ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Friday January 18th, 2019
Posted by Heather Thamm on 01/18/19 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 2500′ for triggering a slab 1-2′ thick. Slabs may be small and isolated or large enough to bury a person. Cornices have been recently loaded and could be triggered by the weight of a person or snowmachine. Identify glide cracks and avoid/limit your exposure time under this unpredictable hazard.

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 has seen recent wind-loading.  

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

  • The Friends of the Chugach National Forest Avalanche Center is an  official Pick. Click. Give. organization.  When you apply for your PFD please consider supporting your public avalanche center. We rely heavily on your support, which allows us to provide the best possible service. Thank you to all of our donors past, present and future!
  • For the Hatcher Pass Avalanche Center mid-week snowpack report click  HERE  and recent observations click 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

Across our region the snowpack is quite variable in the Alpine. Two storms this week left varying amounts of snow across our region favoring coastal areas like Portage, Girdwood, and the Northern end of Turnagain Pass. These new slabs were formed by strong winds and may range from 1-2’ thick. This new snow may be resting on several persistent weak layers (buried surface hoar or near surface facets) formed during the cold clear period last week and between the two storms on Tuesday. Yesterday we received a report of a skier triggered slab near Placer Valley. The slab was around 18” deep, and triggered by a ski cut on a steep terrain feature. No one was caught, but the slab did have two separate layers and was large enough to get someone in trouble in high consequence terrain. Keep in mind there is potential for a slab to be larger and more connected in bigger terrain. Shooting cracks and whumpfing may not be present until a slab releases suddenly. Identify wind-loaded features, evaluate the terrain for consequences, and always us safe travel protocols.

Further away from coastal areas where less snow fell, strong winds also impacted the upper elevations. Triggering a smaller slab in steep terrain remains possible. Be weary of hard supportable snow especially if it feels hollow or drum-like. 

South of Turnagain – Johnson Pass/Summit Lake zone: A very poor snowpack structure exists in these areas.  Multiple mid-pack weak layers of facets and buried surface hoar have been found as well as a facet/crust combination in the bottom of the snowpack. No recent avalanche activity has been observed since the New Year’s storm, but more uncertainty exists for triggering a deeper more dangerous avalanche. If you’re headed this way, it will be important to evaluate terrain and snowpack as you travel. Be on the lookout for signs of instability and maintain extra cautious around wind-loaded slopes.

Skier triggered slab on a West aspect in the Placer Valley. Also notice the large cornices. 

 

Avalanche Problem 2
Cornice
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
  • TYPE
    Cornice

Cornices:  Recent strong winds and new snow have added stress to cornices. Not only can they break farther back than expected, they may be more tender than usual. Give these features lots of space.

Loose Snow: Yesterday several sun triggered wet-loose avalanches were observed on South facing slopes where rocks and vegetation were heating up the new surface snow. This harzard is unlikely today, but something to keep in mind with inverted temps in the upper elevations. It’s still pretty early in the season for any significant radiation affect at our latitude. 

Several sun triggered wet-loose avalanches observed on South facing terrain of Goat Mt near Crow Creek. Photo by James Lyons.

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

Be on the lookout for glide cracks opening up. They may be hard to see and filled in by recent snow and wind this week. 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. This avalanche hazard is unpredictable and can release without warning. Identifying cracks and avoiding exposure time under them is the best way to manage this problem.

Mountain Weather

Yesterday: Skies were clear with valley fog in some areas. Warm air from the day before was mixing in places and slightly inverted. Temps near sea level dropped into the low 20F’s and upper elevations temps were in the low 30F’s upper 20F’s. No precipitation fell yesterday and winds were calm.

Today: Expect clear skies with valley fog. Temperatures will be inverted with valley bottoms dropping from the low 20F’s into the teens and upper elevations will be in the upper 20F’s. Winds will be light and variable. No precipitation is in the forecast.

Tomorrow: Clear skies are expected through the weekend. Temperatures will continue to fall.   Expect upper elevations in the teens and valley bottoms may see low’s in the single digits F. Winds are expected to be light and variable.

 *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′) 29   0   0   51  
Summit Lake (1400′) 17   0   0   20  
Alyeska Mid (1700′) 29   0   0   39  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 28   Variable    4  10
Seattle Ridge (2400′) 32   *N/A   *N/A     *N/A