ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Tuesday January 15th, 2019
Posted by Aleph Johnston-Bloom on 01/15/19 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 1,500′.  Wind slabs 1-2′ thick will be possible to trigger on steep wind-loaded slopes. Evaluate snow and terrain carefully.  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.
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.

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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

After ten days of high pressure the advisory area was impacted by a storm over the weekend. Strong easterly winds scoured ridges and loaded slopes. Overall snowfall amounts from ranged from 4-6″ at Turnagain Pass to a foot at upper elevations in Girdwood Valley. Observers yesterday noted a few naturals that occurred during the storm with more activity in and around the Girdwood area. Although the winds have calmed down wind slabs are still possible today on steep wind-loaded slopes. It will be important to be careful on terrain that looks pillowed or appears “fat”. 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. The snow that fell over the weekend landed on weak surface snow comprised of near surface facets and surface hoar. This weak snow was easy to spot under the new snow yesterday in quick snow pits. Below around 1800′ there is a significant breakable crust from where wet snow and/or rain depending on elevation fell on Sunday and then froze as the skies cleared Monday morning. 

Cornices: As always remember cornices may break farther back than expected and if they fall have the potential to trigger slabs on the slopes below. 

Natural avalanche on the south side of Raggedtop. Note the wind loading pattern. 1-14-19.

Weak snow buried under the weekend storm snow, 1-14-19.

 

 

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

South of Turnagain – Johnson Pass/Summit Lake zone: A poor snowpack structure exists in these areas and strong winds over the weekend have loaded leeward slopes.  In addition to the recently buried weak surface snow the Christmas buried surface hoar has been found; as well as concerning facet/crust combinations in the bottom of the snowpack. Avalanches may initiate near the ground and be quite dangerous. If you’re headed this way, evaluate terrain exposure and the snowpack as you travel. Be on the lookout for signs of instability. 

 

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 got covered with snow/wind filling them in 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.

 

Lipps, 1-14-19. The glide cracks are harder to spot. 

 Lipps glide cracks, 1-8-19.

 

 

 

Mountain Weather

Yesterday: Skies were partly cloudy. Temperatures were in the 40Fs at sea level and 20Fs at upper elevations. Winds were easterly 10-20 gusting into the 40s decreasing overnight. Skies were clear overnight and temperatures cooled slightly.  

Today:  Skies will start off mostly clear and then clouds increase in the afternoon with a chance of snow. Temperatures will be in the 30Fs to mid 20Fs depending on elevation. Winds will be light 5-15 mph increasing overnight with gusts into the 30s. 1-5″ of snow is forecast to fall overnight with rain/snow line around 900′.

Tomorrow: Cloudy skies,  temperatures in the 30Fs to mid 20Fs and continued rain/snow showers. The overall trend for the week is mostly cloudy skies with a chance of rain/snow showers and temperatures in the 20Fs and 30Fs.    

*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   0   0   53  
Summit Lake (1400′)   26     0   0    22    
Alyeska Mid (1700′) 32   0    0     41  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  25  NE 14   43  
Seattle Ridge (2400′)   29    *N/A *N/A   *N/A