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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, December 31st, 2018 - 7:00AM
Expires
Tue, January 1st, 2019 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

Strong winds and new snow have created dangerous avalanche conditions. The avalanche danger is  CONSIDERABLE  at all elevations today. Triggering a large slab avalanche 1-3′ feet thick will be likely on slopes steeper than 30 degrees and natural avalanches are possible. Cautious route-finding and conservative decision-making are essential today. Avoid travel under cornices and glide cracks. Pay attention to changing conditions. If the next storm moves in earlier than forecast the danger will bump up to HIGH  and natural avalanches will be likely.  

GIRDWOOD: Roof avalanches are real hazard with warming temperatures and rain on snow. Pay attention to children and pets and where you park your car.  

SUMMIT LAKE / JOHNSON PASS / LYNX DRAINAGE:    South of Turnagain Pass, keep in mind weak layers near the ground and the potential for large avalanches.  

 

 

Special Announcements

If you are considering Hatcher Pass today, PLEASE check HPAC’s  Saturday avalanche forecast. A close call occurred at Hatcher on Friday and tricky snowpack conditions exist, read the report  HERE.  

  •  There will be intermittent traffic delays today on the Seward Highway for avalanche hazard reduction work between Girdwood and Portage near mileposts 82 to 89. Motorists should expect delays of up to 45 minutes between 12:00 Noon and 1:30 PM. Updates will be posted on the 511 system. http://511.alaska.gov/
Mon, December 31st, 2018
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.
Avalanche Problem 1
  • Storm Slabs
    Storm Slabs
  • 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.
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

Human triggered (skier or snowmachiner) storm slabs are likely today on slopes steeper than 30 degrees. The first in the series of storms forecast to impact the area is moving out this morning. The storm brought sustained strong winds yesterday and last night and heavy snow overnight. Do not let the potential for decent visibility today lure you into avalanche terrain. Look for signs of instability. Recent avalanches are the best clue that the snowpack is unstable. 

24-hour precipitation and wind:

  • Turnagain Pass SNOTEL: 15″ (1.3″ SWE), Sunburst wx station winds: NE, 25-40 mph, gusting to 102
  • Girdwood – Alyeska Midway: 20” (1.5” SWE), Max’s wx station winds: E-NE 25-40 mph, gusting to 92
  • Summit Lake: 3-6″ (0.3” SWE)

With the strong winds and new snow we expect a buffet of storm related avalanche issues today. New snow was landing on a layer of surface hoar and may not bond well to the old snow. The temperatures increased during the storm putting heavier snow over lighter snow. Storm slabs may be very sensitive. Look for cracking and collapsing. With the buried surface hoar potential these slabs may be very reactive even on slopes in the Tincan Trees. Remote triggering may be possible. Avoid travel in runout zones. 

Wind slabs: Strong winds will have the blown snow into the leeward start zones. Due to the high wind speeds slabs may have formed further down the slope than expected. Look for pillowing, drifting and cracking. Be suspect of hollow sounding snow or hard snow over soft snow. 

Cornices: High winds and blowing snow will have added to already large cornices. These may be very sensitive and a cornice fall could trigger a large avalanche on the slope below. Remember these can break further back than expected. 

 Heavy snow overnight on Turnagain Pass.

Surface hoar observed 12-28-18, Photo: Trip Kinney

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 avalanches have been releasing over the past week. Remember these avalanches are not triggered by people and are unpredictable. The most recent release of a glide crack (glide avalanche) was noted Saturday on Lipps. The new snow may have covered some of the glide cracks up. Don’t forget they are looming over popular ski and snowmachine terrain and limit as much time under them as possible! The glide cracks are on the move… Don’t mess with the brown frown!

 Lipps glide avalanche, 12-29-18. Photo: Andy Moderow

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:  As we have been mentioning for some time, a shallow and poor snowpack structure exists in the Summit Lake zone. Buried weak layers of facets associated with crusts sit 1-3′ below the surface near the base of the snowpack. Although Summit did not get much precipitation in this first storm it will be important not to forget the possibility of triggering a larger avalanche that could release near the ground if recreating in that zone. Check out the Summit observations HERE for the most current information. 

Weather
Mon, December 31st, 2018

Yesterday:  Skies were overcast to obscured with snow and rain showers throughout the day intensifying overnight. Rain/snow line was around 500′. Winds were easterly 25-40 mph gusting as high as 102 on Sunburst. Temperatures were in the 20Fs to mid 30Fs.  

Today:  Skies will be mostly to partly cloudy. There is a chance of snow showers during the day. Winds will be easterly10-20 mph with gusts into the 30s. They shift to the north this afternoon and ease up briefly. Temperatures will be in the high 20Fs to mid 30Fs. This evening the next storm moves into the area with easterly winds increasing 20-40 mph gusting into the 50s. Precipitation may be heavy again with rain/snow line around 800′-1000′.  

Tomorrow: Rain and snow in the morning decreasing in the afternoon. Winds will decrease as well. Temperatures will be in the high 20Fs to high 30Fs. The pattern remains active for the week but the model confidence remains low. Stay tuned for storm details.  

*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′) 30   15   1.3   67  
Summit Lake (1400′)  30     3 0.3   15  
Alyeska Mid (1700′) 30    20   1.5   55  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 20   NE   38    102    
Seattle Ridge (2400′) 25   *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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