40*F.  Intermittent rain and snow showers.  Winds out of the East.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 33 9 0.5 29
Summit Lake (1400′) 33 0 0 7
Alyeska Mid (1700′) 33 5 0.7 18

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 24 ENE 28 56
Seattle Ridge (2400′) NA* NA* NA* NA*

*Seattle Ridge weather station is down and as soon as the weather clears we will get it up and running!

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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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