ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Wednesday February 6th, 2019
Posted by Aleph Johnston-Bloom on 02/06/19 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  triggering a large and dangerous hard slab avalanche is still possible.  In addition look for recently wind-loaded slopes and roller balls on solar aspects. Avoid cornices and glide cracks and watch your sluff in steep terrain.    

SUMMIT LAKE / JOHNSON PASS:    A variety of weak layers exist in the snowpack and human triggered slab avalanches 1-3′ thick remain possible. Choose terrain wisely.  

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

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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 lure of sunshine and soft snow today with a lurking buried weak layer (MLK buried surface hoar) is a spooky combination. Before entering avalanche terrain above 2000′ remember the size and consequences of the slides that occurred on Friday on Widowmaker and Jr.’s. Snowpack tests on Saturday in this terrain showed that the layer of surface hoar still has the potential to be triggered. There haven’t been avalanches triggered in other zones but the buried surface hoar has been found throughout the advisory area in snow pits from Crow Creek to Summit Lake. The MLK buried surface hoar is sandwiched between hard layers of snow and in places associated with a melt-freeze crust. In some spots it seems to becoming less reactive in snowpack tests but the fact that the MLK buried surface hoar is so widespread is concerning. Is there a chance of triggering a large unsurvivable avalanche?  At this point we are still saying YES. This is the type of weak layer that can’t be underestimated or forgotten.

What to keep in mind as you travel today: 

   1-  The weak layer is around 2-3 feet under your feet or snowmachine. 
   2-  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…
   3-  If an avalanche is triggered in this deeper weak layer, it can be very large and propagate across the entire slope. 
   4-  There is still a chance of remotely triggering a slab from the ridge, sides or below.
   5-  Use safe travel protocols: One at a time in avalanche terrain, Stop in safe zones, Watch your partners and Avoid terrain traps!
   6-  Assess the consequences if the slope slides. The larger the terrain = the larger the potential avalanche!

Crown profile from one of the Widowmaker avalanches that occurred Friday, 2-1-19.

 

One of the Widowmaker avalanches, 2-1-19. Photo: Jerry Mann

 MLK buried surface hoar in a quick pit on Sunburst at 2800′, 2-4-19.

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

There is obvious wind effect in the Alpine from the winds on Sunday day and Monday night. Lingering wind slabs are still possible today. Look for wind loading patterns and be suspect of loaded slopes. 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 avalanches (dry and wet):  Watch your sluff in steep terrain on shaded aspects. In steep terrain receiving direct sunshine look for roller balls and small wet loose avalanches. 

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 often break farther back than expected. 

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 cloudy with light snow showers in the early morning. Winds were easterly 5-15 mph with gusts into the 20s, decreasing to light winds below 5 mph overnight. Temperatures were in the low 20Fs at upper elevations and hovered near 30F at sea level.

Today:  Mostly cloudy skies with a slight chance of flurries in the morning becoming mostly sunny by this afternoon. Calm easterly winds throughout the day with gusts up to 5 mph. Temperatures will be in the 20Fs at upper elevations and in the 30Fs at sea level.

Tomorrow:  Mostly cloudy skies in the morning, becoming partly sunny in the afternoon. Light westerly winds with temperatures in the 20Fs to low 30Fs. Friday looks to be similar. There is a chance of snow over the weekend and a more active pattern forecast for next week. 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 0   0   59  
Summit Lake (1400′)  27 0   0   27  
Alyeska Mid (1700′)  27  0      0     53  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 21    E 5   21  
Seattle Ridge (2400′) 26    *N/A *N/A   *N/A