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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, December 13th, 2023 - 7:00AM
Expires
Thu, December 14th, 2023 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger remains CONSIDERABLE in the higher elevations (above 2,500′). Another round of east winds and 2-6″ of new snow will make wind slab avalanches the main concern. Natural wind slabs 1-2′ deep will be possible and human triggered slabs likely on slopes with active wind loading. The danger is MODERATE at elevations below 2,500′ where the winds will be less, but still could form slabs in exposed areas. Additionally, a weak layer is buried 3-4′ deep in our minds. It has not produced big avalanches yet, but something we are watching. Sticking to the treed zones in the mid elevations with nothing steeper above is a good option for avoiding these issues.

Special Announcements

The NWS has issued a Special Weather Statement:   “…BANDS OF MODERATE TO HEAVY SNOW SHOWERS LIKELY ACROSS EASTERN KENAI PENINSULA AND EASTERN TURNAGAIN ARM THROUGH THURSDAY MORNING…”

Chugach State Park:  Between 8-10″ of snow fell yesterday along Anchorage’s Front Range, increasing avalanche conditions.

Wed, December 13th, 2023
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
Thu, December 14th, 2023
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
Avalanche risk
Thu, December 14th, 2023
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
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

The last known avalanches were those that occurred during the short but intense storm on Sunday night into Monday. During this event, 1-2′ of snow fell with several small to large storm snow avalanches observed across the forecast area. However, visibility has been hampered to get a really good look around. If you happen to see evidence of large avalanches please send us a note.

Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • Aspect/Elevation
  • 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.

Aspect/Elevation of the Avalanche Problem
Specialists develop a graphic representation of the potential distribution of a particular avalanche problem across the topography. This aspect/elevation rose is used to indicate where the particular avalanche problem is thought to exist on all elevation aspects. Areas where the avalanche problem is thought to exist are colored grey, and it is less likely to be encountered in areas colored white.

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

After 4-10″ of low density snow fell yesterday, unsettled weather is forecast again today bringing intermittent snow showers and cloudy skies. However, models are showing a chance for some clearing midday… Regardless of the 2-6″ that might fall at varying times, it will be the ridgetop winds that will be the main player today for avalanches. These are forecast to blow 10-20mph from the east with gusts near 30mph. Hence, it’s those upper elevation slopes, above the trees, that will be the most likely place to find and trigger a wind slab. Watching for active wind loading and any signs of where the winds have deposited snow will be key. Slopes over 30 degrees with stiffer snow over softer snow, or where cracks are shooting out from you, have the potential to avalanche. Winds slabs are likely to be in the 1-2′ deep range but could be thicker in areas seeing the strongest winds.

Loose snow sluffs:  With that 4-10″ of light snow yesterday and another 2-6″ possible today, sluffs on steeper slopes will be likely. In the big steep terrain, which is not recommended for traveling today, small chunks of cornice or a wind slab could entrain a good degree of loose snow, making a small avalanche quite large.

 

Tincan Trees yesterday after 6″ of new low density snow was reported. Reports were visibility and recent heavy snow with winds kept folks in the trees and letting the higher elevations be. 12.12.23.

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

We have been keeping an eye out for any evidence that the storm Sunday night may have been enough of an impact to trigger avalanches on a growing weak layer that sits on top of the Thanksgiving Crust (3-4′ deep at this point). Data so far is pointing to no avalanches failing on that layer from the storm. Which says it’s not that likely a person will trigger a big slab this week, but there is high uncertainty with that. It also can be that the weak layer isn’t weak enough yet, and maybe the next storm will show its cards. Either way, we need to remember that dragon exists in the basement because an avalanche on that layer could be huge. It’s also good to remember that prior to this storm, there was a remotely triggered avalanche on Eddies at 2200’ that failed on this layer. This is the only avalanche we know of that has released in those facets above the crust. To avoid this problem stick to lower angle slopes and be aware of runout zones below steep terrain.

Snowpack near the top of trees on Tincan just before Sunday night’s storm. 12.09.23.

Weather
Wed, December 13th, 2023

Yesterday:  Mostly cloudy skies with light to moderate snowfall was over the regions yesterday. Snow fell to sea level as temperatures were in the teens and 20’sF. Between 4-8″ of fell in most areas. Ridgetop winds were light and variable through the day before picking up from the east overnight.

Today:  Mostly cloudy skies and intermittent snow showers are expected today. Around 2-6″ of snow is expected with the heavies periods around sunset. Ridgetop winds are blowing 10-20mph with gusts near 30mph from the east this morning and look to remain in this range today. Temperatures are staying cool enough for snow to sea level (teens to 20’sF).

Tomorrow:  A short break in weather will be over the region tomorrow, Thursday, into Friday. Skies may clear up a bit with a few scattered snow showers possible. Ridgetop winds should be 5-10mph from the east with gusts near 20. The next round of stormy weather is forecast to move in Saturday night into Sunday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 19 4 0.3 65
Summit Lake (1400′) 19 4 0.3 34
Alyeska Mid (1700′) 20 8 0.65 56
Bear Valley – Portage (132′) 24 10 0.8
Grouse Ck – Seward (700′) 25 2 0.1 32

 

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 13 var 4 36
Seattle Ridge (2400′) 15 SE 1 14
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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.