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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, December 6th, 2021 - 7:00AM
Expires
Tue, December 7th, 2021 - 7:00AM
Forecaster
Andrew Schauer
High Avalanche Danger
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 as strong winds and heavy snowfall overload a weak snowpack. Large natural and human triggered avalanches 3-6′ deep or deeper are very likely today, and will run long distances into valley bottoms. Travel in avalanche terrain is NOT recommended. This includes flat terrain below steeper slopes.

*Roof Avalanches: Warm temperatures and heavy rain could cause roofs to shed the snow that has been stacking up for the past month. Pay attention to children and pets, and be mindful of where you park your vehicles.

PORTAGE/PLACER: These areas will see heavier snowfall than the Girdwood and Turnagain zones, with 1-2′ by tomorrow morning. Very large avalanches are likely.

SUMMIT LAKE: This will be the first major loading event on the weak snowpack in the Summit Lake area, creating the potential for large natural avalanche activity.

SNUG HARBOR/LOST LAKE/SEWARD: These zones are expected to see the heaviest snowfall during this storm, with over 2′ of snow expected by tomorrow morning.

Special Announcements
  • The National Weather Service has issued a Winter Storm Warning through Tuesday morning for Girdwood, Whittier, Moose Pass and Seward.

  • There will be intermittent traffic delays Monday, December 6, 2021 on the Portage Glacier Road for Avalanche Hazard Reduction work. Motorists should expect delays of up to 45 minutes to 1 hour between 11 AM and 12:00 PM. More info at https://511.alaska.gov/

Mon, December 6th, 2021
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
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
  • Girdwood: Multiple natural avalanches ran in paths along the Seward Highway between Bird Point and Peterson Creek as strong winds loaded start zones yesterday. Many of these ran to sea level. Skiers on Notch easily triggered avalanches in the new snow on small and steep terrain features.
  • Turnagain Pass: We received word of a skier-triggered wind slab avalanche on Eddie’s yesterday. We do not have any details on the size of the avalanche, but it sounds like nobody was caught or carried.

 

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

The much-anticipated storm arrives in force today, with strong winds and heavy snowfall expected through tonight. Some areas (Portage/Placer and Seward) may see 2-3′ of snow, while the core of the advisory area is going to be on the low side of storm totals, with only 4-8″ snow expected by tomorrow morning. We saw natural avalanches yesterday as winds were blowing 15-35 mph with gusts up to 68 mph. We can expect more today as snowfall picks up and winds increase to the 20-40 mph range, gusting 40-60 mph. This combination of new snow and strong wind will make sensitive slabs that fail naturally today, and are likely to step down to deeper weak layers in the snowpack, resulting in very large avalanches (more on this in problem 2 below). These avalanches are expected to run long distances into runout zones at lower elevations. Travel in and below avalanche terrain is not recommended today, including nordic and hiking trails that will be exposed to avalanche hazard from above.

Skiers saw sensitive conditions on Notch yesterday as winds picked up, with multiple shooting cracks and soft slabs releasing on small terrain features like the one pictured above. Photo: Kakiko Ramos Leon. 12.05.2021

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

Although this storm may be somewhat unremarkable on its own by Chugach standards, it will likely be the extra load needed to make very large avalanches on an already dangerous snowpack. Strong winds and new snow will add a lot of weight to a snowpack supported by multiple weak layers, likely pushing them to their breaking point. As we saw towards the end of last week, these weak layers were already capable of producing very large avalanches (take a look at this avalanche from Eddie’s or this one from Magnum for a reminder).

These weak layers have been propagating long distances, and people have been triggering avalanches remotely- from above, below, and to the side of steep slopes. With the added potential for natural triggers, avalanche conditions are very dangerous right now. Very large avalanches are expected to run into flat terrain below steeper slopes, which means users like hikers, bikers, and nordic skiers need to be aware of the overhead hazard. Long story short, today is really not the day to try to get out in the mountains.

Large natural avalanches like this one on the Karlsberg path on Orca from last week are on our mind today. Similar activity is expected as winds continue to load a dangerous snowpack. 

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

As if we didn’t already have enough to worry about with wind slabs, storm slabs, and persistent weak layers, we are also still thinking about the potential for large glide avalanches. Glide avalanches are impossible to predict, but the rapid warm up over the past 24 hours is not likely to improve the glide condition.

Weather
Mon, December 6th, 2021

Yesterday: Strong winds were blowing 15-35 mph, with gusts to 68 mph at Alyeska and up to 59 mph at the Sunburst station. Temperatures have climbed steadily from the mid 20’s yesterday morning to the low 30’s this morning. Snow started falling last night, with around 3-4″ snow at mid and upper elevations as of 6:00 this morning.

Today: Snow is expected to pick up today and continue through tonight. Totals will be around 3-5″ snow near Girdwood and Turnagain Pass during the day and 8″-12″ by tomorrow morning. Heavier snow is expected near Portage and Seward, with 1-2′ expected near Portage and 2-3′ near Seward. Temperatures are expected to hover in the mid 20’s to low 30’s F, with strong easterly winds blowing 20-40 mph and gusting 40-60 mph.

Tomorrow: We will see an abrupt pattern change tomorrow as the low pressure system exits to the east. Winds are expected to shift, blowing 15-25 mph from the northwest. Clouds will start to lift through the day, and temperatures will drop into the low teens to single digits F. Snow is expected to taper off early tomorrow morning.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 27 4 0.4 60
Summit Lake (1400′) 21 3-4 0.3 N/A
Alyeska Mid (1700′) 25 6 0.48 36

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 17 ENE 23 59
Seattle Ridge (2400′) 21 SE 15 28
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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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