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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, December 21st, 2023 - 7:00AM
Expires
Fri, December 22nd, 2023 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger remains MODERATE at all elevations. Dry loose snow sluffs are the main concern. These are likely to be easy to trigger on steeper slopes and generate significant amounts of volume and run further than expected. Additionally, there is a rare chance a person could trigger a large avalanche on weak snow forming near a crust 3-5′ below the snow surface.

Special Announcements

Hatcher Pass:  HPAC forecasts on Thursday’s and Saturday’s, check out this morning’s avalanche forecast at hpavalanche.org.

Chugach State Park:  A bump in northwest winds today may form wind slabs in the 8-10″ of loose new snow that fell Tuesday night.

Thu, December 21st, 2023
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
Fri, December 22nd, 2023
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
Fri, December 22nd, 2023
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
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.
Recent Avalanches

With the clear skies yesterday, above the valley fog that is, evidence of many small to large storm snow avalanches were seen across the region. These were natural avalanches from Tuesday night’s snowfall (6-12″). Also of note, some larger slabs that look to have released on a deeper weak layer (suspecting the Thanksgiving Crust) were seen. One in Girdwood Valley on the shoulder of Goat Mtn and several others in the Summit Lake area. Both these zones have a thinner snowpack than Turnagain. Any avalanche that breaks in old layers has our attention, see more on that in Problem 2 below.

As far as human triggered avalanches, one skier reported being able to trigger high volume and long running loose snow sluffs at Turnagain Pass.

Avalanche Problem 1
  • Dry Loose
    Dry Loose
  • Aspect/Elevation
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Dry Loose
Dry Loose avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.

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

Happy Solstice!

For the first time in years, our main concern for avalanches in the top couple feet of the snowpack is loose snow sluffs. Ridgetop winds have essentially been zero’ed out over the past 24 hours, another rarity – check it out – Penguin Pk, Sunburst, Seattle Ridge, Max’s Mtn, and more.

That said, a light west to north wind should pick up today along ridgetops and blow in the 5-10mph range with gusts in the teens. This isn’t likely to move much snow around except along the highest peaks where some shallow wind slabs may form. AND, in the event the northwest winds pick up more than forecast watch for any areas that see active wind loading. There is ample loose snow on the surface that will be easy moved by wind.

Loose Snow Avalanches:  Sluffs should be just as easy to trigger today due to the cold temperatures keeping the snow surface loose and unconsolidated. Sluffs can gain a lot of debris and momentum on the steeper longer slopes and create a dangerous situation if someone isn’t prepared. Watch our sluff.

 

Sluffing on south facing Cornbiscuit yesterday. Photo by Peter Wadsworth 12.20.23.

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
  • Aspect/Elevation
  • 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.

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

As mentioned above, there were some large avalanches triggered by the storm on Tuesday night. One from Goat Mtn and a few others (no photos) from the Summit Lake area. These are something to clue into because they could be releasing in facets forming around that old buried Thanksgiving Crust. That crust is generally 3-5′ deep and exists region wide, as high as 4,000′ we believe (maybe higher). These two areas that have seen big avalanches have a thinner snowpack and therefore the weak layer could be weaker and generally more unstable. There are a lot of unknowns with this layer.

We do know that snowpits are showing the layer exists but it is hard to get it to fail. Check out Andrew’s video below. This means it’s unlikely that a person could trigger one of these large slabs, but again a lot of unknowns. For now we want to be sure everyone knows we are concerned about this layer and what it could do now and in the future, especially in those shallow snowpack zones. Being mindful of your travel routes and safe zones in the event a surprise avalanche is triggered is a good plan.

 

It has been quite a season so far! As of mid December, the Turnagain Pass SNOTEL has seen 172″ of snowfall, this is more than half the average seasonal snowfall. Photo from John Pearce who was at Eddie’s yesterday. 12.20.23.

Additional Concern
  • Glide Avalanches
    Glide Avalanches
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

Several glide avalanches have been seen scattered across the region over the past week. Keep a lookout, and avoid being under, glide cracks.

Weather
Thu, December 21st, 2023

Yesterday:  Mostly clear skies with varying degrees of valley fog was seen across the region. Ridgetop winds were either calm to very light from the north and west. Temperatures were in the teens at most locations.

Today:  Some high clouds have streamed in overnight and could linger through today. Valley fog is possible again today as well. Otherwise, mostly clear skies are forecast. Ridgetop winds should bump into the 5-10mph range, yet stay fairly light through the day. Temperatures look to remain cool, teens to single digits.

Tomorrow:  One last partly clear sky day is expected Friday before the Christmas storms roll in. Ridgetop winds will turn easterly and pick up through Friday along with the chance for clouds. Models suggest snow, wind, and stormy weather arrive Friday night and may continue well after Christmas. Stay tuned!

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 18 0 0 73
Summit Lake (1400′) 10 0 0 41
Alyeska Mid (1700′) 20 0 0 70
Bear Valley – Portage (132′) 24 0 0
Grouse Ck – Seward (700′) 21 0 0 46

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 15 0 2
Seattle Ridge (2400′) 17 0 0
Observations
Recent Observations for Turnagain Pass
Date Region Location
02/24/24 Turnagain Observation: TinCan Backdoor/ Center Ridge
02/22/24 Turnagain Avalanche: Lynx Creek
02/22/24 Turnagain Observation: Turnagain, Seattle, Mt Ascension
02/21/24 Turnagain Observation: Tincan Trees
02/21/24 Turnagain Observation: Sunburst
02/20/24 Turnagain Avalanche: Tincan
02/20/24 Turnagain Observation: Seward Highway across from Johnson Pass TH
02/19/24 Turnagain Avalanche: Base of Seattle Ridge
02/18/24 Turnagain Observation: Lynx creek
02/18/24 Turnagain Observation: Tincan Trees
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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.