ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Friday February 15th, 2019
Posted by Heather Thamm on 02/15/19 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 2000′. Triggering an isolated wind slab will be possible on steep, leeward terrain features. Triggering a larger slab avalanche 1-3′ thick is becoming more unlikely, but not out of the question for Turnagain Pass. Give cornices a wide berth, and avoid travel under glide cracks.

SUMMIT LAKE / JOHNSON PASS: Areas south of Turnagain Pass harbor a thinner, weaker snowpack with multiple weak layers present including buried surface hoar. This zone has more potential for triggering a large slab avalanche 1-3′ thick. Choose terrain wisely. Whumpfing, shooting cracks and recent avalanches are 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

  • Heading to Hatcher Pass?  Be sure to check out the Hatcher Pass Avalanche Center  mid-week snow and avalanche summary  and  observations page  for current conditions.
  • 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 the CNFAIC. We rely heavily on your support, thank you to all of our donors, past, present and future!
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

Two days of strong Northwest winds 20-40mph blew most of the loose snow either into the atmosphere, eroded old tracks, or packed it into hard wind slabs and wind crusts. Dozens of small wind slabs were triggered naturally during this event, and several human triggered wind slabs were observed including one yesterday in Lynx Creek. Today triggering an isolated wind slab will be possible in very steep terrain. Smooth hard supportable snow on steep convex and unsupported features will be the most suspect. Wind slabs could be small and shallow or larger if they step down to older weak snow. More on this below. Also keep in mind the Northwest wind direction creates unusual wind loading patterns opposite our normal Easterly storm track direction. Aspects that are typically more shallow and scoured may be more loaded than usual.

CORNICES: Cornices are large in some of the Alpine terrain. Winds may have added additional stress this week. Give them an extra wide berth as they often break farther back than expected. 

 

Wind eroded tracks along Seattle Ridge yesterday are a good example of how strong wind over the last few days have moved a lot of snow. 

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

Strong Northwest winds over the last two days were a good test of the snowpack. Summit Lake proved to be the most active, which isn’t a surprise due to an overall thinner and weaker snowpack. Dozens of small natural wind slab avalanches occurred in steep channeled terrain, and some stepped down to older layers producing larger avalanches. Most of this activity occurred on East facing aspects of Fresno Ridge to Gil Patrick and further South to the Sterling Wye. In Turnagain Pass very little avalanche activity was observed other than a few wind slabs in steep terrain and nothing larger that we know of.

What about the MLK buried surface hoar we’ve been talking about for the last few weeks? Is the weight of a person or snow machine still able to trigger this widespread weak layer that sits 1-3’ below the surface? Some uncertainty does remain, but it is becoming more and more unlikely with time. We know buried surface hoar is still lurking, but it has been 10 days since an avalanche was triggered  on this layer in Turnagain. Strong winds over the last few days have added additional stress in some places and this should still be on your mind if you are out searching for soft snow. 

If you’re headed into the mountains keep in mind:

  1. Wind loaded steep features, large connected and unsupported slopes are the most suspect.
  2. Eastern aspects that typically harbor a weaker structure may be more loaded than usual.
  3. There is more potential for triggering this problem in Summit Lake, Johnson Pass and Lynx Creek due to a generally thinner and weaker snowpack.
  4. 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

Pit results yesterday on Seattle Ridge showed the MLK (Martin Luther King Day) buried surface hoar layer present but unreactive.

 

 

Several pockets of wind slab on the flank of two East facing gullies of Summit Peak stepped down into older snow closer to the gut.

 

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 are on the move and becoming more active this week. Two glides have released this week, one near the Hope Wye and another in Girdwood Valley. Several glide cracks threaten popular routes on the South facing slopes of Lipps and Magnum. There is also a new glide crack opening on Seattle Ridge on Repeat Offender in a zone that is commonly high-marked. Glide cracks are unpredictable, not associated with human triggers, and can release without warning at any time. The best way to manage this problem is to identify their location and avoid traveling on slopes directly below glide cracks.

 

New glide crack opening up on Seattle Ridge just South of the up-track. 

 

Mountain Weather

Yesterday: Skies were clear and sunny. Temperatures at all elevation dipped into the single digits F. High temps near sea level were in the teens F. Ridgetops and channeled valleys experienced Northwest winds 5-15mph with gusts 20-30mph. No precipitation fell.  

Today: Expect another day of clear and cold weather. Temperatures will remain in the single digits this morning, but will gradually increase throughout the day in sync with a weather pattern shift. Low pressure moving into our region will bring temperatures back into the teens and low 20F’s overnight. Ridgetop winds 5-15mph from the Northwest will  diminish and shift mid-day to an Easterly directly.  

Tomorrow:   The first of several storms will move into Southcentral Saturday and a second storm will arrive on Sunday. Cold air from the North will keep temperatures at or below freezing. At this point there remains a lot of uncertainty around quantity of snow accumulation, but the Southwest flow typically favors Cook Inlet and Hatcher Pass. Sunday’s storm looks more promising for the Eastern Kenai Mountains. Strong Easterly winds will accompany these storms. Expect ridgetop winds to build to 30-40’s mph by Saturday evening.  

*The Seattle Ridge anemometer was replaced yesterday and has been recording wind data since 1pm, Feb 14.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 11   0   0   57  
Summit Lake (1400′) 6   0   0   24  
Alyeska Mid (1700′) 9   0   0   50  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 3   WNW   8   26  
Seattle Ridge (2400′) 8   *NW   *4   *10