ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Sunday January 27th, 2019
Posted by Wendy Wagner on 01/27/19 at 7:00 am.
The Bottom Line
Considerable Avalanche Danger
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

The avalanche danger remains CONSIDERABLE on slopes above 1,500′.  Human triggered slab avalanches between 2 to 5 feet thick remain likely; nine avalanches were triggered yesterday.  These slabs can be triggered from the bottom, side or on top of a slope while traveling along a ridge. They have the potential to  be large, dangerous and unsurviveable.  Additionally, cornices may break farther back than expected and could trigger an avalanche below.  

*Cautious route-finding and conservative decision-making are essential again today.  A  SPECIAL AVALANCHE BULLETIN  remains in effect through the National Weather Service.  

SUMMIT LAKE / JOHNSON PASS:   Dangerous avalanche conditions exist in these regions south of Turnagain Pass. Strong wind, a poor snowpack structure and recent avalanche activity all point to an unstable snowpack.

3. Considerable
Alpine
/ Above 2,500'
Avalanche risk
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
3. Considerable
Treeline
/ 1,000'-2,500'
Avalanche risk
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
1. Low
Below Treeline
/ Below 1,000'
Avalanche risk
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
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

Yesterday was an interesting day in the mountains. The snowpack proved to be highly unstable and there were 9 confirmed human triggered avalanches. These were all on the back side of Seattle Ridge. Four of them I was able to witness. These four were along Main Bowl (1st Bowl) and were triggered by two snowmachiners traveling along the ridge; releasing the avalanches one after another while they moved along. The ridge is so broad they could not see the slope below and had no idea they were triggering these slides. To our knowledge, there was no one caught in any slides yesterday. 

Most, if not all, of the avalanches triggered were triggered remotely. This means triggered by a person not on the slope, but instead on the ridge above, to the side or at the bottom. The snowpack is set-up exactly for this situation and we can expect remote triggered avalanches as we move forward. The scary thing is, this set-up can allow someone to inadvertently trigger an avalanche onto themselves from below or onto someone else from a safe location. Something for all of us to keep in mind.

The problem is last week’s storm snow fell onto a layer of surface hoar (we are now calling the MLK or 1/21 buried surface hoar). This is a persistent weak layer which is inhibiting bonding between the new and old snow and creating our dangerous snowpack structure. How deep the weak layer and subsequent avalanche will be is dependent upon how much snow fell in that zone. The avalanches yesterday were generally 18″ to 3′ thick. Although yesterday’s activity was all between 2,000 and 3,000′, we saw no evidence or anyone traveling in the higher elevations testing those slopes. Hence, all elevations above 1,500′ should be suspect. Points to keep in mind if headed out today:

1-  The snowpack will be just as touchy, we need to know what is and isn’t avalanche terrain before heading out
2-  The snowpack will be slow to adjust/stabilize with this notorious persistent weak layer 
2-  No obvious signs of instability may be seen, yet avalanches could release easily and remotely
3-  Sticking to lower angle slopes (less than 30 degrees) with nothing steeper above is a great way to avoid these avalanche concerns

 

Slab avalanches in Main Bowl (1st Bowl) triggered remotely by riders moving along ridge. 

 

Two slabs triggered remotely by riders along with a cornice break. The slab on the top of the photo is quite thick, up to 5′ at the crown. This area is between Main and Jr’s Bowl (1st and 2nd Bowl) Thanks to these folks for clearing off the weather station!

 

Snowpack structure at 2,300′, West aspect along Seattle Ridge. Clearly seen is the ‘thin gray line’ of the 1/21 (MLK) buried surface hoar.

 

Video linked HERE.

 

South of Turnagain – Johnson Pass/Summit Lake zone: 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. See the video below (link HERE) as Don Sharaf shows the recent weak layer reacting as well as an older weak layer. Avalanches triggered in this area could step down into deeper layers creating a much larger avalanche. Although this region received less snow (6-12″) last week, the snowpack is still very dangerous. 

    
Don Sharaf with the American Avalanche Institute performs an Extended Column Test in the Silvertip area south of Turnagain Pass. A big thanks to the AAS/AAI Pro 2 avalanche course for their information this week.

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
  • Chance
    Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size
    Historic
    Very Large
    Large
    Small

Cornices have grown and may still be quite tender and teetering on the brink of failure. Give them a wide berth from above and limit exposure underneath them. Any cornice fall is not only dangerous itself, but is likely to trigger a slab avalanche below. 

Mountain Weather

Yesterday:   Mostly clear to partly cloudy skies were over the region. Ridgetop winds were light from the east and bumped up slightly to the 15-20mph range overnight. Temperatures have been near 30F at sea level and the lower elevations, while ridgetops have remained near 20F.  

Today:   Mostly cloudy skies are expected today with a chance for a few snow flurries. Up to an inch of snow could fall tonight. Ridgetop winds are slated to keep climbing and reach the 20-30mph range this afternoon from the east. Temperatures look to remain near 30F at the lower elevations and near 20F along ridgelines.

Tomorrow:   Cloudy skies, increased easterly winds and a chance for a few inches of snow is on tap. Temperatures should remain cool enough for snow to sea level.  

*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′) 27   0   0   59  
Summit Lake (1400′) 20   0   0   20  
Alyeska Mid (1700′) 28   0   0   44  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 21   ESE    9 28  
Seattle Ridge (2400′) 25   *N/A   *N/A     *N/A