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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, December 11th, 2023 - 7:00AM
Expires
Tue, December 12th, 2023 - 7:00AM
Forecaster
Andrew Schauer
Avalanche Warning
Issued: December 10, 2023 6:00 pm
Travel in avalanche terrain is not recommended. Avoid being on or beneath all steep slopes.
Avalanche risk The Bottom Line

The avalanche danger is HIGH today. We received 1-1.5 feet of snow during the past 12 hours, making it very likely a person can trigger a very big avalanche today. The storm is passing this morning, but fresh storm and wind slabs will be extra touchy during the day, and there is the potential for very large avalanches failing deeper in the snowpack running far into flat runout zones. If you get out today, be careful to avoid traveling on or below steep terrain.

We have issued an Avalanche Warning through the National Weather Service for the areas around Girdwood, Portage, Turnagain, Summit Lake, Moose Pass, and Seward.

Special Announcements

Become a Member in December! The Friends of the Chugach Avalanche Center is a non-profit, which means we need your help to keep our avalanche center running. This is our most crucial time of year for fundraising, so if you haven’t yet, please consider becoming a member. Did you know that nearly half of our Forest Service avalanche specialists’ positions are funded by community donations to the Friends, as well as 100% of the new Chugach State Park Avy Specialist’s position? Everyone who donates will be entered to win some awesome prizes (like Dynafit bindings, a Voile splitboard, and more!) at our Girdwood Brewery Forecaster Chat on January 19.

Mon, December 11th, 2023
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
Tue, December 12th, 2023
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
Avalanche risk
Tue, December 12th, 2023
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
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 human-triggered avalanche was Saturday on Eddie’s Ridge, where a skier remotely triggered a small but deep pocket failing on the Thanksgiving crust. The avalanche was around 2 feet deep on average and 50 feet wide. Nobody was caught or carried. It is very likely there was natural avalanche activity last night; we will hopefully be able to get a look at some of the activity today.

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

A fast-moving storm brought around 1.5 feet of snow to the mountains near Girdwood and Turnagain Pass over the past 12 hours, with only 4-6″ near Summit and Seward. The heavy snowfall was accompanied by very strong easterly winds, with average speeds of 50-78 mph and gusts as high as 117 mph recorded on Sunburst last night. While this is less snow than we were predicting, it is still a major load that will likely produce big avalanches. Light precipitation should pass quickly this morning, but the avalanche conditions will remain very dangerous through the day today. 

This period right after the storm is the most common time for people to get in trouble. New storm and wind slabs will be very tender and reactive, making large to very large human-triggered avalanches very likely, even if we aren’t seeing widespread natural avalanches during the day. A few things that make this storm particularly scary are the way the snow came in and the snowpack it was falling on. The storm started cold and finished warm, which means even in areas that weren’t affected by the winds (which may be few and far between), there will be a layer of denser snow sitting on top of a layer of softer snow. That will be more likely to produce avalanches. In addition to the high likelihood of avalanches failing within the new snow, this storm also just added a lot of stress to a buried weak layer, which may have the potential to produce destructive avalanches (see Problem 2 below for more on this).

Large avalanches failing in upper elevations will have the potential to run far distances into low-angle runout zones. If you are planning on getting out to check out all of this new snow, be sure to avoid traveling on or below steep terrain today. Even places like the motorized uptrack or the south face of Sunburst that are often relatively benign have the potential to produce big avalanches today.

Last night was intense- stations recorded wind speeds of up to 117 mph, with almost 1.5 feet of snow falling in just 12 hours. Data from the Sunburst weather station (top) and Center Ridge Snotel site (bottom).

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

The layer of faceted snow on top of the Thanksgiving crust has been a big question mark for us over the past week, and this loading event will be the first big test of that weak layer. Immediately prior to the storm, a skier triggered a small but deeper avalanche failing on this layer on Eddie’s Ridge. We also received a report of a large natural avalanche that likely failed on this layer about a week ago on the back side of Seattle Ridge. Based on what we’ve been seeing in snowpits, the layer is most concerning in middle and low elevation bands, which will see the most traffic today. There is a lot of uncertainty with just how reactive this weak layer will be today. Given the depth (it’s now 3-4′ deep on slopes that haven’t been wind-loaded) and the fact that the crust/facet layer is very well connected across terrain features, if the layer does awaken it has the potential to produce massive avalanches. These can be triggered remotely- which means you could trigger an avalanche from flat terrain that is connected to steeper terrain.

The high uncertainty and high consequences of triggering one of these avalanches are very concerning. Conditions will be dangerous today, and travel on or below steep terrain is not recommended.

This skier-triggered avalanche on Eddie’s from Saturday may be a precursor of things to come, or it may be an outlier. Time will tell, but with 1.5″ water adding stress to the snowpack today is not the day to test the waters. Photo: Andy Moderow, 12.09.2023

Weather
Mon, December 11th, 2023

Yesterday: Last night’s storm brought 1-1.5 feet of snow to Girdwood and Turnagain Pass, and 4-6” to Summit and Seward. The storm started with snow to sea level, with rain or mixed precipitation up to 700-1000 feet as precipitation is finishing this morning. Ridgetop winds were howling at 50-78 mph, with gusts as high as 117 mph recorded on Sunburst late last night. Temperatures were coldest yesterday morning in the single digits to mid teens F, warming through the day and last night to the mid 20’s to low 30’s F this morning.

Today: Precipitation should shut off early this morning for most of the forecast area, with another 1-3” snow possible and the rain line creeping up to 1000-1500’ as the storm passes. Seward will be the exception, with more precipitation expected through today and around 10” possible at upper elevations. Winds should back down but still blow 20-35 mph with gusts of 40-50 mph out of the south. The warmest temperatures will be this morning in the mid to upper 20’s F, dropping steadily into the teens F tonight. Skies should be mostly cloudy.

Tomorrow: We should see a break in the action tomorrow during the day before things pick up a bit overnight into Wednesday. Skies will be mostly cloudy with light and variable winds. High temperatures should be in the mid teens F, with temperatures rising slightly overnight as another round of snow moves in, bringing 4-6” by Wednesday morning and snow to sea level.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 20 16 1.5 67
Summit Lake (1400′) 21 4 0.3 30
Alyeska Mid (1700′) 22 15 1.14 52
Bear Valley – Portage (132′) 21 N/A N/A
Grouse Ck – Seward (700′) 23 6 0.4 32

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 16 ENE 49 117
Seattle Ridge (2400′) 19 SE 16 50
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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.