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Issued
Tue, December 26th, 2023 - 7:00AM
Expires
Wed, December 27th, 2023 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger remains CONSIDERABLE above 2,500′ and is MODERATE below 2,500′. A bump in northwest winds this morning and southeast winds this afternoon may be enough to drift snow into fresh wind slabs on a variety of aspects. Additionally, there is still a lot of uncertainty as to how well the storm snow from the weekend has bonded. Therefore, triggering a slab avalanche 2-3′ deep is possible. This type of avalanche could also be triggered remotely, from the top, side, or below a slope.

A cautious mindset is recommended along with keeping a close eye out for red flags, such as whumpfing in the snowpack.

Special Announcements

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Tue, December 26th, 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
Wed, December 27th, 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
Wed, December 27th, 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

The only avalanches we heard of yesterday were two glide releases on Seattle Ridge, one of them pictured below. There were no known human triggered avalanches. The last avalanches seen/reported were from Saturday and Sunday when a widespread natural avalanche cycle occurred during the Holiday storm that deposited 2-3 feet of snow with strong east winds.

Recent glide avalanche, likely released Dec 24 or Dec 25. Photo by Andy Moderow 12.25.23.

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

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

It was two days ago this morning when the Holiday storm ended along with the end of the dramatic natural avalanche cycle. Because there were so many avalanches from Girdwood to Turnagain Pass and to Summit Lake, we are suspect that the storm snow didn’t bond well and could still be an issue. With a bump in winds today, there will be two avalanche issues in the top 3 feet of the snowpack:

Lingering Storm Snow Avalanches:  The Holiday storm snow fell on loose older snow with a layer of surface hoar on top. That surface hoar is now buried 2-3′ deep and we don’t yet have a good handle on how big of a problem it may be in the snowpack. Yesterday on Tincan, a skier around 3,000′ in elevation triggered a whumpf big enough that their partner felt it along with another group 300′ away. This is bulls eye data that the snowpack was not stable and the group stuck to low angle terrain. If this type of red flag continues, then we have to expect we can trigger avalanches on any aspect at elevations above 1,500′, where that new crust disappears. It might be too deep to do simple hand pits to assess this layer, and even if we could these types of layers are often hard to assess anyhow. Sticking to the smaller terrain and mellower slope angles, away from large committing slopes, is a good way to stack the odds in our favor. Please if you get out, let us know what you see, or don’t see!

New Wind Slabs:  There is some discrepancy with the weather models on how strong the NW winds will be this morning and the SE winds this afternoon. The key will simply be to pay close attention to any wind effect on the snow surface and/or if you are seeing active wind loading. Fresh wind slabs should be shallow, maybe up to a foot thick. However, this issue will be forming on top of the storm snow issue just mentioned. Hence, any new wind slab could be more dangerous than expected if it steps down and triggers a larger storm slab below.

The storm warmed up on Sunday and rained as high as 1,500′ in places. Reports yesterday stated that a rain crust sits under 2-6″ of loose snow and goes away around 1,500′.

Tincan on Christmas Day during a moment of sun hitting Seattle Ridge. Not a lot of folks were out and parking, as of yesterday afternoon, was still very limited and many areas remained unplowed. 12.25.23.

Avalanche Problem 2
  • Glide Avalanches
    Glide Avalanches
  • Aspect/Elevation
  • 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.

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

Glide cracks are opening up and continue to release every now and again. There were two new glide avalanches seen on Seattle Ridge yesterday that likely released in the past two days. As the photo below shows, some ridgelines and slopes are littered with cracks. If you see these, it’s always best to avoid being under them. They can release into an avalanche at anytime.

Glide cracks covering the steeper slopes of the north end of Seattle Ridge. Photo taken from Tincan by Andy Moderow, 12.25.23. 

Additional Concern
  • Deep Persistent Slabs
    Deep Persistent Slabs
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.
More info at Avalanche.org

Sitting under 4-6 feet or more of snow is that old Thanksgiving crust with facets that were, and maybe still are, forming around it. With hampered visibility again yesterday, we are still waiting to see if any of the avalanche activity from the Holiday storm broke in those facets on the crust. This would have created a very large slide with a big crown face. Due to the depth, the layer is not only very difficult to assess at this point, but also would be very difficult to trigger. We are not forgetting about it however and will continue to look for any evidence that it is reactive.

Weather
Tue, December 26th, 2023

Yesterday:  Overcast to sunny skies were over the region yesterday. Ridgetop winds were light and variable, except when they picked up from the west a hair last night (5 gusting 15mph from the west). Temperatures were cold (Between -15 in Summit Lake to around 10F in Girdwood Valley).

Today:  Mostly clear skies with some high clouds are expected today. Models are showing ridgetop winds turning easterly midday and picking up into the 10-20mph range, possibly this afternoon. Temperatures should remain cold, in the single to minus single digits. Tonight there in a chance for a trace to a couple inches of light snow to fall.

Tomorrow:  Partly cloudy skies are forecast with a chance for valley fog on Wednesday. No precipitation is expected. Ridgetop winds should be light from the south and west in general. Temperatures look to warm into the teens on Wednesday before dropping to the single digits again later this week.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 5 0 0 82
Summit Lake (1400′) -3 0 0 N/A
Alyeska Mid (1700′) 8 0 0 72
Bear Valley – Portage (132′) 17 tr 0.07
Grouse Ck – Seward (700′) 10 0 0 51

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) -1 W 4 16
Seattle Ridge (2400′) 3 N/A N/A N/A
Observations
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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.