ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Friday January 4th, 2019
Posted by Heather Thamm on 01/04/19 at 7:00 am.
Avalanche risk
The Bottom Line
Moderate Avalanche Danger
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

The avalanche danger is  MODERATE  in the Alpine and Treeline zones for Turnagain Pass. Triggering a slab avalanche 2-3′ thick is possible and could be large enough to bury or kill a person. Natural glide avalanches are possible today and could release without warning. Give cornices a wide berth and avoid being under glide cracks.

SUMMIT LAKE / JOHNSON PASS / LYNX DRAINAGE:  South of Turnagain Pass, keep in mind multiple buried weak layers exist and recent avalanche activity from the New Years storm was significant. More potential for triggering a large slab avalanches exists in this zone.  

2. Moderate
Alpine
/ Above 2,500'
Avalanche risk
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
2. Moderate
Treeline
/ 1,000'-2,500'
Avalanche risk
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
1. Low
Below Treeline
/ Below 1,000'
Avalanche risk
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

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.

Avalanche Problem 1
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.
  • TYPE
    Persistent Slabs
  • Chance
    Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size
    Historic
    Very Large
    Large
    Small

Yesterday we received a report of a remotely triggered large slab avalanche in Warm-Up Bowl of Seattle Creek. This avalanche was triggered by a snowmachine from Seattle ridge without incident the day before (Wednesday.) The weak layer in question was confirmed to be buried surface hoar. We have been tracking this layer since it formed on Christmas Eve and have found its size and distribution to vary across the region with no consistent pattern.  This is a tricky problem to manage. Multiple people could ride or ski a slope before someone finds a pocket or just the right trigger spot. The incident on Tuesday is proof that this layer is reactive and a slab could be large enough to bury or kill a person. Obvious clues like whumpfing may not be present. Digging a test pit adjacent to a slope may or may not be representative of the presence or reactivity of this weak layer.

As we move away from the New Years Storm that brought hurricane force winds and 2-3’ of snow, we must be diligent with our terrain choices and travel protocols. This means avoiding terrain traps, identifying safe zones and traveling one at a time in avalanche terrain. 

South of Turnagain:  The New Years storm caused some notable avalanche activity throughout Summit Lake on many West and South aspects. This isn’t a big surprise. Observers over the last month have been tracking a poor snowpack structure in this region. Buried weak layers of facets associated with crusts sit near the base of the snowpack and buried surface hoar has also been found similar to Turnagain Pass. The Lynx Creek and Johnson Pass area are also suspect for deeper instabilities and also warrants a cautious mindset. Evaluate terrain and snow as you travel and remember ‘whumpfing’ and recent avalanches are obvious clue of instability.

 

Remotely triggered avalanche on a North aspect of Warm-up Bowl in Seattle Creek that occured on Wednesday. Photo taken yesterday by Wendy Wagner. 

 

 

Crown profile of Warm-up bowl avalanche. 

 

Avalanche Problem 2
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. The 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.
  • TYPE
    Glide Avalanches
  • Chance
    Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size
    Historic
    Very Large
    Large
    Small

Several glide cracks have avalanched over the last few days and we expect more to release through the weekend. Glide cracks have been seen in popular ski and snowmachine terrain and some are covered by new snow from the New Years storm. If you see a glide crack the best way to manage this problem is to avoid traveling under them. They can release at any time and are not typically associated with human triggers. Known glide avalanches have occurred on Lipps,  Seattle Ridge, Penguin Ridge,  Lynx Creek and Johnson Pass this week.  

There are also some very large cornices along many ridgelines across our region. Give them lots of space and similar to glide cracks avoid being directly under them.

This glide crack on Lipps continues to release chunks of the slab. Photo taken yesterday.  

Mountain Weather

Yesterday: skies were clear and sunny. Temperatures dropped from the low 20F’s into the single digits throughout the day at Turnagain Pass. Areas near the coast remained in the 20F’s most of the day. Westerly Ridgetop winds were light, 5-10mph. No precipitation occurred.

Today: Expect clear and sunny skies in the alpine with valley fog. Temperatures will be slightly inverted with cooler air (single digits) at lower elevation and in the teens F near ridgetops. Winds should remain be calm to light from the West.

Tomorrow: An arctic high is positioned over mainland, AK and will continue to bring cold, clear and dry conditions through the weekend. Expect temperatures to range from single digits to teens.

*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′) 15   0   0   62  
Summit Lake (1400′) 2   0   0   21  
Alyeska Mid (1700′) 16   0   0   48  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 15   W   6   19  
Seattle Ridge (2400′) 15   *N/A   *N/A     *N/A