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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, January 3rd, 2019 - 7:00AM
Expires
Fri, January 4th, 2019 - 7:00AM
Forecaster
Heather Thamm
Avalanche risk The Bottom Line

The avalanche danger is  MODERATE in the Alpine and Treeline zones for triggering a large slab 2+’ thick in steep terrain. Give cornices a wide berth and avoid being under glide cracks. Natural glide avalanches are possible today and could release without warning.

There is LOW avalanche danger below 1000′ where a surface crust has strengthened the snowpack.

SUMMIT LAKE / JOHNSON PASS / LYNX DRAINAGE:    South of Turnagain Pass, keep in mind old buried weak layers exist and there is potential for triggering a large slab avalanche that breaks near the ground.

Special Announcements

In Chugach State Park and Hatcher Pass dangerous avalanche conditions have been reported this week. Check out the Hatcher Pass Avalanche Information Center mid-week summary click  HERE and recent observations from the Front Range HERE.

Thu, January 3rd, 2019
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.
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

UPDATE: We just recieved a report of a large remote triggered avalanche in the Seattle Creek drainage that occured yesterday. We don’t have much info at this time and its unknown what layer this avalanche failed on. We do know that remote triggered avalanches are a sign of a persistent slab problem and it may be buried surface hoar. Keep this new info on your mind if you’re heading to Turnagain Pass.  

Cooling temps and light winds are helping to improve stability of wind slabs since the New Year’s storm dumped 2-3’ of snow and blasted our snowpack with hurricane force winds. Clear and sunny weather today will make it easy to identify smooth pillowed convexities, cross-loaded gullies, and hollow sounding snow – wind slab habitat. What makes this avalanche problem challenging is it’s transition into becoming a persistent slab. As this storm snow problem strengthens, a layer of buried surface hoar from Christmas remains on our minds. Hand pits yesterday were challenging to find this layer due to how deeply buried (2+’) it is in places. Many observations over the last week have been documenting the location of buried surface hoar and its presence and reactivity have been variable. With that said – triggering a wind slab on a mid-storm density change or on buried surface hoar are both possible today.

If you’re headed out, ease into steeper terrain with a conservative mindset. Evaluate terrain and snow as you travel and remember ‘whumpfing’ is an obvious clue of instability.

CORNICES:  There are some very large cornices along many ridgelines across our region. Triggering a cornice with the weight of a person or snowmachine is possible today. Remember these can break further back than expected. Give cornice features lots of space and avoid being directly under them.

 

A storm triggered slab from the New Years storm below Hippy Bowl on SW aspect of Tincan. Also note the wind sculpted snow in the alpine and cornice along the ridgeline.  

 

Avalanche Problem 2
  • Glide Avalanches
    Glide Avalanches
  • 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.
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 exist in popular ski and snowmachine terrain and a may be tricky to identify with new snow covering them. Several glide cracks have avalanched this week and it’s possible more will release in the coming days. The best way to manage this problem is to avoid being under slopes with cracks opening up. They can release at any time and are not typically associated with human triggers. Glide avalanche have occurred in Lynx Creek, on Lipps, and Seattle Ridge this week.

 

 

A glide avalanche on Lipps SW face that released just before the New Years Storm is not covered by new snow and looks very different

Photo of Lipps glide crack taken yesterday (1-2-19). Although part of this crack has released the additional portion can still avalanche without warning.  

 

Additional Concern
  • Persistent Slabs
    Persistent Slabs
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.
More info at Avalanche.org

South of Turnagain:  A shallow and poor snowpack structure exists in the Summit Lake zone. Buried weak layers of facets associated with crusts sit near the base of the snowpack. An observation from Lynx Creek on Friday also found a reactive layer of facets mid-pack. Keep in mind Summit Lake has received additional loading from the New Years storm – strong winds and a foot of new snow. Recent avalanche activity and ‘whumpfing’ will be good reminders to keep terrain choices conservative in these zones. Triggering a larger avalanche that releases near the ground is not out of the question. Check out the Summit observations HERE for the more snowpack details.

Weather
Thu, January 3rd, 2019

Yesterday: Snow showers were observed in the morning with a trace of new snow at Turnagain Pass. Rain/snow line was near 300′. Light to moderate ridgetop winds from the East shifted to a NW direction by mid-day. Temperatures cooled in the upper elevations to low 20F’s/upper teens (F) as skies cleared in the afternoon.  

Today: A cooling trend will continue today as a high-pressure system establishes itself over Southcentral, Alaska. Expect temps at 3000′ to reach low teens/upper single digits today. Temperatures near sea level will drop into low 20F’s to teens by this evening. Northwest ridgetop winds will be in 5-15mph range. No precipitation is expected.

Tomorrow: A similar pattern of clear skies, cold temps and light NW winds will continue into the weekend.

*Seattle Ridge weather station was heavily rimed and the anemometer (wind sensor) was destroyed.    

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 25   trace   0   63  
Summit Lake (1400′) 23   0   0   22  
Alyeska Mid (1700′) 25 1   0.12   50  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 16   NW   5   26  
Seattle Ridge (2400′) 21   *N/A   *N/A     *N/A    
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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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