ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Monday February 11th, 2019
Posted by Aleph Johnston-Bloom on 02/11/19 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  above 2000′. Look for signs of recent wind loading. Triggering a small wind slab will be possible on steep, leeward terrain features. There is also still a chance of triggering a large slab avalanche 2-3′ thick on slopes 35 degrees and steeper. Give cornices a wide berth, avoid travel under glide cracks and watch for sluffing.

SUMMIT LAKE / JOHNSON PASS:  Areas south of Turnagain Pass harbor a thinner, weaker snowpack with multiple weak layers present including the MLK buried surface hoar. Choose terrain wisely and look for signs of instability.  

 

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

Saturday, February 2nd a snowshoer triggered an avalanche and was caught, carried and  suffered a minor head injury on the Harding Icefield trail. For more details read the accident report  HERE.   We really appreciate the individual involved sharing their story. This is a good reminder that summer trails often travel through avalanche terrain in the winter.  

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

Yesterday for most of the day the region saw sustained northeasterly winds 10-20 mph. Sunburst weather station recorded gusts into the 30s and 40s. Although only a trace of new snow fell there was old soft snow available for transport. Look for signs of wind loading in the Alpine. Fresh wind slabs maybe tender on steep, unsupported, leeward slopes. Watch for cracking in the surface snow and stiffer snow over softer snow. Although wind slabs are likely to be shallow, they could be more dangerous if they were to step-down and trigger a large slab that breaks in the MLK buried surface hoar. On steep, protected slopes watch for sluffing. 

CORNICES: Cornices are looming large in some of the Alpine terrain. Give them an extra wide berth as they often break farther back than expected.

Note the increased wind speeds during the day on Sunburst yesterday. This was enough to move snow in the Alpine.

 

Avalanche Problem 2
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

As time goes on the likelihood of triggering a large avalanche on the MLK buried surface hoar has decreased. Over the last few days many people have been pushing into steeper terrain without incident. However, don’t forget the lingering concern that this is the kind of avalanche problem where several tracks may be on a slope before someone finds a trigger point and the whole slope avalanches. The MLK buried surface hoar that is roughly 1-3′ deep has been responsible for 13 human triggered avalanches since January 26th. The last one was Wednesday February 6th on Eddies. On that day snowpack tests were pointing toward a stabilizing weak layer and then the 2′-3′ deep avalanche was triggered remotely from the ridge on a steep, unsupported slope. 

What to keep in mind today:

1- This weak layer is widespread in the region and seems to be particularly suspect between 2000′-2500′ due to a melt-freeze crust associated with it. 
2- Use safe travel protocol. Expose only one person at a time (this includes paying attention to other groups in the area), watch partners, stop in safe zones and be rescue ready.
3- Wind loaded steep features, large connected and unsupported slopes are the most suspect. As always, one can simply avoid high consequence terrain and stick to slopes under 35 degrees with nothing steeper above to avoid the issue. 

The MLK buried surface hoar over a melt-freeze crust in a snow pit on Repeat Offender, 2-7-19. 

Additional Concern
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

Glide cracks continue to creep open and are scattered across the region. The last glide crack to release into an avalanche was over a week ago in the Summit zone just north of Manitoba. Glide cracks are unpredictable, not associated with human triggers, and can release without warning at any time. Look out for glide cracks and limit exposure under them.

Lipps glide cracks threaten the skin track and ski terrain, 2-8-19.

 

Mountain Weather

Yesterday: Mostly cloudy skies with very light snow flurries. Winds were northeasterly 10-20 mph with gusts into the 40s. Temperatures were in the 20Fs to 30Fs. Overnight temperatures dropped slightly and winds shifted to the west and eased off, averaging 5-10 mph gusting into the teens.

Today:   Partly cloudy skies with some valley fog becoming mostly cloudy tonight. There is a chance of snow showers overnight, 1-3″. Winds will be light and westerly and temperatures will be in the 20Fs and 30Fs.  

Tomorrow: Skies are forecast to clear in the early morning and become mostly sunny. Winds will be light and westerly and temperatures will be in the 20Fs to low 30Fs.   There is a slight cooling trend with sunshine on tap for most of the week. The next chance for snow looks to be over the weekend.  

*The Seattle Ridge anemometer (wind sensor) was destroyed. We have a replacement on the way and it should be operational by mid February.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 29   0   0   57  
Summit Lake (1400′) 25   0   0   25  
Alyeska Mid (1700′)  28     trace   0.03   50  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  21  NE  10 48  
Seattle Ridge (2400′)  26 *N/A   *N/A   *N/A