ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Wednesday January 16th, 2019
Posted by Wendy Wagner on 01/16/19 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 2,000′. Old wind slabs 1-2′ thick, along with shallow fresh wind slabs less than a foot thick, will be possible to trigger on wind-loaded slopes. In addition, glide cracks may release into avalanches; limit/avoid exposure under them. Give cornices a wide berth.

SUMMIT LAKE / JOHNSON PASS:  Keep in mind buried weak layers exist in the middle and base of the snowpack. More potential for triggering a large slab avalanche exists in this zone, especially in terrain that was recently wind-loaded.  

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

Special Announcements

  • Head down to Alaska Mountaineering and Hiking this evening for a discussion on the State of the Snowpack! Presentation by CNFAIC, from 7-8pm, more details HERE!
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

Wind slabs and cornices will be the main concern today as warm temperatures and a quick moving system rolls through. Ridgetop winds bumped up into the ‘strong’ category overnight from the east. With only a few inches of new snow accompanying the wind, fresh slabs should be shallow and less than a foot thick. Between 1-4″ of snow fell last night at the upper elevations and another 1-3″ is expected today. There is little old loose snow available for transport due to the strong winds last weekend, hence new slabs should be mainly composed of the new snow. Rain falling up to 1,800′ will limit slab development in the mid-elevations. 

Evaluating surface conditions will be key if venturing into the backcountry. Look for loading patterns, i.e. is the slope cross-loaded or top-loaded? Watch for shooting cracks and listen for hollow sounds. These are signs of hard snow over softer snow and indicate wind slab potential. Wind slabs could be stiff enough to allow a person onto them before releasing. Old slabs could still be reactive as they may be sitting on weak snow underneath (near surface facets and surface hoar). 

Cornices:  The warming temperatures at the upper elevations can help destabilize cornices. Not only can they break farther back than expected, they may be more tender today and could also trigger a slab avalanche on the slope below. 

Old natural wind slabs on Raggedtop in the Girdwood Valley. Triggered by cornice falls likely last Sunday, 1/13. 

 

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

South of Turnagain – Johnson Pass/Summit Lake zone: A poor snowpack structure exists in these areas and strong winds over the weekend loaded leeward slopes. Multiple mid-pack weak layers of facets and buried surface hoar have been found as well as concerning facet/crust combinations in the bottom of the snowpack. If you’re headed this way, evaluate terrain exposure and the snowpack as you travel. Be on the lookout for signs of instability and be suspicious of any wind loaded slope.

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

Many glide cracks are covered with recent snow/wind and may be hard to spot. Remember the known areas with cracks are Eddies, Tincan, Sunburst, Magnum, Cornbiscuit, Lipps, Seattle Ridge, Johnson Pass, Lynx Creek, Summit Lake, Petersen Creek, and Girdwood. Avoiding/limiting time under these features is prudent as they can release into an avalanche at any time and are completely unpredictable.

Mountain Weather

Yesterday:   Mostly clear skies were over the region with clouds and light snow/rain moving in overnight. Between 1-2″ of snow has been seen at the mid-elevations. The rain/snow line is hovering between 1,500 – 1,800′. Ridgetop winds increased overnight and are blowing in the 30’smph this morning with gusts near 50mph from the east. Temperatures are warm, upper 20’s to 30F along ridgelines, upper 30’s at 1,000′ and 40F at sea level.  

Today:   Mostly cloudy skies with light rain/snow showers are expected. Only 1-2″ of new snow should fall in favored zones with light rain below 1,500′. Ridgetop winds are expected to stay strong, 20-30mph with gusts to 50mph from the east. Temperatures should also stay warm with ridgelines reaching 30F and sea level 40F.  

Tomorrow:   Clearing skies and decreasing winds are on tap as the system over the area today moves out. Mostly sunny skies are expected to last through Friday and possibly into the weekend.

*Seattle Ridge weather station was heavily rimed and the anemometer (wind sensor) was destroyed. We are currently working to replace it.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 33   1    0.2 53
Summit Lake (1400′) 29   1   0.1   21  
Alyeska Mid (1700′) 32   0.5   0.2   50  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 27 ENE   19   49  
Seattle Ridge (2400′) 30   *N/A   *N/A     *N/A