ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Tuesday February 5th, 2019
Posted by Aleph Johnston-Bloom on 02/05/19 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 remains a ‘scary’  MODERATE  on slopes above 2,000′. A weak layer of buried surface hoar is lurking  2-3′ below  the surface and  it remains possible to trigger a large and dangerous hard slab avalanche; similar to those triggered last Friday. In addition look for recently wind-loaded slopes, avoid cornices and glide cracks and watch your sluff.    

SUMMIT LAKE / JOHNSON PASS:    A variety of weak layers exist in the snowpack and human triggered slab avalanches 1-3′ thick remain possible on slopes with recent wind loading.  

LOST LAKE:   New snow and wind on Sunday in the Lost Lake and Seward area is expected to have increased the avalanche danger. 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

  • New snow and wind has created dangerous avalanche conditions regionally.  There was a partial burial at Hatcher Pass yesterday. For preliminary information click HERE and check out the Hatcher Pass Avalanche Center Facebook page for current conditions.  
  • GET  your tickets now! SNOWBALL is this Thursday – February 7th:  This annual fundraiser supports CNFAIC and Alaska Avalanche School with a silent auction and a raffle. Come celebrate the winter season with the funk band  Superfrequency  at the 49th State Theater in Anchorage!!!
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

The problem with the MLK buried surface hoar sitting 2-3′ below the surface is the potential consequences and the uncertainty.  It was the culprit in all of the scary avalanches on the Seattle Ridge side. The most recently triggered avalanches were four days ago and snowpack tests on Saturday showed that the layer still has the potential to be triggered on that side of the road. On the non-motorized side there haven’t been avalanches triggered.  The buried surface hoar has been found in snowpits but recently has not shown the same potential for triggering. We don’t have a lot of data on this side. The fact that the MLK buried surface hoar is so widespread above 2000′ in the advisory area is concerning. Is there still a chance of triggering a large unsurvivable avalanche on both sides of the road? Or up Lynx or in Placer?   At this point we are still saying YES. This the type of weak layer that can’t be underestimated. Additional snow fall and wind loading also could add stress to a layer like this.

 What to keep in mind:

   1-  Around 2-3 feet under your feet or snowmachine sits a weak layer
   2-  The weak layer may or may not be reactive, this is the tricky part
   3-  If an avalanche is triggered in this deeper weak layer, it can be very large and propagate across the entire slope. 
   4-  No obvious signs of instability are likely to be seen before a slope releases and it may be the 1st track on the slope or the 20th…
   5-  Remotely triggering a slab from the ridge, sides or below is possible
   6-  Use safe travel protocol and assess the consequences if the slope slides. The larger the terrain = the larger the potential avalanche!

To complicate matters the Girdwood Valley has another weak layer of facets associated with a crust. This is now roughly 2-3′ below the surface. All this said, it’s important to keep in mind that the snowpack harbors buried weak layers. 

Buried surface hoar at 3200′ on Sunburst, 2-4-19.

 Large avalanches on the backside of Seattle Ridge, 2-2-19.

Avalanche Problem 2
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

Both the winds last night and on Sunday were enough to move the soft new snow around. Look for signs of wind loading in the Alpine. Fresh wind slabs maybe tender. 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 buried surface hoar discussed above. 

Loose snow sluffs:  Sluffs on steep slopes within the new snow should be expected.

Cornices:  We had a report of a large cornice fall in upper Seattle Creek drainage over the weekend. Give cornices an extra wide berth as they continue to grow. 

Cross-loading and wind texture on Seattle Ridge, 2-4-19. Photo: Nikki Champion

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 opening again. We know of one glide avalanche that has released recently in the Summit zone just north of Manitoba. Look out for glide cracks and limit exposure under them! 

Mountain Weather

Yesterday:  Mostly clear skies with valley fog between 1000-2000′ decreasing throughout the day. Easterly winds were light during the day but increased to 10-15 mph overnight with gusts into the 20s. Clouds moved in late in the day.  

Today:  Mostly cloudy skies and light snow showers starting early this morning are expected to continue on and off throughout the day as a front pushes in from the southwest, 1-3″ of snow forecast.   Easterly winds 10-15 mph with gusts into the 20s. Temperatures should remain in the 20Fs at upper elevations, and continue to hover near 30F at sea level.

Tomorrow:  Mostly clear skies and calm winds with temperatures in the 20Fs. The next chance for snow looks to be over the weekend. Stay tuned!

*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′)  28 1    0.1 59  
Summit Lake (1400′)  25  1    0.1  28    
Alyeska Mid (1700′)  25  2   0.2   53  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  21  E  8 25  
Seattle Ridge (2400′)  25   *N/A   *N/A   *N/A